morphology - heart · findings on the morphology ofthe sinus node, this being the most reliable...

Br Heart J 1995;74:437-442

Morphology of the sinus node in human andmouse hearts with isomerism of the atrialappendages

Siew Yen Ho, Jeong-Wook Seo, Nigel A Brown, Andrew C Cook, Nuala L K Fagg,Robert H Anderson

AbstractBackground-The location of the sinusnode is known to be at best abnormal, orat worst unknown, in patients with iso-merism of the morphologically left atrialappendage. In contrast, the sinus node isknown to be an excellent histologicalmarker of the morphologically rightappendage, being duplicated in those withright isomerism. The aim ofthe study wasto investigate this condition further infetal human and mouse hearts.Methods-Serial histological sections ofthe area anticipated to contain the sinusnode were studied in hearts with iso-merism of the atrial appendages takenfrom 14 human fetuses and 13 ivliv mice,using 12 mouse hearts with normallyarranged or mirror imaged atrial cham-bers for controls.Results-All hearts with isomerism of theright appendages (two human and fourmouse) had bilateral sinus nodes. Thecases with isomerism of the leftappendages (12 human and nine mouse)showed absence of a recognisable sinusnode except in four cases (19%) in which asmall remnant of the node was found. Inthree of these cases, it was related pos-tero-inferiorly to the superior cavoatrialjunction.Conclusions-The concept of isomerismof the atrial appendages is endorsed byfindings on the morphology of the sinusnode, this being the most reliable histo-logical criterion for existence of a mor-phologically right atrium. A smallproportion of hearts with left isomerismhad a structure resembling the sinusnode, but it was hypoplastic and displacedpostero-inferiorly, distant from itsexpected position had the hearts pos-sessed an incompletely formed morpho-logically right appendage.

(Br Heart Jr 1995;74:437-442)

Keywords: sinus node morphology; isomerism of atrialappendages; situs inversus; conduction system

Since the introduction of the concept of iso-merism to describe the morphology of heartswith the so called "heterotaxic syndrome",'arguments have continued as to whether iso-merism is really present or whether the con-stellation of lesions would better beunderstood in terms of partial expression of

either the usual or mirror imaged atrialarrangements.23 One of the ways to confirmthe presence of an isomeric arrangementwould be to provide morphological proof insuch cases of the presence of a histologicallyrecognisable sinus node.The sinus node in the human is a distinct

and obvious anatomical structure foundsubepicardially at the junction between thevenous component and the appendage of themorphologically right atrium.4 The node isfunctionally important in initiating the heartbeat, which begins very early in life and con-tinues for a long period without changing itsbiological and physiological characteristics.5 Itmay reasonably be presumed that the sinusnode is one of the original anatomical featuresof the morphologically right atrium, and isunlikely to be affected by lesions involving itsvenous component. With this premise inmind, we searched histologically for the sinusnode in human fetal hearts which were part ofa series recognised morphologically as havingan isomeric arrangement of the atrialappendages.6 We have also previously identi-fied a group of iv/iv mouse hearts with iso-meric arrangement of the atrial appendages.7Since the mouse heart is small enough to bereadily studied in its entirety by histologicaltechniques, we supplemented the humanstudy with murine hearts to determinewhether the sinus node is also a marker ofright atrial morphology in the mouse.

MethodsWe studied 14 human hearts ranging from12-25 weeks gestation, and from newborn toone week in age. These formed part of a studygroup reported previously.68 Left isomerismwas recognised when both atrial appendageswere finger-like in shape and had narrow junc-tions with the venous component of the atria.In addition, pectinate muscles were confinedwithin the atrial appendage. In contrast, bothatrial appendages were triangular in shapewith broad junctions in hearts with right iso-merism. Furthermore, the pectinate muscleswhich radiate from the terminal crest extendedaround the vestibule of the atria to meet at thecrux. Using these criteria, 12 of these fetalhearts had isomerism of the morphologicallyleft atrial appendages, while the remaining twohad isomerism of the morphologically rightappendages.Twenty five iv/iv mice of the SI/col back-

ground, originally from the JacksonLaboratory (Bar Harbour, USA), were also

Department ofPaediatrics, NationalHeart and LungInstitute, LondonS-Y HoJ-W SeoR H AndersonMRC ExperimentalEmbryology andTeratology Unit, StGeorge's HospitalMedical School,LondonN A BrownDepartment ofPathology, Guy'sHospital MedicalSchool, LondonA C CookN L K FaggCorrespondence to:Dr S Y Ho, Department ofPaediatrics, National Heartand Lung Institute,Dovehouse Street, LondonSW3 6LY.

