structure of anterior dorsal ventricular ridge in a turtle (pseudemys scripta elegans)

Structure of Anterior Dorsal Ventricular Ridge in a Turtle (Pseudernys scripta elegans)

CAREY D. BALABAN Department of Anatomy, University of Chicago, 1025 E, 57th Street, Chicago, Illinois 60637

ABSTRACT The dorsal ventricular ridge (DVR) is a subcortical, telencephalic structure in reptiles and birds that protrudes into the lateral ventricle. The structure of DVR has been studied in the red-eared turtle (Pseudemys scripta elegans) in Nissl and Golgi preparations. The DVR in Pseudemys is divided into the anterior dorsal ventricular ridge (ADVR) and the basal dorsal ventricular ridge (BDVR) by the dorsal branch of the middle ventricular sulcus. The structure of ADVR has been examined in detail.

The ADVR is divided into four regions with distinct boundaries termed dorsal area, medial area, ventral area and central area. Dorsal area, medial area and ventral area border on the lateral ventricle; central area lies deep to the other areas. Three classes of neurons are found in Golgi preparations of ADVR. Jux- taependymal cells have somata near the perikarya of ependymal cells; their dendrites are found primarily in a periventricular fiber zone. Aspiny neurons were observed only in the dorsal half of ADVR and appear to be restricted to deep regions of the ridge. These multipolar neurons are rarely encountered in Golgi preparations, and the observed distribution may not represent their actual distribution in ADVR. The majority of the cells observed in ADVR are spiny neurons with dendritic fields that range from stellate to double-pyramidal. Cells in this class may be subdivided on the basis of axonal morphology into a t least two groups, but further studies are needed to determine the range of axonal morphology exhibited by these neurons.

An analysis of the distribution of these cell types in Golgi material shows that dorsal area, medial area and ventral area are organized in four zones concentric with the ventricular surface. Central area apparently lacks a concentric pattern of organization. Zone 1 is a periventricular fiber band that contains juxtaependymal neurons and ascending dendrites of zone 2 spiny neurons, and it may serve as a structural substrate for segregated input onto these cell populations. Zone 2 contains clusters of spiny neurons with apposed somata, which vary in size and distribution between areas. Dendrites of zone 4 neurons are also found in the deep half of zone 2. Zone 3 is a cell-poor region which lies a t the center of a region of overlapping dendritic fields of zone 2 and zone 4 neurons. Zone 4 contains predominantly spiny neurons (aspiny neurons are found only in the dorsal half of ADVR) which are either isolated or in small clusters with apposed somata. Dendrites of zone 2 cells extend superficially into zone 4, so that the deep portions of zone 4 may be a substrate for segregated input to zone 4 neurons. These zones are differentially elaborated in each area. Central area, by contrast, consists of scattered spiny and aspiny neurons among fibers connecting ADVR and the lateral forebrain bundle.

A comparison of these findings with the ADVR of snakes (Ulinski, '78a,b) shows both similarities and differences in DVR organization in the two taxa. Although snakes lack areal divisions, ADVR is organized in four concentric zones (zones A-D). Zones A and B resemble zones 1 and 2 in turtles, consisting of a superficial fiber zone and a subjacent cell cluster zone. The clusters are smaller in snakes than in turtles. However, snakes lack a cell-poor band deep to zone B, and dendrites of cells in zone C enter zone A. Thus, there are differences in both areal and zonal dimensions of ADVR organization in turtles and snakes.

J. MORPH. (1978) 258: 291-322. 29 1

292 CAREY D.

The dorsal ventricular ridge (DVR) is a subcortical, telencephalic structure which receives discrete thalamic projections in reptiles and birds (Karten, '69; Hall and Ebner, '70a; Northcutt, '78). Experimental studies have demonstrated that nucleus rotundus, a tectorecipient thalamic nucleus, projects to a discrete, rostrolateral portion of DVR in emydid turtles (Hall and Ebner, '70a,b; Kosareva, '74; Parent, '761, a crocodilian, Cai- man crocodilus (Pritz, '75) and a monitor lizard, Varanus benegalensis (Distel and Eb- besson, '75). Nucleus rotundus of pigeons projects to a rostrolateral portion of DVR, the ectostriatum (Karten, '69; Benowitz and Karten, '76). Auditory projections from the thalamus to DVR have also been identified. A discrete, caudomedial region of DVR receives a projection from nucleus medialis in Iguana iguana (Foster and Peele, '75; Foster, '76), from nucleus reuniens in Caiman (Pritz, '74) and in Varanus (Distel and Ebbesson, '75), and from nucleus ovoidalis in pigeons and ca- naries (Karten, '69; Nottebohm et al., '76). In addition, a spinothalamic projection to DVR via nucleus medialis posterior has been reported in Caiman crocodilus (Northcutt and Pritz, '78). Butler and Ebner ('72) report that two discrete fields of degeneration are present in DVR following large thalamic lesions in Iguana. One region is located in the medial third of DVR, in the position occupied by the nucleus medialis projection (Foster, '76) ; the second is located in the middle third of DVR. There is physiological evidence that the latter region receives visual input in another igua- nid, Dipsosaurus dorsalis (Peterson and Rowe, '76). However, it has not been determined whether this reflects a rotunda1 projection to DVR or a projection from the dorsal lateral geniculate nucleus as reported in snakes (Wang and Halpern, '77) and Tupinambis ni- gropunctatus (Lohman and van Woerden-Ver- kley, '78). Thus, it appears that reptiles and birds have a common pattern of thalamic afferents to DVR in which visual information is directed rostrolaterally and auditory information is directed caudomedially.

The cytoarchitecture of DVR in different reptilian groups, however, does not reflect this common pattern of thalamic projections. Sev- eral distinct patterns of neuronal distribution can be identified in different reptiles in Nissl preparations. For example, turtles have a DVR which can be divided into two concentric layers, a superficial cell plate and a core nu-

BALABAN

cleus (Johnston, '15; Riss et al., '69; North- cutt, '70). Northcutt ('78) has reported similar divisions of DVR in most families of lizards. The superficial cell plate lies just deep to the somata of ependymal cells and tends to contain clusters of neurons with apposed somata. The size of the clusters varies between species, and clusters are formed by cells of different sizes in some taxa. The centrally-positioned core nucleus is a more diffusely organized cellular field which contains fewer and smaller clusters than does the peripheral cell plate. In addition, Northcutt ('78) has reported that cells in the core nucleus may be smaller than those situated peripherally in some species. The DVR of Sphenodon differs from this pattern, consisting of only a superficial cell plate (Cairney, '26; Durward, '30; Smith, '19; Northcutt, '78). By contrast, DVR of snakes (Ulinski, '76), chameleons, agamids, teiids, iguanids and varanids (Northcutt, '78) cannot be divided into a superficial cell plate and a core nucleus, although these taxa tend to have superficially placed neuronal clusters. Thus, these studies suggest that the internal organization of DVR may differ between the various groups of living reptiles.

Nissl preparations provide an incomplete picture of the organization of a neural structure because they only give information about the relative sizes and the spatial distributions of neuronal somata. A more detailed under- standing requires information about dendritic and axonal morphology and orientation of DVR neurons. An analysis of this type has thus far been reported only for DVR of snakes (Ulinski, '76, '78a,b). Four concentric zones (A, B, C and D) are present in the anterior dorsal ventricular ridge (ADVR) of snakes. Zone A is a cell-poor band deep to the ependyma. Zone B is a band 100-150 p wide that contains clusters of two to six neurons with apposed somata. Zones C and D, distinguished on the basis of the axonal morphology of the constituent neurons, tend to contain isolated neurons rather than cell clusters. The ADVR of snakes contains spiny neurons with dendritic trees that range from double pyramidal to stellate, and cells in zone B tend to have a higher density of dendritic spines than do those in zones C and D. Cells in zone C are located more dorsally than those in zone D and have axons which lack preferred orientations. Zone D contains a population of neurons that project out of ADVR into the subjacent s triatum.

ADVR IN PSEUDEMYS 293

The wide variability in cytoarchitecture of reptilian DVRs suggests that the zonal pattern in snakes may not be the only pattern of DVR organization in reptiles. This paper ana- lyzes the organization of DVR in Nissl and Golgi preparations in the red-eared turtle (Pseudemys scripta elegans '1, a representative of a second reptilian order, and then com- pares the pattern of DVR organization in turtles with that of snakes. The gross morphology of DVR is similar in both taxa and similar populations of neurons are present in the rostra1 division, ADVR. The ADVR in both groups is organized as a series of zones oriented concentric with the ventricular border of the structure and perpendicular to fibers of the lateral forebrain bundle. However, there are differences in the details of zonal organization. A major difference in ADVR of turtles is the presence of four areas with distinct borders in both Nissl and Golgi preparations. Three of these areas show different variations of the basic zonal pattern of organization; the fourth lies at the center of ADVR and lacks zones.

MATERIALS AND METHODS

His to1 ogy Red-eared turtles (Pseudemys scripta ele-

guns) ranging from seven to ten inches in carapace length were obtained from Nasco, Fort Atkinson, Wisconsin. Six turtles were perfused intracardially with 0.9% saline solution followed by 10% formosaline solution for cell and fiber staining procedures. Four brains were embedded in either celloidin or paraffin, and were sectioned a t 15 p or 25 p in the transverse (2 brains), horizontal or sagittal plane. Serial sections were stained with either cresyl violet (3 brains) or with the Weil method for myelin sheaths. In addition, two forebrains were embedded in either gelatin-albu- min (Ebbesson, '70) or 12% gelatin-30% su- crose (R. Switzer, personal communication), sectioned a t 25 p or 60 p in the transverse plane, and every eighth section was stained with either the Fink-Schneider reduced silver method (Schneider, '69) or cresyl violet, respectively.

