HISTOGENESIS OF THE TOOTH TISSUES

Brillante, Charmagne

Busto, Treblig

Cambe, Estephanie

De Leon, Janine

De Los Santos, Andrea Laura

Macainag, Mary Louisse Christine

DOH 121- DBA

Dentinogenesis

Dentinogenesis is the formation of dentin by the odontoblasts. It begins at late bell stage. The presence of preameloblast will induce the peripheral cells of the dental papilla to differentiate. This peripheral cells are star-shaped, have rounded nuclei and have small cytoplasmic volume. Their nuclei gradually migrate toward the cell pole. They will now change in shape and become short columnar cells. They move closer together at the periphery of the papilla. This is now the preodontoblasts, which are columnar cells which exhibit also short cytoplasmic processes on their distal poles.

The membrana preformativa, (basal lamina) the basement membrane between the preameloblast and dental papilla, will thicken. The outer side of the wavy basal lamina follows the cytoplasmic membrance of the preameloblast, while the inner side lines against the fibrillar material. The wavy basal lamina will determine the later contour of the dentino-enamel junction.

The Terminal Bar Apparatus, formed at the distal pole, keeps the individual preodontoblasts in contact with one another and seal off the intercellular spaces. As it moves towards the center, towards the pulp, it will become a highly specialized cell, Odontoblasts, now a slender columnar cell with thick cytoplasmic processes called odontoblastic processes.

High RNA content and marked oxidative and hydrolytic enzyme activity The cells will have well-developed endoplasmic reticulum, golgi apparatus with

numerous mitochondria, with many vascular structure and well-developed microtubular system

Odontoblast changes from oval to columnar, length is 40µm, width is 7µm

The first dentin formed is at the incisal or cusp area of the tooth that progresses in a rootward direction.

Production of collagen by the cellular elements of the sub-odontoblast layer:

The collagen molecules link together extracellulary so that distinct fiber bundles, fibers of von Korff (Alpha Fibers), appear to spiral between the odontoblast and are described as ‘fanning out’ against the basement of the lamina of the internal enamel epithelium where they form the organic matrix of the first formed dentin

o This fibers contain type III collagen associated initially by fibronectino With the formation of the von Korff fibers, the odontoblasts and sub-

odontoblast cells move away from the basement membrane.o The odontoblast leave behind one or more slender cytoplasmic

odontoblast processes.o Initially, daily increment is 4µm per day

While the collagen fibers are being formed, the ground substance of the dentin matrix may either be contributed by acid mucopolysaccharide (noncollagen elements such as phosphoprotein and other glycoaminoglycan like chondroitin sulfate) from the dental papilla which becomes progressively smaller with continued dentin formation or alternatively, and more likely, be secreted by the Beta Fibrils by the Odontoblast.

The von Korff fibers and ground substance form the organic matrix of the dentin which, in its non-mineralized state, is termed predentin.

Mineralization of the mantle dentin is thought to be initiated by matrix vesicles. These membrane bound organelles are budded off from the odontoblast. They contain a variety of enzyme (including alkaline phosphatise) and other molecules that lead to the formation of the first mineral crystals of hydroxyapatite within the vesicles. The crystals then break out of the vesicles and subsequent mineralization of the remainder of the dentin occurs without the presence of matrix vesicles. Similar matrix vesicles have been implicated in the initial mineralization of bone and calcified cartilage.

Once the initial thin layer of mantle dentin has formed collagen fibrils that is being formed will be oriented parallel to the dentino-enamel junction. This is the formation of the circumpulpal dentin. When the predentin reaches a thickness of about 10-20µm it attains a state of maturity that will allow it to mineralize. The fully differentiated odontoblast continue moving pulpward, trailing out an odontoblast process around which the odontoblast continues to secrete the predentin associated with circumpulpal dentin.

Higher power showing the first formed mantle dentin stained red, adjacent to pre-odontoblasts.

Active dentinogenesis. Note pulp on the left and odontoblast layer at the periphery of the pulp, the pale predentin layer with mineralized dentin beyond. Note the mineralisation front with calcospherites between predentin and dentin. There is a trace of enamel at top right.