Accepted for publication2 May 1995

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Morphology of the sinus node in human and mouse hearts with isomerism of the atrial appndages

*<rMf Th'TerminalVe*g- -Rig9ht atrium-K.",,cresttZ ;.*. :. %Figure 3 A section in the coronal plane ofan adult mouseheart with usual atrial arrangement shows the entrance ofthe superior caval vein into the right atrium. The sinusnode (within dotted lines) is seen as a collection of spindlecells around a central artery at the junction between rightappendage and the superior caval vein. The node partlyencircles the vein to reach to the posterior side of the atriumwhere there are lobules of brown fat.

studied. There were five adults, 18 fetuses at18 or 19 days gestation, and two neonates lessthan two days of age. As in the human hearts,right isomerism was recognised on the basis ofthe bilateral presence of broad appendagesand terminal crests with pectinate musclesextending round the vestibule. The atrialappendages in left isomerism were also broadbilaterally, but were distinguishable from mor-phologically right appendages by their narrowjunctions with the venous atrial componentsand the absence of pectinate muscles roundthe vestibule. Four hearts were identified ashaving isomerism of the right atrialappendages, while nine hearts had isomerismof the left atrial appendages. The remaining 12hearts, with usual or mirror imaged arrange-ment of the atrial appendages, were used todetermine the normal morphology of the sinusnode in the ivliv mouse.

All heart specimens were fixed in formalde-hyde and then processed in their entirety forhistological sectioning, blocking them inparaffin wax for serial sectioning at 1O ,umthickness. The sections were collected oncardboard trays and numbered sequentially.Every 25th section of the human hearts, andevery fifth section from the mouse hearts, wasmounted on glass slides and stained with amodified Masson's trichrome technique.9 Allthe stained sections were examined and addi-tional sections mounted and stained wherenecessary. The sinus node was identified inthe human hearts using well established histo-logical criteria.4 10 11 For the mouse hearts, thecontrols with lateralised atrial arrangement

Figure 4 The heart ofa normalfetal mouse sectioned in acoronal plane. The sinus node (within dotted line) ishorseshoe shaped and draped across the junction betweenthe superior caval vein and the right atrium. The nodalcells are less darkly stained than ordinary myocytes.

were used to determine the histological char-acteristics of the sinus nodes.

ResultsHUMAN HEARTSIsomerism of the right atrial appendagesThere were bilateral superior caval veins inthese two hearts. Bilateral sinus nodes wereidentified in each heart. The nodes wererecognised by their composition of small inter-weaving cells within a fibrous matrix, usuallysurrounding a small artery.4 The nodal cellsstained slightly less intensely than the sur-rounding atrial myocytes. In one heart, eachnode was spindle shaped and located sub-epicardially in the terminal groove, lateral tothe cavoatrial junction (fig 1). In the otherheart, the sinus node on the right side was nor-mally located and supplied by an artery whichpassed through its centre. The node on the leftwas hypoplastic and situated more inferiorlyand posteriorly than usual along the terminalgroove. It was recognised by its composition ofsmall cells and fibrous network around anartery.

Isomerism of the left atrial appendagesThe sinus node was identified as a recognis-able structure in only three of the 12 hearts. Inone case, the node was well formed and situ-ated on the crest of the right sided atrialappendage (of left morphology) but in the

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Figure 5 The thorax ofa fetal mouse (18 day gestation) with isomerism of the right atrial appendages is sectioned in a transverse plane. Enlargements ofthe boxed areas (B and C) show sinus nodes (within dotted lines) represented by vacuolated cells around the cavoatrialjunctions on both sides.LPA, left pulmonary artery; RA, right atrium; RPA, right pulmonary artery.

absence of a superior caval vein. In the secondcase, the node was hypoplastic, represented bya tiny cluster of small cells around a prominentnodal artery. It was located in the posteriorwall of the right sided atrium, inferior to itsjunction with the superior caval vein. Therecognisable sinus node in the third case wasalso hypoplastic and situated at the posteriorjunction between the right sided atrium andthe right superior caval vein (fig 2). Bilateralcaval veins were present in this case.