Thirteen turtles were used for Golgi preparations. Four brains were processed with the rapid variant of the Golgi-Cox method (Ramon-Moliner, '701, embedded in celloidin and sectioned a t 80 p in either the transverse (3 brains) or the sagittal plane. Eight turtles were used for Golgi-Kopsch material accord-

ing to the method of Colonnier ('64). Following impregnation, the tissue was dehydrated through a graded series of acetone solutions (70% aqueous, 95% aqueous and loo%), cleared in xylene, and embedded in paraffin. The material was sectioned a t 7 0 p in either the transverse (5 brains), sagittal (2 brains) or horizontal plane. One brain was used for rapid Golgi material. This brain was singly impregnated according to the procedure of Valverde ('70) with the dichromate impregnation time reduced to five days. The tissue was then dehydrated through a graded acetone series, embedded in soft epon (Epon A mixture) and sectioned in the transverse plane a t 50 p on a sliding microtome. The acetone dehydration was found to minimize deposition of silver in the background in both Golgi-Kopsch and rapid Golgi material.

Sampling of neuron types in ADVR Camera lucida drawings were made of rep-

resentative cells from the different Golgi preparations in order to sample the diversity of neuron types in ADVR. The majority of the drawings were made a t 625 x with a 40 x plan-apochromatic objective, although several panoramic drawings were completed a t 390 x . All cytological details were checked a t 1,250 X with an oil-immersion, 100 X plan- apochromatic objective. A large sample of drawings of well impregnated neurons permit- ted both the identification of different morphological categories of cells and the assess- ment of morphological variability within each category. Although drawings were usually made of the most completely impregnated neurons, both poorly impregnated neurons and those with truncated processes were sampled to determine whether they were consistent with the identified categories. Table 1 summarizes the total number of well-impregnated examples of each cell type, by area, drawn in the course of the study. The position of each sampled neuron in ADVR was noted on each drawing, and some were subsequently used in the reconstructions of the areas of ADVR in figures 9, 11, 13 and 15.

RESULTS

General anatomy of dorsal ventricular ridge

The dorsal ventricular ridge (DVR) in Pseu-

' Although several investigators have considered Pseudemys and Chrysemys as congeneric (McDowell, '64; Zug, ,711, the generic distinction is common in the literature and is, therefore, retained here.

294 CARE

ADVR, Anterior dorsal C, Central i 3, Medial forebrain bundle

BVDR, Basal dorsal L, Lateral cw LZA

BT, Basal telencephalon M, Medial area V, Ventral area

ventricular ridge D, Dorsal a Olfactory bulb

ventricular ridge LFB, Lateral forebrain bundle P Th, Pallial Thickening w I , Optic tectum

A

5 mm

Fig. 1 Gross morphology of dorsal ventricular ridge (DVR) in Pseudemys. A. Reconstructed medial view of DVR following removal of the medial aspect of the forebrain. DVR is shown in stipple. The planes of section for figures 3A, 3B and 3C are indicated a t the appropriate level. B. Reconstructed dorsal view of DVR following removal of the dorsal aspect of the cortex. DVR is stippled, and the plane of section of figure 2 is illustrated. These drawings show that DVR is divided into two parts by the middle ventricular sulcus (MVS). The bifurcation of the MVS is indicated by an arrow in (A). The olfactory bulb (OBI and optic tectum (OT) are included for reference.


Fig. 2 Sagittal section of DVR. The bifurcation of the middle ventricular sulcus (MVS) is shown in a section stained with cresyl violet. Arrows indicate the position of the MVS, which divides DVR into the anterior dorsal ventricular ridge (ADVR) and the basal dorsal ventricular ridge (BDVR). The level of the section is indicated in figure 1B. The optic tectum (OT), pallial thickening (P Th) and basal telencephalon (BT) are also labelled. Calibrating bar is 1 mm.

TABLE 1

Summary of the number of well-impregnated examples of each cell type drawn in the course of the study

Dorsal area Medial area Ventral area Central area Total

Juxtaependymal neurons 4 14 1 - 19

Spiny neurons 54 43 31 14 142 11 28 12 5 Aspiny neurons

70 62 32 25 189 - - - - - -

demys is a subcortical, telencephalic structure which protrudes medially into the lateral ventricle from the lateral telencephalic wall. Me- dial and dorsal views of DVR are shown in figure 1. In figure lA, the forebrain has been divided sagittally and parts of the septum and medial cortex removed to expose the medial surface of the ridge. The dorsal surface of DVR following removal of the dorsal aspect of the cortex is shown in figure 1B. These illustrations of the gross morphology of DVR show that it is comprised of two distinct parts, the anterior dorsal ventricular ridge (ADVR) and the basal dorsal ventricular ridge (BDVR).

The ADVR lies both rostra1 and dorsal to BDVR and is the largest part of DVR. The ADVR is surrounded dorsally, medially and ventrally by the lateral ventricle over much of its anteroposterior extent (figs. 1-31. The ventricular surface of the dorsolateral and ventrolateral surfaces of ADVR is folded to form the dorsal and middle ventricular sulci. These sulci are shown in transverse sections through three different levels of ADVR in figure 3. The dorsal ventricular sulcus (DVS) marks the junction of ADVR and the lateral telencephalic wall; further caudally it forms the dorsal border of attachment of BDVR (fig. 1).

296 CAREY D. BALABAN

Fig. 3 Transverse sections through DVR. The level of each section is indicated in figure 1A. ADVR protrudes into the lateral ventricle from the lateral wall of the telencephalon, bordered dorsally by the dorsal ventricular sulcus and ventrally by the middle ventricular sulcus. The sulci are indicated by arrows. The areas of ADVR present in each section are indicated on a mirror-image tracing of the accompanying photo- graph from cresyl violet sections. The calibrating bar is 500 p. A. Section through the rostral pole of ADVR. This section shows medial area (MI wrapping around the rostral pole of ADVR. B. Section through mid- ADVR. This section illustrates the four areas, dorsal area (D), medial area (MI, ventral area (V) and central area (C), as they appear over most of ADVR. C. Section near the caudal pole of ADVR. The ventral boundary of ADVR is the dorsal branch of the middle ventricular sulcus, marked by the medial arrow in this section. Only dorsal area (D), medial area (MI and central area (C) are found at this level dorsal to BDVR. Lateral cortex (L) and the pallial thickening (PTh) are also labeled in (A) and (B).

The ventral margin of attachment of ADVR is formed by the middle ventricular sulcus (MVS), which bifurcates at the rostral pole of BDVR into dorsal (MVSd) and vent ra l (MVSv) divisions (fig. 1A). This division of the MVS is illustrated in a parasagittal section (fig. 2). The MVSd marks the ADVR-BDVR boundary at the caudal pole of ADVR, where i t curves medially toward the DVS. The MVSv forms the ventral border of BDVR. The MVS and DVS, then, serve as external landmarks delimiting ADVR and BDVR in Pseudemys.

Areas of anterior dorsal ventricular ridge

The ADVR in Pseudemys contains four cytoarchitectonic regions: dorsal area, medial area, ventral area and central area. These areas, named for their positions in ADVR, are regions with distinct boundaries in cresyl violet preparations. Dorsal, ventral and medial areas are situated around the ventricular surface of the ridge. Central area, on the other

hand, is positioned deep to the other areas. The extent of each area is shown in transverse sections in figure 3.

Dorsal area is a half-crescent shaped region which extends ventrolaterally from the ventricular surface to the deep margin of lateral cortex in transverse sections (figs. 3B,C). I t is characterized by the presence of scattered, small clusters and single neurons in its deep ventrolateral wing. Dorsal area is bounded by medial area rostrally at the rostral pole of ADVR and medially at more caudal levels. The rostral two-thirds of dorsal area lies both ventral and medial to the pallial thickening and the DVS. The DVS is deepened caudal to the bifurcation of the MVS, so tha t the ventricular surface forms the dorsolateral border of dorsal area at the caudal pole of the ADVR (fig. 3 0 .

Medial area is a region deep to the ventricular surface of ADVR which contains a distinctive population of large clusters of neurons superficially and smaller clusters centrally

ADVR IN PSEUDEMYS

Figure 3

297

298 CAREY D.

(figs. 3A-C). I t begins rostrally a t the cell free zone separating ADVR and the pallial thickening and covers the entire rostral pole of ADVR. Figure 3A is a transverse section through the rostral cap of ADVR which shows the neuronal clusters characteristic of medial area. Further caudally, medial area occupies a large region bounded laterally by ventral area, central area and dorsal area and limited on all other sides by the ventricular surface (fig. 3B). Ventral area is not present between the bifurcation of the MVS and the caudal pole of ADVR, so that the ventral boundary of medial area is formed both by MVSd and by BDVR (fig. 3C).

Ventral area is a roughly ovoid region which contains cell clusters and individual neurons adjacent to the MVS (fig. 3B: V). I t extends rostrally from the bifurcation of the MVS to the deep border of medial area at the rostral pole of ADVR. It is bounded laterally by the MVS, ventrally by the ventricular surface, and medially by medial area. The dorsal border of ventral area is formed by a large fascicle of fibers which courses medially into the ventral one-third of central area from the lateral forebrain bundle. Unlike medial and dorsal areas, the largest neuronal clusters in ventral area are not restricted to a clearly demarcated superficial zone. Rather, they extend deep into ventral area.

Central area is the only region of ADVR which does not border on the ventricular surface (figs. 3B, C). I t is easily distinguished from the periventricular areas in cresyl violet materials on the basis of i ts extremely low cell density. Large fascicles of fibers coursing between the lateral forebrain bundle and the periventricular areas of ADVR are observed in reduced silver and Golgi preparations in this region. Large blood vessels also cross central area to reach the periventricular regions, so central area seems to constitute the hilus of ADVR. Central area is bounded dorsally, medially and laterally by dorsal and medial areas between the rostral and the caudal poles of ADVR. I t lies in close proximity to the striatum laterally.