Higher power of dentinogenesis. Dentin with tubules at right; note the mineralization front with calcospherites. Observe the odontoblasts with processes passing through the predentin into dentin. Note capillaries in the odontoblast layer.

This section shows dentin forming on the left and enamel forming on the right. The amelodentinal junction separates the dark purple enamel on the right from the light purple dentin on the left. Notice the ameloblast layer immediately to the right of the enamel.

Higher power of dentin, pulp, odontoblasts, calcospherites, predentin.

Root dentin formation

Formation of dentin in the root portion is the same as that of the crown, with few differences. These are the following

1. Differentiation of odontoblast in the root portion is due to the presence of Hertwig’s epithelial root sheath.

2. The epithelial root sheath does not deferentiate and remains only as cuboidal cells.

3. Initially, the migrating odontoblast (pulpward) does not trail behind a process.

Hyaline layer• A thin, initial, organic predentin layer in root dentin that will mineralize.• Continuous with the mantle dentin of the crown.• nontubular , structurless band which appears whitish in color.

Granular layer of Tomes

Following the formation of the hyaline layer, the migrating odontoblasts trail behind their odontoblastic processs. These branch, loop and appear

dilated and, when the dentin matrix around them become mineralized, give rise to granular layer beneath the hyaline cartilage

A: Granular layer of TomesB: Hertwig’s epithelial root sheath

C: Hyaline layer

Interglobular dentinThere are two distinct patterns of dentin that can occur: a linear or a

spherical (calcospherite) pattern. *In calcospherites, the crystallites are arranged in a radial pattern and,

despite complete mineralization of dentin, this pattern still be discerned using polarized light. Failure of calcospherites to fuse may result in the appearance of interglobular dentin, representing small regions of unmineralized matrix.

Globules of Calcospherites

Dentinal tubules• S shaped or straight canal that contains the odontoblastic process• In the formation of the odontoblastic process curvatures may arise. These

curvatures are due to the following:a) Primary curvature results from the oscillation of the odontoblast which

arises from their crowding as the volume of the pulp decreases (coronal direction).

b) Secondary curvatures are hypothesized to be a result of the inequality of the distance moved by the odontoblast and formed length of odontoblast process in unit time. It is said that in unit time the formed length of the odontoblast process is greater than the distance moved by the odontoblast towards the papilla (apical direction).

2 products of odontoblastA. Peritubular dentin

Little is known about the genesis of peritubular dentin. Scientist believes that it is form due to the presence of microtubules and vesicle in odontoblastic process. Such structures in the odontoblastic process explain how peritubular dentin is formed within the depths of already formed dentin. By these structures the materials synthesized by the body of the odontoblast could pass to the site of peritubular dentin formation.

B. Intertubular dentinIt is the primary secretory product of the odontoblast between dentinal

tubules. Not like the peritubular dentine, intertubular dentin consists of Type I collagen fibers.

Secondary dentinSecondary dentin is formed by the same odontoblast that formed the primary

dentin, and is laid down as a continuation of the primary dentin after root formation. It is formed the same way as primary dentin but at a much slower pace. Secondary dentin is easily distinguished from primary dentin due to its changed in direction and also by the presence of the demarcation line between the secondary dentin and primary dentin.

Tertiary dentinIt is a dentin that is deposited at specific sites in response to injury or trauma. Its

formation depends on the degree of the injury; the more severe the injury, the more rapid the rate of dentin deposition. Because of the rapid deposition tubular patterns are distorted.

*tertiary dentin is poor in collagen and enriched in noncollagenous matrix proteins such as sialoprotein and osteopontin

Incremental linesThe rate of dentin formation varies, producing incremental lines. These are the following:

a) A diurnal rhythm of formation produces short-period lines approximately 4µm apart (von Ebner lines), resulting from slight differences in composition or orientation of dentin matrix.

b) Contour lines of Owen

It is the result from coincidence of the secondary curvatures between neighboring dentinal tubules.

Root Formation

Root formation occurs after the crown has completely formed and shaped. Therefore, tooth begins to form from crown to root. It involves interactions between the Enamel organ, Dental papilla and Dental Sac.