MOUSE HEARTSUsual and mirror image arrangement of the atrialappendagesThe sinus node was located in subepicardialposition at the upper part of the junction ofthe terminal groove with the superior cavalvein in the morphologically right atrium. Incontrast to the usual lateral position in humanhearts, the sinus node in the murine heart wasmore extensive and straddled the crest of themorphologically right appendage (fig 3). Inkeeping with this, the node was related to the

left sided appendage in hearts with mirrorimage arrangement of the atrial appendages.The node in the fetus was relatively biggerthan the node in the adult. In the fetus, itpartly or almost encircled the junction of thesuperior caval vein with the morphologicallyright atrium (fig 4). Histologically, the nodewas composed of compactly arranged, palestaining, non-striated short cells which werenot noticeably smaller in diameter than ordi-nary atrial myocytes. Unlike the situation inthe human, the nodal cells were not arrangedwithin a prominent fibrous tissue matrix. Thenodal cells in the fetal hearts were slightly big-ger than ordinary atrial myocytes and theirnuclei and cytoplasm were vacuolated.Collections of ganglion cells were frequentlyfound close to the nodes.

Isomerism of the right atrial appendagesTwo distinct bilateral sinus nodes were foundin each case. They were bigger than usual,occupying a transmural position at the lowerend of the superior caval vein at its junction

Figure 6 A transverse section through the thorax of a fetal mouse (19 day gestation) with left isomerism. Collections of vacuolated cells are lacking onboth sides. The left cavoatrial junction is enlarged in panel B.

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Morphology of the sinulis niode in human and mouse hearts with isomlerismii of the atrial appndages

B Mirror imagedatrial arrangement

C Right isomerism D Left isomerism

Bilateral sinus node

N)

Absence orhypoplasia of sinus

Figure 7 Diagramii shozing the disposition of the sinus node in the four variaarrangement. (A) anid (B) shozw lateralised arrangement, while (C) and (D)isomeric arrangemlenit of the atrial appendages.

with the appendage (fig 5). Each n

dled the crest of the cavoatrialextending medially and posteriorly.

Isomerism of the left atrial appendagesIn only one case was it possible topoorly developed sinus node at thibetween the right sided superior cavsthe atrium, this remnant being sitl:rior to the position usually seen in Eusual atrial arrangement. The other (

showed no evidence of nodal ce

cavoatrial junctions (fig 6), nor cc

cells be found elsewhere in the atria.

DiscussionIt has been argued that the arrangenatrial chambers, a feature which ethe first step in the logical analyEpatient with congenital cardiac defe(determined according to the morpthe atrial appendages. It is also ai

this feature is sufficiently consiappendages always to be recognise(of either right or left morphologythis approach, all hearts can be placeof four groups, namely those showinmirror imaged arrangements, ormerism of either the morphologicalleft appendages.2 Others argue, in

that all hearts have a basicallyarrangement according to the venou

tions, and that hearts with "aspleniasplenia" (heterotaxy syndrome)categorised into partial forms of tlalised patterns.3 14 15

This latter concept, however, hsevidence from earlier studies in bo

specimens'2 13 and animal models'6 17 which

Left sidedshowed that recognition of the morphology of

sinus node each appendage depended on the nature of itsjunction with the rest of the atrial chamber,rather than relying merely on its size andshape, or upon the venoatrial connections.The ultimate difference between the two typesof atria, therefore, still remains to be deter-mined. At the microscopic level, evidence todate suggests that the presence or absence of asinus node' could be the most recognisabledistinction between the two types of atrialappendage. With this in mind, we have stud-ied the location and morphology of the sinusnode in both human and mouse hearts in

node which isomerism of the atrial appendages hadbeen diagnosed grossly, as judged from mor-phological criteria. Our objectives were toexamine the usefulness of recognising thepresence of the sinus node in the determina-tion of atria of right morphology, and itsabsence in atria of left morphology, and to re-evaluate the concept of isomerism in terms ofits functional significance.Our material, therefore, included hearts

with isomerism of right or left types fromnts of atrial human fetuses and iv/iv mice. Our earliershow studies offer evidence that the arrangement in

the human fetus can be generalised into thesituation in childhood and adulthood.4 Theexperimental model of our iv/iv mice, how-

iode strad- ever, needed to be assessed further to establishjunction, its validity in this context. The iv/iv mouse is a

model of abnormal laterality which has beensaid to show mirror image arrangement inapproximately half of adult animals.18 More

identify a critical examination, nonetheless, has showne junction that many iv/iv mice show so called hetero-al vein and taxy.'6 17 19-21 Our previous study on iv/iv mice,iated infe- for example, showed that 1-5% and 3-3% hadiearts with right and left isomerism, respectively.7eight cases Morphologically right and left atrial appen-

lls at the dages could be distinguished using criteriauld nodal very similar to those applied on human hearts.