Types of neurons i n ADVR Three types of neurons a re present in Golgi-

Kopsch and rapid Golgi preparations of ADVR. These cell types are designated juxtaependymal neurons, spiny neurons and aspiny neurons on the basis of both their position in

BALABAN

ADVR and their morphological characteristics. The cytology of each type of neuron is described in this section. The following section considers the organization of the neurons into zones in different areas of ADVR.

1. Juxtaependymal neurons Juxtaependymal cells a r e distinguished

from the other cell types by their position, adjacent to the ependymal cells, and by their dendritic morphology. These neurons have somata either adjacent to or embedded within the layer of ependymal perikarya which line the ventricular surface of ADVR (figs. 4, 9, 11) and are found in this position in dorsal, medial and ventral areas in cresyl violet sections.

The somata of juxtaependymal cells give rise to four main dendritic trunks which divide into secondary and tertiary branches within the fiber layer deep to the ependymal somata (fig. 4). These branches extend within the fiber layer to about 200 p from the cell body in rapid Golgi and Golgi-Kopsch material. The pattern of the dendritic fields varies with a cell’s position in ADVR and two major patterns seem to be correlated with the width of the periventricular fiber band. The fiber layer is relatively thin over most of dorsal, medial and ventral areas, and the identical appearance of juxtaependymal cells in these regions in transverse, horizontal and sagittal sections suggests t ha t the dendritic fields of these neurons are discs up to 400 p in diameter and 60 p thick, oriented parallel to the ventricular surface (fig. 4: 1-41. A second form of dendritic tree is found in dorsal area, along the border of tha t region and layer 3 of pallial thickening (figs. 4, 11). The dendrites of juxtaependymal cells in this region extend about 200 p deep to the ventricular surface, into the thickened fiber layer which occupies that region. However, juxtaependymal cells have somata near the ependymal elements and dendritic fields restricted primarily to the subjacent periventricular fiber band independent of their position in ADVR. The dendrites of juxtaependymal cells bear a sparse population of long, pedunculated spines along their lengths. The shafts of t he spines are poorly impregnated in Golgi-Kopsch preparations, so that some juxtaependymal neurons appear to be aspiny. However, the dendritic branching patterns a re consistent with those seen in rapid Golgi materials and the distal swellings of the


Fig. 4 Morphology of juxtaependymal neurons. Camera lucida drawings from Golgi-Kopsch preparations are shown for five representative juxtaependymal neurons. These cells span the range of variability observed in the total sample. Cells 1. 2 and 3 are from zone 1 of medial area. cells 4 and 5 are from zone 1 of dorsal area. The solid line represents the ventricular surface

spines are clear. Axons of juxtaependymal neurons were not found in any of our preparations.

2. Spiny neurons Spiny neurons are a heterogeneous class of

multipolar cells which constitute the most common cell type of ADVR. These neurons are found deep to juxtaependymal neurons in all regions of ADVR. The region immediately subjacent to the juxtaependymal cells is characterized, in cresyl violet preparations, by the presence of large clusters of neurons with apposed somata in dorsal, medial and ventral areas. Spiny neurons with apposed somata are present in this region in both Golgi-Cox and Golgi-Kopsch preparations, and only spiny cells have been found in the region of large neuronal clusters, suggesting that they contribute to these large neuronal clusters. More centrally, the smaller clusters of neurons visi- ble in Nissl material lie in a region which contains both spiny and aspiny neurons, and clus- tering has not been frequently observed in any of our Golgi material. The distribution of spiny neurons is discussed in greater detail in the following section.

The dendritic fields of spiny cells are arranged along a continuum from stellate to double-pyramidal patterns (figs. 5 , 6). The somata, which vary in cross-section from pyramidal or fusiform to circular profiles, give rise to three to five main dendritic trunks.

Only one spiny cell with six primary dendrites was found. Primary trunks bifurcate sever- a l times to form secondary and tert iary branches. Although the number of branch points is variable, completely impregnated spiny neurons show a t least tertiary branches. Dendritic spine density, low on the primary trunks, increases distally. Although the density of pedunculated and simple spines on distal segments is variable, it is considerably higher than that of the associated primary dendritic shafts. Spine density is reduced on the most distal branches, so that the last 50 c~ of a dendrite either is aspiny or shows a very low density of pedunculated spines.

At least two populations of spiny neurons can be distinguished on the basis of axonal morphology. Some spiny cells, restricted to a region approximately 300 p deep to the ventricular surface in dorsal, medial and ventral areas, give rise to a single, unmyelinated axon which extends radially toward the ventricular surface (figs. 5 , 9, 11, 13). These axons divide into a t least two branches in the cell-poor zone deep to the somata of juxtaependymal neurons. The branches travel parallel to the ventricular surface and bear varicosities up to 8 /1 in length. Near the periphery of the cells’ dendritic trees, some axons curve back toward the deeper portions of ADVR. Although examples of these axons were found in both Golgi- Kopsch and rapid Golgi preparations, it has not been possible to trace their total extent.


Other spiny neurons located throughout ADVR show a different axonal branching pattern. The axons of these cells divide, near the cell body, into a series of collateral branches bearing smaller varicosities than those of radial axons (figs. 6, 9, 11, 13, 15). In some cases, the collaterals appear restricted to the neuron’s dendritic field, but it is not clear if this is the general case. It is also not clear if these different axonal types are associated with distinct classes of spiny neurons, or if they lie a t the ends of a spectrum of axonal branching patterns.

3. Aspiny neurons Aspiny neurons are multipolar cells found

in deep parts of dorsal, medial and central areas in Golgi-Kopsch preparations (figs. 7, 9, 11, 15). Although they were only found in the dorsal half of ADVR in these areas, i t is not certain whether this reflects the actual distribution of these cells or whether it is an artifact of the Golgi methods employed. The somata of aspiny neurons are the same size as those of spiny cells, but they give rise to four to nine primary dendrites. The proximal dendritic trunks of ADVR neurons do not stain in cresyl violet sections, so that the relative fre- quency of spiny and aspiny neurons cannot be determined. The proximal dendrites of aspiny cells branch repeatedly. Their distal branches are restricted to a roughly spherical zone about 2 0 0 ~ in diameter, although a few longer dendrites were observed. Axons of aspiny cells could only be traced for short dis- tances. Thus, further study is needed to assess both the within-class variability in dendritic morphology and the axonal branching patterns of these cells.

Zonal organization of ADVR Comparisons of the distribution of neurons

in Golgi and Nissl preparations reveal that dorsal area, medial area and ventral area are each divided into four concentric zones. Zone 1, located deep to the ependymal perikarya, is a fiber layer which contains the somata and dendrites of juxtaependymal cells and dendrites of zone 2 spiny neurons. Zone 2 is a band which contains a distinct population of cell clusters. These clusters appear to be composed exclusively of spiny neurons. Zone 3 is a cell- poor region separating zones 2 and 4. Zone 4, the deepest layer, contains both spiny and aspiny cells in the dorsal half of ADVR. Zone 4 also contains smaller neuronal clusters than

zone 2. These zones vary in relative thickness between the areas. Central area, by contrast, apparently lacks a zonal organization.

1. Medial area The arrangement of the four zones in medial

area is shown in a cresyl violet section from mid-ADVR in figure 8 and in a reconstruction from camera lucida drawing of single neurons from Golgi preparations in figure 9.

Zone 1 Zone 1 is a cell-poor band 30-50 p wide

which lies between the somata of the ependymal cells and the cell clusters in zone 2. The major constituent of this zone is a dense band of unmyelinated fibers oriented concentric with the ventricular surface. Axons frequently leave this fiber band and curve radially toward deeper zones of medial area. Although some of these fibers arise from spiny neurons in zone 2, the course and the origin of the vast majority of the fibers could not be determined. Both juxtaependymal cells and dendrites of zone 2 spiny neurons are found among the fibers in zone 1. Juxtaependymal cells in medial area tend to have dendritic trees restricted to zone 1, so that both their somata and their dendrites lie among the fibers. Sev- eral distal dendrites, though, extend into the superficial part of zone 2. No axons of juxtaependymal cells have been found in Golgi preparations.

Zone 2 Zone 2 is a region 200-300 F wide which con-

tains large clusters of neurons with apposed somata, scattered within a cell-poor neuropil (fig. 8). The largest clusters are found in the dorsal half of medial area, with cluster size tapering off ventrally. Spiny neurons are the only cell type present in Golgi impregnations of zone 2. In addition, spiny cells with apposed somata are found in Golgi-Cox impregnations of zone 2, suggesting that the large clusters are probably composed exclusively of spiny cells. Figures 5 and 9 show the variety of dendritic fields present in the zone. At least one dendrite of each zone 2 cell extends into zone

Fig. 5 Morphology of superficial spiny neurons. Repre- sentative camera lucida drawings of spiny neurons from Golgi-Kopsch preparations of zone 2 of dorsal area (D) and medial area (M) and from rapid Golgi preparations of ventral area (V) are shown. These cells represent the range of variability observed in the course of study. Some axons of these cells ascend and ramify near the ventricular surface, which is represented by solid lines.


3 0 0 u

D

M

V

302

1 0 0 p

Fig. 6 Morphology of deep spiny neurons. Five camera lucida drawings which reflect the morphological variability in these cells are shown. Cell 1 is from zone 2 of medial area (Golgi-Kopsch method); cells 2 and 3 are from zone 4 of ventral area (rapid Golgi method); cell 4 is from zone 4 of dorsal area and cell 5 is from zone 4 of medial area (Golgi-Kopsch method)

1. The dendrites of the more superficial cells tend to travel in zone 1 parallel to the ventricular surface for a distance, while those of deeper cells tend to radiate into zone 1 perpendicular to the surface. The remaining dendrites of zone 2 cells either remain within zone 2, or extend through zone 2 into zone 3, and some of the deeper zone 2 cells send dendrites into the superficial half of zone 4. Those den-

drites which remain in zone 2 tend to ramify parallel with the ventricular surface. Thus, zone 2 contains clusters composed of a homogeneous population of neurons. All of these neurons have dendrites which extend into zone 1, and the zonal relations of the remaining dendrites appear to be partially a function of the depth of the soma in zone 2.

Some of the variety of axonal branching


D

M

Fig. 7 Morphology of aspiny neurons. Representative camera lucida drawings are shown from Golgi-Kopsch preparations of aspiny neurons in dorsal area (D) and medial area (M). Examples of aspiny neurons from central area are illustrated in figure 15.

patterns observed for zone 2 cells is illustrated in figure 9. The axons of some cells ascend into zone 1 and divide into two or more branches bearing small varicosities. Other axons of zone 2 cells travel parallel with the ventricular surface in zone 2 and give off descending branches, which could not be traced in adjacent sections. A third branching pattern is illustrated by axons which descend through zone 3, giving off collateral branches in zones 2 and 3. It was not possible to follow these axons to their complete extent. This is only a cursory overview of axon types present. Fur- ther studies with other Golgi preparations are

needed in order to describe the range of axonal variability of zone 2 spiny neurons.

Zone 3 Zone 3 is a cell-poor lamina which separates

zones 2 and 4. It varies in width between 15 p and 50 p both between and within sections. This variation does not display any systematic pattern and, as a result, zone 3 is more distinct in some sections than in others. Zone 3 contains dendrites of zone 2 and zone 4 cells and axons with all orientations. I t has not been possible to determine which cell types in Golgi material correspond to the zone 3 cells in


Fig. 8 Cytoarchitecture of medial area. This is a high power view of medial area from a transverse section at the level of figure 3B stained with cresyl violet. The four zones are indicated by the numbers 1-4. Zone 1 is a cell- poor region containing somata of juxtaependymal neurons; zone 2 contains isolated, large clusters of neurons with apposed somata; zone 3 is a cell-poor band and zone 4 contains smaller clusters of neurons that are more evenly packed than in zone 2. Calibrating bar is 100 p.

cresyl violet sections. This is probably a sampling problem due both to the selectivity of Golgi techniques and to the small cell population in question.

Zone 4

Zone 4 is a 250-300 1 wide band of small cell clusters and single neurons, which constitutes the deepest layer of medial area. The deep margin of this zone abuts upon central area, and this boundary is marked by the sudden de- crease in cell density characteristic of the latter region. In contrast to the island-like appearance of the large neuronal clusters in zone 2, the smaller clusters in zone 4 tend to be distributed evenly among single neurons, giving this layer a fairly homogeneous appearance in Nissl sections. Zone 4 tends to be wider dorsally than ventrally in transverse sections, and clusters tend to be larger dorsally. Studies of intrinsic and extrinsic connections of medial area are needed in order to determine if the dorsoventral differences justify the division of medial area into dorsal and ventral parts.

Both spiny and aspiny neurons are found in zone 4 (fig. 9). Spiny neurons are distributed throughout zone 4 and show a range of dendritic fields from stellate to double pyramidal forms. The majority of these neurons send dendrites into zone 3 and deeper parts of zone 4. Superficial and deep cells, though, extend processes either superficially in medial area or into central area, respectively. The apical dendrites of superficial zone 4 spiny cells terminate in zone 2; none extend into zone 1. Den- drites of deep cells, may extend into the border region with central area but the degree of overlap could not be determined in Golgi materials. Aspiny cells were also found a t all depths of zone 4 in the dorsal half of medial area, and it has already been stated that their apparent absence ventrally may be a result of sampling from Golgi preparations. It was not possible to determine which cell types contribute to the zone 4 clusters, both because aspiny and spiny neurons are indistinguishable in cresyl violet material and because neurons with touching somata were not found in Golgi preparations.

2. Dorsal area The orientation of zones in dorsal area is

shown in a transverse, cresyl violet section from mid-dorsal area in figure 10, and in a reconstruction from camera lucida drawings of single cells in figure 11. Zone 2 is narrower


2

4

Fig. 9 Golgi reconstruction of medial area. This is a reconstruction from Golgi-Kopsch materials that shows the zonal organization of medial area. Zone 1 contains dendrites of zone 2 spiny neurons and somata and dendrites of juxtaependymal cells. Zone 2 contains clusters of spiny neurons, some of which send axons into zone 1, and some ascending dendrites of superficial zone 4 cells. Zone 3 is a region of overlapping dendritic fields of zone 2 and zone 4 cells. Zone 4 contains both spiny and aspiny neurons, and some descending dendrites of zone 2 cells are found superficially in the region.

than in medial area and contains smaller neuronal clusters. Zone 4, on the other hand, is extremely wide, forming the wedge-shaped lateral wing of dorsal area. Zone 1 tends to be wider than its medial area counterpart and zone 3 is of the same dimensions in both regions. Thus, dorsal and medial areas represent

two variations of a common pattern of zonal organization.

'One

Zone 1 is a cell-poor band 40-90 p wide. I t is continuous medially with zone 1 of medial area and laterally both with layer 3 of the


pallial thickening (Balaban, '78) rostrally and with a periventricular fiber band that wraps around the DVS a t all levels (fig. 3). As in medial area, zone 1 contains a band of fibers, some of which recurve towards deeper zones of dorsal area. Some of the latter fibers originate from zone 2 spiny neurons; the source of the others is unknown. Laterally, some of the fibers that course parallel with the ventricular surface in zone 1 either curve ventrolaterally toward the deep margin of layer 3 of the pallial thickening, or proceed in a periventricular fiber band that wraps around the DVS to merge with layer 3 of the lateral part of dorsal cortex. The sources and extent of these fibers could not be determined. Simi- larly, fibers of indeterminate origin were traced across the border between dorsal and medial areas in this zone. Thus, the possibility remains that some extrinsic connections leave dorsal area in zone 1.

Ascending dendrites of zone 2 spiny neurons and somata and dendrites of juxtaependymal neurons are found in zone 1 (fig. 11). Juxtaep- endymal cells are identical with those of medial area over most of the mediolateral width of the zone. Near the DVS, though, the dendrites of juxtaependymal cells extend up to 200 p deep to the ventricular surface. These deep dendrites are located near the border of dorsal area and layer 3 of the pallial thickening and seem to lie among fibers continuous with zone 1. Axons of juxtaependymal cells have not been found.

Zone 2 Zone 2 is a region, roughly 100-200 p wide,

which contains island-like neuronal clusters scattered in a cell-poor neuropil (fig. 10). Its clusters are smaller than those in zone 2 of medial area. Only spiny neurons have been found in Golgi impregnations of zone 2 of dorsal area, and spiny cells with apposed somata appear in Golgi-Cox material. The dendritic fields of zone 2 spiny cells are the same in dorsal and medial areas (figs. 5,9,11). As in medial area, the dorsal area zone 2 spiny cells send at least one dendrite into zone 1. The dendrites from more superficial cells tend to travel in zone 1 parallel to the ventricular surface, while those from deeper zone 2 cells tend to reach into zone 1 perpendicular to the ventricle. Other dendrites of spiny cells either extend within zone 2 parallel to the ventricular surface, or extend into zone 3. Dendrites of

some deeper zone 2 cells reach superficially into zone 4.

Axonal branching patterns of zone 2 spiny neurons are shown in figures 5 and 11. One axon type ascends into zone 1 and splits into a number of branches bearing varicosities up to 8 p in length. Some of these branches recurve and descend into zone 2, but the total length of these recurved branches could not be determined. Other zone 2 cells have axons which branch near the soma into a series of collaterals restricted within a cell's dendritic tree (fig. 11). Some of these collaterals extend into zone 1, travel for a short distance parallel with the ventricular surface, and then curve back into zone 2. Others either ascend or descend in zone 2; none can be traced beyond a cell's dendritic field. A third pattern is found where the axon initially descends in zone 2 and then divides into descending and ascending branches (fig. 11). The first branch descends toward zone 3, while the ascending branch divides into collaterals which could not be traced beyond zone 2. This is by no means a complete catalogue of axons of zone 2 cells and further studies are needed to specify the morphological range of these processes.

Zone 3 Zone 3 is a cell-poor region, 30-50 p wide,

situated between zones 2 and 4 (fig. 10). Its width is less variable than that of zone 2. Zone 3 contains dendrites of zone 2 and zone 4 neurons and axons coursing both radially and cir- cumferentially in ADVR. No cells have been found in zone 3 in Golgi materials. As in medial area, zone 3 of dorsal area is a region of overlap between the dendritic fields of zone 2 and zone 4 spiny neurons.

Zone 4 Zone 4, the widest portion of dorsal area, ex-

tends ventrolaterally from the deep margin of zone 3 to the deep margin of lateral cortex (figs. 3B, 10). I t contains a relatively dense, uniformly packed population of small neuronal clusters and single cells. Golgi preparations show that zone 4 contains spiny and aspiny neurons embedded in a dense fiber plexus. While some fibers could be traced to a fascicle of fibers from the dorsal peduncle of the lateral forebrain bundle on the ventromedial border of dorsal area, the sources and overall morphology of the fibers could not be determined.