The sinus nodes in our control group ofmouse hearts were also histologically recognis-able structures, consistent with previousdescriptions.22 We are not aware of any previ-

nent of the ous study on the sinus node of murine heartsconstitutes with isomeric arrangements of the atrialsis of any appendages.cts, can be On studying our human and murine hearts,)hology of we discovered bilateral sinus nodes in allrgued that hearts with isomeric arrangement of morpho-istent for logically right appendages, while a single sinusd as being node was observed in the superior cavoatrial,12 Using junction of each morphologically right atriumd into one in hearts with lateralised arrangement (fig 7).tg usual or Our study has shown, therefore, that the mor-else iso- phologically right appendage is accompanied

Ily right or by a sinus node irrespective of its location.contrast, The node was almost always found at the

lateralised junction of the superior caval vein with theis connec- appendage, albeit that one node was displacedand poly- posteriorly and inferiorly along this junction incan be a heart with isomerism of the morphologically

hese later- right appendages. Clinical studies on atrialexcitation in hearts with isomerism of right

as ignored atrial appendages have also shown the pres-)th human ence of bilateral sinus nodes,23 24 while the his-

A Usual atrial arrangement

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tological proof of this gross finding has longbeen established by Van Mierop andWiglesworth.1 Taken together, these findingssupport the notion that the junctions of theappendages with the rest of the atrium aretruly isomeric in those hearts we have charac-terised grossly as showing bilateral rightappendages. This is also supported by the con-sistent association of other cardiac lesions,such as the absence of the atrial septum andthe coronary sinus, and abnormal pulmonaryvenous connections.' 25

Bilateral absence of the sinus node was afeature in most hearts with isomerism of themorphologically left appendages. In threecases, however, a small nodal replica was dis-covered in an abnormal position at the pos-tero-inferior wall of the right sidedmorphologically left atrium, while in one casethe node was located more superiorly.Absence, or virtual absence, of the sinus nodecan therefore be taken as a marker of the mor-phologically left atrium. Clinical studies onpatients with left isomerism have shown thepresence of a P wave which was almost alwaysabnormal in its axis.232426 The abnormal loca-tion of a hypoplastic node, or absence of thenode, is an attractive morphological findingwith which to explain this abnormal P wave.Our morphological criteria on the recogni-

tion of the atrial arrangements are useful foranticipating the functional activity of the sinusnode, and conversely have shown that iso-merism of the atrial appendages exists in con-junction with the arrangement of the sinusnode. If there is a narrow tubular zonebetween the venous component and theappendage, with pectinate muscles confined tothe appendages, the atrium is of left morphol-ogy and no well formed specialised junctionalnode is expected. In the morphologically rightatrium, in contrast, the transition from thesuperior caval vein to the appendage is markedexternally by the terminal groove, which corre-sponds to the terminal crest on the endocar-dial aspect. The sinus node can be anticipatedin the terminal groove, even in the absence of asuperior caval vein.27 Further studies on theprocesses involved in development of atria ofleft or right morphology will provide moreunderstanding of the differences betweenright and left morphologies, as well as thosebetween right and left isomerism.

1 Van Mierop LHS, Wiglesworth FW. Isomerism of the car-diac atria in the asplenia syndrome. Lab Invest 1962;11:1303-15.

2 Becker AE, Anderson RH. Isomerism of the atrialappendages-goodbye to asplenia and all that. In: ClarkEB, Takao A, eds. Developmental cardiology: morphogenesisand functions. Mount Kisco: Futura Publishing Co,1990:659-70.

3 Van Praagh R, Van Praagh S. Atrial isomerism in the het-erotaxy syndromes with asplenia, or polysplenia, or nor-mally formed spleen: an erroneous concept. Am J Cardiol1990;66: 1504-6.