The dendritic fields of zone 4 spiny neurons are arranged along a continuum from stellate to double pyramidal forms (fig. 11). No relationship is evident between a neuron’s position in zone 4 and the shape of its dendritic field; cells from all points on the morphological spectrum appear throughout the zone. While aspiny cells seem restricted to the dorsal part of zone 4 in Golgi material, this may be a sampling artifact. The dendrites of most zone 4 spiny cells are restricted to zone 4; only those cells near zone 3 send dendrites into zones 3 and 2 (fig. 11). As in medial area, ascending dendrites of zone 4 cells do not extend into zone 1. Zone 4 spiny neurons with apposed somata are found in Golgi-Kopsch preparations indicating that a t least some of the small clusters in cresyl violet materials are composed of spiny cells. The possibility that aspiny cells contribute to clusters cannot be ruled out. Only axon hillocks of zone 4 spiny cells were impregnated, suggesting that the axons of these cells may be myelinated.

3. Ventral area The zonal organization of ventral area is

best illustrated in sagittal sections (fig. 12). Zone 2, which contains large cell clusters, varies in thickness across the region, making it difficult to discern the narrow zone 4 in transverse sections. For this reason, it was impossible to draw a precise boundary between zone 2 and deeper zones in the rapid Golgi material that is the basis for the reconstruction in figure 13. Zone 3 was particularly difficult to identify in these sections; cells from zones 2 and 4 were differentiated both by observation of unstained, osmicated profiles in the Golgi material and by the distance of an impregnated neuron from the ventricular surface. The zonal position of neurons illustrated in figure 13 is, therefore, accurate within the limitation of these methods. These particular cells were used for the illustration because their location was unambiguous in the material.

Zone 1

Zone 1 is a periventricular fiber zone, 50- Fig. 10 Cytoarchitecture of dorsal area. This is a high

power view of dorsal area from a section of the level of figure 3B. The four zones are indicated by numbers 1-4. The zones have the same characteristics as those in medial area except that zone 2 is narrower and zone 4 is ex- panded to form the deep ventrolateral wing of the area. Calibrating bar is 100 p.

Figure 10

m

0

m


100 p wide, which contains both juxtaependymal cells and dendrites of zone 2 spiny neurons. The fiber band is continuous medially with zone 1 of medial area; its lateral relations are not understood a t this time. Some fibers in zone 1 clearly originate from zone 2 spiny cells, but the sources and courses of the remainder are unknown.

Zone 2 Zone 2 of ventral area contains large clus-

ters of neurons and varies in width between 250 p and 300 /I. It tends to be wider laterally than medially, and a large cluster of neurons is frequently observed a t its lateral margin in cresyl violet material. Spiny neurons in zone 2 display a variety of dendritic field shapes (fig. 13). As in the other periventricular areas, these spiny cells send dendrites into zones 1 , 3 and 4. Other dendrites extend through zone 2 parallel to the ventricular surface. Dendrites of neurons proximal to zone 1 tend to travel in that zone parallel with the ventricular surface, while more distal zone 2 cells send dendrites into zone 1 perpendicular to the ventricular surface (fig. 13). Individual dendrites which remain in zone 2 may extend up to half the mediolateral width of ventral area, because of the relatively narrow mediolateral dimensions of the region. A consequence of the narrow width of zone 4 is that some dendrites of zone 2 neurons extend through the depths of that zone. Thus, the variation of zonal organization in ventral area allows for the distribution of dendrites of zone 2 neurons in all zones of the area.

Two axonal branching patterns are found among zone 2 spiny neurons in the sample. One axon type proceeds toward zone 1, occasionally giving rise to a collateral branch in zone 2 (figs. 5, 13). The axon then divides in zone 1. Most of these branches travel concentric with the ventricular surface in zone 1, although some recurve into zone 2. Varicos- ities are observed on these axons in both zone 1 and zone 2. The second type of axon divides near the soma into ascending and descending limbs. The ascending branch travels radially toward zone 1; the descending branch proceeds toward zone 3, occasionally leaving a collateral in zone 2. As in other areas, these axons require further study.

Zone 3 Zone 3 is a narrow, cell-poor band, 10-20 p

wide, that separates zones 2 and 4. Although the boundaries of the zone are clear in sagit-

Fig. 12 Cytoarchitecture of ventral area. This is a high power view of ventral area from a sagittal section through the middle of the area. The four zones are labelled with the numbers 1-4. Zone 1 is a cell-poor hand that contains somata of juxtaependymal cells. Zone 2 contains large neuronal clusters, packed more densely than in medial area or dorsal area. Zone 3 is a narrow, cell-poor band and zone 4 is a relatively narrow layer that contains small clusters and single neurons. Calibrating bar is 100 /L.

tally-sectioned material, (fig. 12) it is difficult to specify its boundaries in transverse sections. It was, therefore, impossible to determine whether cells were in zone 3 or 4 in transverse Golgi sections. One such cell is shown in figure 13 with dendrites in zones 2 , 3 and 4. Studies with counterstained Golgi preparations are needed to identify zone 3 cells reliably. As in the other periventricular areas, zone 3 of ventral area contains dendrites of zone 2 and 4 spiny cells and a network of radial and concentric axons.

Zone 4 Zone 4 is a narrow band of evenly-packed,

small neuronal clusters and single neurons

3 10

2

CAREY D. BALABAN

Fig. 13 Golgi reconstruction of ventral area. This reconstruction from rapid Golgi preparations illustrates the zonal organization of ventral area. Zone 1 is a fiber layer containing juxtaependymal cells (not illustrated) and ascending dendrites of zone 2 spiny neurons. Zone 2 contains spiny neurons and ascending dendrites of zone 4 spiny cells. Zone 3 is a narrow, cell-poor region. Zone 4 contains spiny neurons which extend dendrites into a bundle of fibers along the deep margin of the zone and descending dendrites of zone 2 neurons.

which occupies the deep margin of ventral area (fig. 12). It varies in width from 150 p to 250 p in transverse sections. Unlike the other two periventricular areas, only spiny neurons were found in zone 4 of ventral area (fig. 13). This reflects the fact that no aspiny neurons were found in the ventral half of ADVR. Some

dendrites of zone 4 spiny cells extend through zone 3 into zone 2; others ramify within zone 4, spanning a t least half of the mediolateral extent of the zone (fig. 13). Descending dendrites extend dorsally into central area to in- tersect a fascicle of fibers from the lateral bundle which occupies the ventral aspect of


that region. As in dorsal and ventral areas, dendrites of zone 4 cells are not found in zone 1. Three types of axons are observed in zone 4, two of which branch near the cell body (fig. 13). The first type divides into ascending and descending branches, each bearing varicosities. The ascending branches can be traced through zone 3 into zone 2, and collateral branches are present in both zones. The descending branches remain in zone 4 and follow a trajectory parallel to the ventricular surface. It was impossible to trace descending branches between sections, and none could be traced out of zone 4 into either central area or the lateral forebrain bundle. The second axon type that divides proximal to the soma is illustrated by a medial zone 4 cell in figure 13. These axons divide several times within zone 4 into collateral branches which could not be traced into other zones or areas. One branch on the cell illustrated appears to ascend to zone 3 near the periphery of ventral area, but the indefinite borders of that zone in Golgi materials make a determination impossible. Other axons of zone 4 cells ascend radially toward zone 3.

4. Central area Central area appears in transverse sections

as a wedge-shaped region deep to zone 4 of dorsal, medial and ventral areas (fig. 14). Thus, it lies a t the core of the concentric organizational pattern. Central area contains both spiny and aspiny neurons, sparsely distributed among bundles of fibers which radiate through the area from the dorsal peduncle of the lateral forebrain bundle. The orientation of cells in central area is shown in a reconstruction from rapid Golgi and Golgi-Kopsch material in figure 15. Figure 15A shows three representative neurons from the dorsomedial part of central area near its border with dorsal and medial areas. The dendrites of both spiny and aspiny cells tend to be restricted to central area, but i t is possible that some dendrites of more superficial cells may extend for a short distance into zone 4 of dorsal and medial area. These cells are separated from ventral area by a fiber bundle. Figure 15B illustrates the morphology and orientation of cells from ventromedial central area, deep to medial and ventral areas. Only spiny neurons were found in this region. Their dendrites are restricted to a region among the fiber bundles in central area. The axons of some of these cells branch near the initial segment and ramify in the

Fig. 14 Cytoarchitecture of central area. This is a higher power view of central area at the level of figure 3B. Central area contains scattered neurons in a cell-poor neuropil which contains bundles of fibers connecting ADVR and the lateral forebrain bundle. Calibrating bar is 100 I*.

vicinity of the cell body. One axon was traced to the deep margin of zone 4 of ventral area (fig. 151, but most could not be traced for any great distance in central area.

5. Summary The ADVR of Pseudemys contains four

areas with distinct boundaries in Nissl and Golgi preparations. The three periventricular areas, dorsal area, medial area and ventral area, have a common zonal pattern of organization. Central area, by contrast, lacks a zonal pattern of organization, and it contains cells scattered among large bundles of fibers connecting ADVR and the lateral forebrain bundle.

312

B

Fig. 15 Golgi reconstruction of central area. This figure depicts the organization of dorsomedial (A) and ventromedial (B) portions of central area in reconstructions from Golgi-Kopsch and rapid Golgi preparations, respectively. The region contained within each reconstruction is indicated in an inset a t right. Both spiny and aspiny cells were found in the dorsal half of central area; only spiny neurons were observed ventrally. The spiny neurons in (B) were interspersed among fascicles of fibers coursing between the lateral forebrain bundle and medial and ventral areas. Calibrating bar is 100 F.