4 Anderson KR, Ho SY, Anderson RH. The location andvascular supply of the sinus node in the human heart. BrHeartj 1979;41:28-52.

5 Mandel WJ, Lak MM, Obayashi K. Sinus node function:evaluation in patients with and without sinus node dis-ease. Arch Intern Med 1975;135:388-92.

6 Ho SY, Cook A, Anderson RH, Allan LD, Fagg N.Isomerism of the atrial appendages in the fetus. PediatrPathol 1991;11:589-608.

7 Seo JW, Brown NA, Ho SY, Anderson RH. Abnormal lat-erality and congenital cardiac anomalies: relationships ofvisceral and cardiac morphologies in the ivliv mouse.Circulation 1992;67:316-20.

8 Ho SY, Fagg N, Anderson RH, Cook A, Allan L.Disposition of the atrioventricular conduction tissues inhearts with isomerism of the atrial appendages-its rela-tionship to congenitally complete heart block. J Am CollCardiol 1992;20:904-10.

9 Smith A, Ho SY, Anderson RH. Histologic study of thecardiac conducting system as a routine procedure. MedLab Sci 1977;34:223-9.

10 James TN. Anatomy of the human sinus node. Anat Rec1961;141: 109-16.

11 Truex RC, Smythe MQ, Taylor MJ. Reconstruction of thehuman sinoatrial node. AnatRec 1967;159:371-8.

12 Macartney I;, Zuberbuhler JR, Anderson RH. Morpho-logical considerations pertaining to recognition of atrialisomerism: consequences for sequential chamber locali-sation. BrHeartj 1980;44:657-67.

13 Sharma S, Devine W, Anderson RH, Zuberbuhler JR. Thedetermination of atrial arrangement by examination ofappendage morphology in 1842 heart specimens. BrHeartJ 1988;60:227-31.

14 Van Praagh S, Kreutzer J, Alday L, Van Praagh R.Systemic and pulmonary venous connections in visceralheterotaxy, with emphasis on the diagnosis of the atrialsitus: a study of 109 postmortem cases. In: Clark EB,Takao A, eds. Developmental cardiology: morphogenesis andfunction. Mount Kisco: Futura Publishing Co, 1990:671-727.

15 Van Praagh R, Santini F, Geva T. Segmental situs in con-genital heart disease: a fundamental concept. G ItalCardiol 1990;20:246-53.

16 Icardo JM. Development of the outflow tract: a study inhearts with situs solitus and situs inversus. Ann N YAcadSci 1990;588:26-40.

17 Icardo JM, Sanchez de Vega MJ. Spectrum of heart malfor-mations in mice with situs solitus, situs inversus, andassociated visceral heterotaxy. Circulation 1991 ;84:2547-58.

18 Hummel KP, Chapman DB. Visceral inversion and associ-ated anomalies in the mouse. J Hered 1959;50:9-13.

19 Layton WM. Random determination of a developmentalprocess: reversal of normal visceral asymmetry in themouse. JHered 1976;67:336-8.

20 Layton WM. Heart malformations in mice homozygous fora gene causing situs inversus. Birth Defects 1978;14:277-93.

21 Layton WM. The biology of asymmetry and the develop-ment of the cardiac loop. In: Ferrans VJ, ed. Cardiacmorphogenesis. Amsterdam: Elsevier, 1985:134-40.

22 Lev M, Thaemert JC. The conduction system of the mouseheart. Acta Anat 1973;85:342-52.

23 Dickinson DF, Wilkinson JL, Anderson KR, Smith A, HoSY, Anderson RH. The cardiac conduction system insitus ambiguus. Circulation 1979;59:879-85.

24 Wren C, Macartney FJ, Deanfield J. Cardiac rhythm inatrial isomerism. AmJ Cardiol 1987;59: 1156-8.

25 Rose V, Izukawa T, Moes CAF. Syndromes of asplenia andpolysplenia. A review of cardiac and non-cardiac malfor-mations in 60 cases with special reference to diagnosisand prognosis. Br HeartJ 1975;37:840-52.

26 Momma K, Takao A, Shibata. Characteristics and naturalhistory of abnormal atrial rhythms in left isomerism. AmJCardiol 1990;65:231-6.

27 Lenox CC, Hashida Y, Anderson RH, Hubbard JD.Conduction tissue anomalies in absence of the rightsuperior caval vein. IntJ7 Cardiol 1985;8:251-60.

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