Each of the periventricular areas contains four zones oriented concentric with the ventricular surface, which vary in relative width between areas. Zone 1 is a periventricular fiber layer that contains somata and dendrites of juxtaependymal neurons and ascending dendrites of zone 2 spiny neurons. Dendrites of neurons in deeper zones do not appear to reach zone 1. Thus, this zone serves as a possible substrate for segregated input to two specific cellular components, juxtaependymal cells and zone 2 cells. Zone 2 contains clusters of spiny neurons with apposed somata, which vary both in size and in distribution between the areas. The largest clusters are found in medial area, where they are isolated in a cell-poor neuropil. The smallest clusters, also isolated, are found in dorsal area. The clusters are more densely distributed in ventral area, and they appear to be smaller than their medial area counterparts. Zone 2 also varies in width between areas; the zone is narrower in dorsal area than in either medial area or ventral area. Zone 3 is a cell-poor band separating zones 2 and 4. This zone contains primarily circumferential and radial fibers and dendrites of zone 2 and zone 4 neurons. It is, therefore, a t the center of a region of overlapping dendritic fields of zone 2 and zone 4 neurons that extends both into the deep half of zone 2 and into superficial part of zone 4. Zone 3 is of roughly the same dimensions (15-50 wide) in dorsal and medial areas, and i t is extremely narrow in ventral area. Zone 4 contains an evenly packed population of small neuronal clusters and single neurons. The majority of cells in this zone are spiny neurons; aspiny cells were found only in the dorsal half of ADVR in this zone. Ascending dendrites of superficial zone 4 cells extend into zone 2, and conversely, descending dendrites of zone 2 neurons are found superficially in zone 4. The dendrites of most zone 4 cells though are restricted to the zone. Thus, only dendrites and somata in deeper parts of zone 4 are in a position to receive afferents segregated to that zone. Zone 4 shows the greatest size variability of any zone between areas. The zone forms the bulk of the wide, lateral wing of dorsal area, it is of intermediate size in medial area, and it is greatly reduced in ventral area.

Thus, each periventricular area shows a different elaboration of the basic zonal pattern. Dorsal area has a wide zone 4; medial area has large, isolated clusters in zone 2 and a narrower zone 4; and ventral area has reduced

zones 3 and 4 and has an increased packing density of neuronal clusters in zone 2. Im- plications of these organizational differences will be discussed in the next section of the paper.

DISCUSSION

This paper has analyzed the structure of ADVR of the red-eared turtle, (Pseudemys scripta elegans) on the basis of data from Nissl and Golgi preparations. These analyses provide information about the types of neurons present in ADVR of this species and the geometry of their distributions in the structure. Since similar analyses have been reported for the ADVR of snakes (Ulinski, '76, '78a,b), it is possible to compare the structure of ADVR in representatives of two major reptilian orders that have cytoarchitectonically distinct modes of ADVR organization. These comparisons can help to identify which characteristics reflect a common pattern of organization of ADVR in reptiles and which are specializations associated with a particular group. In addition, they can assist in deter- mining significant aspects of neuronal organization in each taxon. The comparisons are summarized in figures 16 and 17. Figure 16 shows transverse sections of ADVR of a snake and a turtle; figure 17 depicts, in diagrammatic form, features of cellular organization of ADVR in turtles and snakes.

General anatomy of dorsal ventricular ridge

The division of DVR of Pseudemys into ADVR and BDVR is consistent with previous observations of turtle forebrains. These structures can be differentiated in Pseudemys both by the position of the middle ventricular sulcus (MVS) and by a cell-poor lamina that extends laterally from the sulcus in Nissl preparations (fig. 3C). Illustrations of the gross morphology comparable to figure 1 have been published for Chelonia (Smith, '19), Ter- rupene (Johnston, '15) and Testudo (Hewitt, '67). They show that these turtles also have a sulcus a t the ADVR-BDVR border that corresponds with the MVS. Although Riss et al. ('69) did not recognize an ADVR-BDVR distinction, their illustrations suggest that a division is marked by the MVS in Chelydra, Chrysemys and Podocnemis. Since this division appears to be a characteristic of both cryptodiran and pleurodiran turtles, the term ADVR is proposed as standard nomenclature


Fig, 16 Transverse sections of ADVR in a turtle (Pseudemys, left) and a snake (Coluber constrictor, right). The ADVR of snakes lacks areal divisions present in turtles. but both taxa show a zonal pattern of organization. Calibrating bar is 500 p.

T U R T L E S S N A K E S

Fig. 17 Diagrammatic view of zonal organization in turtles and snakes. The distribution of neuronal types in ADVR in Pseudemys and in snakes is emphasized to compare organizational characteristics. The superficial zones (zones 1 and 2 in turtles; zones A and B in snakes) are similar in both groups. Zones 1 and A are fiber layers; zones 2 and B are characterized by cell clusters which are largest in turtles. The cell populations in the two taxa display similar axonal and dendritic morphology in these zones. The deep zones are different in that dendrites of some zone C neurons in snakes enter zone A, while turtles have a cell-poor zone 3 intercalated between zones 2 and 4. Dendrites of zone 4 neurons extend only into the deep portion of zone 2.


for the rostal division of the turtle DVR. This terminology is consistent with its use for the thalamoreceptive, anterior DVR of squamates (Senn and Northcutt, '73; Northcutt, '78; Ulinski, '76, '78b). Synonyms of turtle ADVR used by earlier workers are epistriatum (Her- rick, '101, dorsal ventricular ridge (Johnston, '151, caudate nucleus (Johnston, '231, hypopal- lium (Smith, '19, Herrick, '261, nucleus laminaris + striatum dorsale (Filimonoff, '64) and upper elevation (Hewitt, '67). In the terminology used by Northcutt ('701, ADVR includes the DVR pars anterior, the core nucleus and rostral and dorsal portions of nucleus cen- tralis amygdalae. Among investigators who did not make an ADVR-BDVR distinction, ADVR corresponds to a rostrodorsal portion of the striatum (Gage, 18951, a similar portion of the occipito-basal lobe (Herrick, 1893), a rostral portion of Field K (Rose, '23) and with a t least the rostral part of the mesostriatum (Edinger, 1896). Similarly, ADVR includes zone 7 and a rostrodorsal portion of zone 8 in the terminology of Riss e t al. ('69). Portions of ADVR have also been labelled as anterior amygdaloid area + amygdala (Carey, '67) and as hyperstriatum (Carey, '701, but the grounds for these designations are not clear.

Previous investigators have typically labelled the Chelonian BDVR as the amygdaloid complex (Johnston, '15, 23; Smith, '19; Hewitt, '67; Carey, '70). More recently, North- cutt ('70) has distinguished a DVR pars posterior and portions of a nucleus basalis amygdala and a nucleus central amygdalae in BDVR of Chrysemys. Riss et al. ('691, on the other hand, have labelled BDVR of Chrysemys as zone 8. In light of these terminological differences, "BDVR" has been chosen as a neu- tral term that describes the anatomical position of the posterior division of DVR in adult turtles. Both ADVR and BDVR are embryo- logical derivatives of the dorsal compartment of the telencephalon (Holmgren, '25; Kallen, '51), and the use of "basal" in the latter term is not intended to imply different embryonic origins for the two structures. This new terminology is proposed as an alternative to the terminology used in squamates and mammals because the relationships between the BDVR, the posterior portions of the squamate DVR, and the mammalian amygdala are unclear. The posterior division of BDVR of squamates contains two distinct structures, posterior dorsal ventricular ridge (PDVR) and nucleus sphericus (Senn and Northcutt, '73; Ulinski

and Kanarek, '73; Northcutt, '78). Nucleus sphericus receives a discrete projection from the accessory olfactory bulb in snakes (Hal- pern, '77) and is most pronounced in squamates with a well-developed vomeronasal system (Ulinski and Kanarek, '73; Northcutt, '78; Rudin, '74). Turtles apparently lack a nucleus sphericus (Rudin, '74; Balaban, personal observations), and results from experimental studies with Fink-Heimer techniques suggest that no portion of the olfactory bulb projects to BDVR in Pseudemys and Graptemys (Bala- ban, '77). Connections of the squamate PDVR and the turtle BDVR are not known. Thus, additional studies of connections and intrinsic organization of the posterior division of DVR in both groups are needed to determine patterns of organization and their relationship to specializations of each group.

Areas in anterior dorsal ventricular ridge

This study is the first to identify non-concentric, areal divisions in turtle ADVR. Areas are operationally defined as regions with distinct boundaries in Nissl and Golgi preparations, while zones are slab-shaped regions with indistinct boundaries, oriented perpendicular to fibers connecting ADVR and the lateral forebrain bundle. Thus, areas tend to be anatomically segregated organizational units, while neurons in a zone have considera- ble overlap of dendritic fields with neurons in other zones. Previous cytoarchitectonic studies of the turtle ADVR have emphasized the concentric organization of the structure. Since a consistent terminology for regions of ADVR has not emerged in the literature, previous schemata will be described in relation to the present areal and zonal divisions.

Johnston ('15) was the first investigator to divide the ADVR of a turtle (Terrapene) into two concentric regions, a superficial cell layer and a core. The core was divided into a dorsally located core nucleus, equivalent to zone 4 of dorsal area, and a more ventral region that corresponds to central area. Johnston also noted differences in the superficial clusters with position in ADVR. In contrast to the isolated arrangement of cell clusters over most of the superficial cell layer, clusters near the middle ventricular sulcus were described as "closely aggregated in large area without special clusters." This region corresponds to ventral area in the present nomenclature. Johnston did not differentiate medial area


from zones 1 and 2 of dorsal area in his superficial cell layer. Although subsequent studies have maintained a superficial cell layer-core distinction, the terms have not been consist- ently applied to the same structures. Thus, Filimonoff ('64) divided the ADVR of Testudo into a nucleus laminaris and a striatum dorsale, equivalent with Johnston's superficial cell layer and core region, respectively. Nu- cleus laminaris was longitudinally subdivided into a anterior and posterior regions and areally into a series of hypopallial plates. Since the criteria for these divisions were not described in the paper, it is not possible to re- late the longitudinal divisions and hypopallial plates to the present nomenclature. Filimon- off also described medial and lateral nuclei in the anterior part of the striatum dorsale which appear to be zone 4 of dorsal area and central area, respectively. A similar superficial cell layer-core division has been presented by Riss et al. ('69) in the description of zone 7 and zone 8 in ADVR of Chelydra, Chrysemys and Podocnemis. Zone 7 is identical with zone 4 of dorsal area; zone 8 includes ventral area, medial area and zones 1 and 2 of dorsal area. Central area was not labelled in the paper. Al- though Northcutt ('70) maintained the superficial cell layer-core distinction, dorsal area, medial area and ventral area were placed in the superficial cell layer and central area was labelled as the core nucleus. The earlier concentric divisions of ADVR, then, correspond poorly with the areal and zonal divisions presented in this study.

Experimental data in the literature suggest that areas may be differentiated on the basis of afferent relations. Following large thalamic lesions in Pseudemys, Hall and Ebner ('70a,b) observed degeneration concentrated in rostral dorsal area and a t the rostral cap of medial area. Discrete lesions of nucleus rotundus in Pseudemys (Hall and Ebner, '70b) and Emys (Kosareva, '74) result in degeneration which appears to be restricted to dorsal area. This rotunda1 projection has been confirmed by injections of HRP into ADVR (Parent, '76). Medial area, by contrast, appears to receive a catecholaminergic projection that terminates around clusters in zone 2 (Parent and Poitras, '74). The present evidence, though, is not detailed enough to permit any firm conclusions about differences in afferent relations of areas in ADVR.

Although Northcutt ('78) has divided ADVR of lizards into areas on the basis of cytoarchitectonic and histochemical evidence,

there is no evidence supporting the existence of areal divisions in snakes (Ulinski, '78a,b). In lizard families where ADVR is divisible into a superficial cell layer and a core nucleus in Nissl preparations, the superficial cell plate has been divided into a t least three regions on the basis of mediolateral cytoarchitectonic differences (Northcutt, '78). Since these regions correspond with loci of intense SDH activity, it is suggested that they reflect sites of termination of discrete visual, somatosensory and auditory pathways from the thalamus. In lizard groups that lack these cytoarchitectonic divisions (iguanids, chameleonids, agamids, teiids and varanids), the areas are defined as discrete regions of SDH activity.

These observations on afferent connections and histochemistry raise two main questions concerning the possible significance of areas, the first in reference to their anatomical definition and the second regarding the nature of input to different areas. Areas in turtles are defined as regions with distinct borders in Nissl and Golgi preparations. Intense SDH activity has only been reported in a region corre- sponding to dorsal area (Baker-Cohen, '681, so that there does not appear to be a one-to-one correspondence between anatomically distinct areas in ADVR of turtles and loci of intense SDH activity. Thus, the areas identified in Iguana by Northcutt ('78) do not qualify as areas under the definition in this paper because a boundary is not present in Nissl preparations. Investigations using Golgi techniques are needed to assess their status as anatomically distinct units. The cytoarchitectonic areas identified in most families of lizards may be areas under the present definition. However, Golgi studies are needed to determine if the cytoarchitectonic differences reflect inter-areal boundaries. The second point involves the hypothesis that these areas represent sites of termination of modality-specific thalamic afferents. Physiological studies have reported that many visual units also re- spond to somatosensory stimuli in both nucleus rotundus and DVR of turtles (Bele- khova, '791, indicating that the thalamic afferents may be multimodal. Thus, it is also possible that the discrete pathways to DVR in reptiles carry multimodal rather than unimo- dal information to the forebrain.

Types of neurons in ADVR 1. Juxtaependymal neurons

Juxtaependymal cells have not been previ- ously reported in turtles. A morphologically


similar class of cells is found in zone A of ADVR of snakes (Ulinski, ’78a,b). As in turtles, the somata of these cells are typically found near the neuroependymal perikarya and their dendritic fields are restricted to the periventricular fiber zone and the superficial part of the subjacent cell cluster zone. However, there are differences in the shape and orientation of dendritic fields of these cells in the two taxa. In snakes, the dendrites of zone A cells tend to extend obliquely through zone A into zone B without arborization parallel with the ventricular surface. Juxtaependymal cells in turtles have significant numbers of dendritic arborizations oriented parallel with the ventricle in zone 1 (fig. 5). Axons of these cells have not been identified in either taxon. The present data, then, suggest that juxtaependy- ma1 neurons in turtles and zone A neurons in snakes are similar in their relationship to a periventricular fiber zone but different in some aspects of dendritic field morphology. While dendrites of cells in snakes extend obliquely through the fiber zone, those in turtles show marked branching parallel with fibers suggesting that local circuitry of these dendrites may be different in the two taxa. Further studies with both anterograde and retrograde tracing techniques are needed to clarify their role in ADVR organization and implications of the observed morphological differences.

2. Spiny neurons The three classes of ADVR neurons de-

scribed in Chrysemys by Northcutt (’70) lie along the continuum of variability of spiny neurons observed in this study. The first group consisted ofpyramidal neurons with long apical dendrites projecting into the “core nucleus” and two or more basal dendrites extend- ing ventrolaterally within the “superficial cell layer.” These cells were primarily found along the medial surface of ADVR (probably medial area) and their axons were traced into the “core nucleus.” The seond neuronal class was composed of small projecting neurons. These cells have multiple dendrites which either ramify within the “superficial cell layer” (dorsal area + medial area + ventral area) or extend into the “core nucleus” (central area). Axons of these cells either contribute to the periventricular fiber layer (zone l), project into central area, or project to the dorsolateral division of dorsal cortex, a region that includes the pallial thickening (John- ston, ’15; Balaban, ’78). The third class was

described as stellate cells that interconnect the other elements. The results of the present investigation suggest that these cell classes are not distinct; rather, they represent arbi- trary divisions of the continuous range of variability in the spiny neuron sample. Since axons of spiny neurons could not be traced out of ADVR in my Golgi preparations, North- cutt’s observations on axons of “projection cells” could not be confirmed. However at least two types of spiny neurons can be distinguished on the basis of axonal morphology. Axons of some zone 2 spiny neurons extend radially into zone 1, where they divide into collateral branches parallel with the ventricle. Some branches recurve toward zone 2 within the dendritic field of the cell; the course of most branches could not be followed. A different type of branching pattern is observed in axons which divide near the cell body into ascending and descending branches. This is by no means a complete list of the diversity of axonal branching patterns of spiny neurons in ADVR of Pseudemys, but it suggests that populations of spiny cells may be distinguished on the basis of axonal morphology.

As is the case in Pseudemys, spiny neurons appear to constitute the principal cell population in ADVR of snakes (Ulinski, ’76, ’78a,b) and chameleons (Ramon, 1896). Ramon (’17, ’19) illustrated a similar class of neurons from Golgi preparations of Iguana and Lacerta. The neurons in the species show the same degree of variability in dendritic morphology as spiny neurons in Pseudemys. In addition, their axons either ascend to a periventricular fiber zone to ramify parallel with the surface or divide near the initial segment into ascending and descending branches. Spiny neurons which send axons out of ADVR into the subjacent striatum have also been found in both snakes and chameleons. Spiny neurons in turtles show a variable density of dendritic spines, but there does not seem to be a systematic relationship between spine density and zonal position in ADVR. In snakes, though, neurons with high spine density tend to be located more superficially than those with low spine density (Ulinski, ’78b). Dia- mond et al. (‘70) have suggested that the stalk of a dendritic spine has a relatively high resistance with respect to the remainder of the dendritic membrane. Given this assumption, the electrotonic activity in the spine is ex- pected to be relatively isolated from activity in the remainder of the cell. One result would


be that the activity in the spine is attenuated in transmission to the dendritic shaft, facili- tating a linear summation of afferent activity. A related proposal by Rall ('70) suggests that spines may be a means of weighting heterogeneous input to a cell by differential attenuation of groups of afferent terminals. Thus, i t is possible that observed variation in dendritic spine density in the spiny neuron population and the distribution of spiny neurons in turtles and snakes may be correlated with functional differences in dendritic integration.

3. Aspiny neurons Aspiny neurons form a distinctive cell class

in zone 4 of dorsal area and medial area. Since these cells are relatively rare in Golgi preparations and are indistinguishable from spiny neurons in Nissl preparations, it was not possible to determine if their observed distribution in the dorsal half of ADVR reflects their actual distribution or a sampling bias of the methods employed. Ramon (1896) described a class of aspiny neurons in chameleons which have dendritic trees similar to those in turtles. One cell is illustrated in figure 1M of his paper. These cells were encountered less frequently than spiny cells and were described as showing a globular to piriform cell body and a large number of aspiny dendrites. Axons of these cells divide repeatedly within a cell's dendritic field into collaterals of unequal diameter. A second class of aspiny cells described by Ramon (1896: fig. 1P) has small somata and a very large number of short, radiating dendrites. Both soma sizes and dendritic fields of these cells are consistent with descriptions of oligodendrocytes in lizards (Inoue et al., '741, so that they are probably glial elements. Although aspiny neurons have not been reported in snakes, this may be a function both of their scarcity in ADVR and of a sampling bias of the Golgi methods employed. Further studies are needed to determine the distributions of aspiny cells and their role in ADVR organization.

Zonal organization of ADVR The presence of a periventricular fiber zone

superficial to a cell cluster zone is a common organizational feature of ADVR in turtles and snakes. However, there are differences in the detailed organization of the cell cluster zones. Zone 1 in turtles and zone A in snakes are both relatively narrow fiber bands deep to somata

of ependymal cells. While many fibers can be traced in this region parallel to the ventricular surface in both taxa, others curve radially into deeper zones of ADVR. Some of the latter axons originate from spiny neurons in the subjacent cell cluster zone, termed zone 1 in turtles and zone B in snakes. The superficial fiber zone also contains a distinctive population of neurons termed juxtaependymal cells in turtles, and differences between these cells and their counterparts in snakes have been, discussed in the preceding section.

The largest neuronal clusters in ADVRs of turtles or snakes are found in a cell zone subjacent to the superficial fiber layer. Zone B in snakes contains a fairly uniform distribution of clusters of two to six neurons with apposed somata. Specialized junctions resembling gap junctions have been observed along the apposed membranes of adjacent cells and so- matic membranes may also be separated by a 100 A extracellular space without any associated membrane specializations (Ulinski, '76). In contrast, zone 2 clusters in Pseudemys contain larger numbers of neurons than those in snakes and tend to be isolated in a cell-poor neuropil in dorsal and medial areas (fig. 16). A denser packing of adjacent clusters is observed in ventral area. Neurons are too densely packed in 15 f i sections to permit cell counts of clusters in turtles, and a combination of electron microscopy and light microscopy with plastic embedded materials will be necessary to determine both the number of cells con- tributing to a cluster and the morphology of associated membrane appositions. Thus, zone 2 in turtles and zone B in snakes show differences both in the number of neurons con- tributing to a cluster and in the distribution of clusters in the zone. All squamates also appear to have a zone of neuronal clusters near the ventricular surface, which show a large range of interspecific variation both in the number of cells in a cluster and in the relative sizes of those cells (Northcutt, '78). Although the function of these clusters is not clear at this time, both gap junctions and unspecial- ized regions of membrane apposition (such as the 180 A dense space in dendritic bundles) have been implicated in the mediation of electrotonic coupling between adjacent neurons (Magherini e t al., '76; Scheibel and Scheibel, '75; Bennett, '72; Sotelo and Llinas, '72). In addition, regions of casual apposition without associated membrane specialization may ac- company gap junctions between mammalian


inferior olivary neurons (Sotelo et al., '74; Llinas et al., '74) and intermediary junctions with a 100 A gap have been observed in con- junction with gap junctions in the mammalian cerebellum (Sotelo and Llinas, '72). Thus, there is a likelihood that the electron micro- scopic findings in snakes represent a substrate for electrotonic interactions between apposed cells in zone B clusters. Berry and Pentreath ('77) have reviewed physiological evidence that electrotonic interactions medi- ate processes such as input filtering, burst for- mation and generation of synchronous output by electrotonically coupled neuronal aggre- gates. I t also appears that inhibitory input can decouple individual neurons from the remainder of the cells in a cluster (Spira and Bennett, '72; Llinas et al., '74), creating the possibility for more complex interactions. One line of research indicates that the time constant and input resistance of individual neurons in an electrotonically coupled cluster de- crease as the number of coupled neurons is increased (Getting, '74). In the case of ADVR, this suggests that functional differences are associated with the observed interspecific variability in cluster size and the interareal variability found in turtles. For example, if all neurons in a cluster have equal input resistance, a cluster will act as an input filter for near-synchronous activation of a proportion of its neuronal elements (Getting, '74; Getting and Willows, '74). Since larger clusters would require input to a large number of neurons than smaller clusters to facilitate this tem- poral summation of spatially distinct input, one would expect to see these different requirements reflected in patterns of afferent organization. The ADVR of Psydemys pro- vides a favorable preparation for the study of such relationships because it shows interareal variability in cluster size, and the catecholaminergic projection to the large, isolated clusters in zone 2 of medial area (Parent and Poitras, '74) is a candidate for one such activating system. In species where neurons of different sizes are found in individual clusters, one would also expect to find differences in filtering properties because input resistance varies as the inverse of cell surface area (Hubbard et al., '69). Thus, it is likely that differences in morphology of neuronal clusters in turtles and squamates are associated with differences in integrative activity in specific zones of ADVR.

A major difference between the deep zones

in turtles and snakes is the presence of a cell- poor region, zone 3, between zones 2 and 4 in turtles. No comparable cell-poor zone is found in ADVR of snakes (figs. 16, 17). Zone 3 of Pseudemys is a region of dendritic overlap of zone 2 and zone 4 cells that partially segre- gates the distribution of dendrites of neurons in those zones. Since the dendritic arborizations of zone 2 and zone 4 spiny neurons are of approximately the same extent, one consequence of intercalating zone 3 is that the deep dendrites of zone 2 cells reach only into superficial regions of zone 4 and that superficial dendrites of zone 4 cells do not reach zone 1. As a result, neurons from both zones are in a position to receive projections that terminate between superficial zone 2 and superficial zone 4. Afferents to zone 1 and to deeper portions of zone 4, though, would be restricted to ascending dendrites of zone 2 neurons and zone 1 juxtaependymal cells and to deep dendrites of zone 4 cells, respectively. This distribution of dendrites was found in dorsal, medial and ventral areas independent of the thickness of zones 2, 3 and 4. In ventral area, where zone 3 is reduced in width to a narrow, cell-poor band, the relative thickness of zone 2 appears to exclude dendrites of zone 4 neurons from zone 1. Thus, the presence of zone 3 does not appear to be the sole factor underlying the observed distribution of dendritic arborizations. Snakes, on the other hand, lack a cell- poor region between superficial and deep cell zones (B, C and D). This is correlated with the observations that superficial neurons in zone C extend dendrites into zone A and that dendrites of deep zone B cells reach into zone D. This implies that afferents to zone A may terminate on dendrites from cells in zones deep to the cell cluster layer (zone B). Similarly, i t is not possible to identify a single zonal population of cells that receives thalamic or midbrain afferents in zones B, C and D (Ulinski, '78a,b). Further studies of afferent patterns in both turtles and snakes are needed to determine the significance of these differences in patterns of dendritic distribution in ADVR.

CONCLUSION

Dorsal ventricular ridge (DVR) is a subcortical, telencephalic structure which has been identified in living reptiles and birds. Experi- mental studies in several species suggest that there is a common pattern of thalamic sensory projections to DVR such that visual information projects rostrolaterally and auditory


information projects caudomedially. By contrast, observations of the cytoarchitecture of DVR suggest that there are significant differences in the intrinsic organization of the structure between different groups. A detailed study of ADVR organization has been reported for snakes (Ulinski, '76, '78b), and this paper has analyzed the organization of DVR in a representative of another reptilian order, the turtles.

The analyses presented in this study reveal several aspects of DVR organization common to turtles and snakes as well as several strik- ing differences. DVR is divided into rostral (ADVR) and caudal (BDVR in turtles or PDVR + nucleus sphericus in snakes) in both groups, and neurons in ADVR fall into two main categories. The first group is composed of juxtaependymal neurons, which have somata near the ependymal elements and dendrites restricted primarily to a periventricular fiber zone. The second is composed of a heterogeneous population of spiny neurons with dendritic fields that range from stellate to double-pyramidal forms. Spiny neurons are most frequently observed in both taxa, suggesting that they constitute the principal cell class of ADVR. ADVR in each taxon is organized into a series of concentric zones oriented perpendicular to fibers of the lateral forebrain bundle. The two superficial zones in both turtles and snakes consist of a periventricular fiber layer and a subjacent cell cluster zone, and the clusters tend to be larger in turtles than in snakes. In addition, the deep cell zone is separated from the cell cluster zone in turtles by a cell-poor region. Neurons in zone 4 of turtles do not extend dendrites into the periventricular fiber zone. Snakes lack a cell-poor band between superficial and deep cell zones and dendrites of cells from zone C are observed in the periventricular fiber band. Another major difference is that ADVR in turtles is divided into four areas with distinct boundaries. Three of these areas (dorsal area, medial area and ventral area) show different elaborations of the basic zonal pattern of organization; the fourth lacks zones and contains neurons scattered among fibers of the lateral forebrain bundle. No areal divisions have been identified in snakes.

These anatomical similarities and differences in DVR organization suggest several hy- potheses about basic features of the reptilian ADVR and specializations associated with particular groups. The division of DVR into

rostral and caudal components, the similarity of neuronal types in ADVR and the presence of zones perpendicular to fibers of the lateral forebrain bundle may be structural patterns common to all reptiles. On the other hand, the presence or absence of anatomically distinct areas may be a specialization of particular groups of reptiles. Clearly, detailed structural analyses of representatives of other groups of reptiles are needed to assess both common structural features and the scope of elabors- tions upon patterns of DVR organization. Such comparisons also provide a basis for studies of the significance of various elaborations of the zonal pattern of organization in turtles and snakes. Since the three periventricular areas in turtles show differential elaborations of different sets of zones, a detailed comparison of connections and intrinsic organization of these regions in experimental preparations may clarify the role of a particular zone in ADVR organization. Similarly, detailed comparisons of results of future experimental studies in turtles with results from snakes may clarify the consequences of the observed differences in zonal organization in the two taxa. In particular, the differences in the size of neuronal clusters and the differences in dendritic distribution are likely to be associated with differences in organization of afferents in the two groups.

ACKNOWLEDGMENTS

This work was supported by USPHS Grant NS 12518 and a NSF Graduate Fellowship. The author wishes to thank Doctor P. S. Ulinski for advice throughout this project and for helpful suggestions during the preparation of the manuscript. Technical assistance and secretarial help were provided by Mary Helen Barcellos, Shirley Aumiller, Patricia Tate and Dorothy Ann Crowder. Solveiga Buchbinder, L. M. H. Larramendi and W. Todd Tainey read and criticized the manuscript.

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