A. Enamel OrganB. Dental PapillaC. Dental Sac / Follicle

The cervical loop, derived from the region of the enamel organ, has external and internal enamel epithelia begins to grow down into the dental sac forming a double layered epithelial root sheath (Hertwig’s epithelial root sheath). Epithelial root sheath proliferates apically to shape the future root except at the basal portion of the pulp which will serve as the apical foramen. As it proliferates it will enclose the dental papilla.

The mesenchymal cells of the dental follicle which lies external to the root sheath will differentiate into cementoblast that deposit cementum on the developing root, to the fibroblast of the developing periodontal ligament and possibly to the osteoblasts of the developing alveolar bone.

Formation of Periodontal Ligament

Formation of the periodontal ligament occurs after the cells of the Hertwig’s epithelial root sheath have separated, forming the known as the epithelial rest of Malassez.

This separation permits the cells of the dental follicle to migrate to the external surface of the newly formed root dentin. Other cells of the dental follicle will differentiate into fibroblast. Fibroblast will make the fibers and ground substances of the periodontal ligamnet by secreting collagen. The fibers will then be embedded in the surface of newly developed adjacent cementum and alveolar bone. The attachment of the periodontal ligament fibers in the cementum and alveolar bone holds the tooth securely in the socket . As the tooth errupts , the periodontal ligament fibers are reoriented. The different orientations are alveolar crest group, oblique fiber group, apical fiber group,horizontal fiber group and interradicular fiber group. The orientation of the fibers is due to the occlusion with the opposing tooth.

The five fiber groups of periodontal ligament:

This diagram shows the location of some of the principal fibers of the periodontal ligament.

AC: alveolar crest fibers; H: horizontal fibers; OBL: oblique fibers; PA: periapical fibers; IR: Interradicular fibers.

1. Interradicular fiber group

2. Apical Group

3. Oblique fiber group

4. Horizontal fiber group

5. Alveolar crest group

Cementogenesis

Cementogenesis is the formation of primary (acellular) cementum and the secondary (cellular) cementum. The process begins at the cervical loop and extends apically as the root grows downwards. It begins shortly after the fragmentation of Hertwig’s epithelial root sheath. Figure 2 below shows the cervical root area with the Hertwig’s epithelial root sheath and its extended diaphragm that will out line the root formation.

(figure 1) (figure 2)

Fragmentation of root sheath permits penetration of the connective tissue cells of the follicle so that they come to lie between the remnants of the root sheath and the surface of the newly formed root. Figure 3 below shows the fragmentation/disintegration of Hertwig’s epithelial root sheath. Figure 4 below shows the penetration of connective cells.

(figure 3) (figure 4)

The ectomesenchymal cells of the follicle after penetration the root sheath differentiate into cement-forming cells or cementoblast. Present in these cells are numerous mitochondria, a roughed surface endoplasmic reticulum, and a prominent Golgi complex. The factor responsible for cementoblast differentiation is unknown.

(figure 5)

The fibrous connective tissue in contact of the roots contributes to the first formed cement matrix. When sufficient organic matrix has been formed it becomes mineralized. As matrix formation proceeds, the cement-forming cells can be incorporated within the developing cement where they become cementocytes, or may remain on the surface of the forming cement as more rounded cells lacking processes. Two types of cement are then recognized, cellular and acellular cementum. Cementocytes are characterized by processes radiating towards the periodontal ligament and their cytoplasm shows a drastic reduction in the number of organelles when compared to cementoblast.

After eruption of the tooth the fibers of the periodontal ligament lie oblique to the root surface and it is obvious that they must be incorporated within the cement, otherwise no attachment would be made. Figure 6 shows the incorporation of cementum and periodontal ligament.

(figure 6)

Once incorporated within the cellular cement they become fully mineralized and indistinguishable from the few other fibers of cement matrix. Acellular cement serves the purpose of anchoring the tooth in the alveolus and explains why it is found applied to

the coronal two-thirds of the root. Cellular cementum, in the other hand, has only few collagen content derived from Sharpey fibres.

(figure 7) (figure 8)

(figure 9)

Histogenesis of the Pulp

The central cells of the dental papilla,

which is ectomesenchymal in origin, gives

rise to the pulp.

Tooth pulp, or simply, pulp was

initially called the dental papilla. It is only

designated as “pulp” only after dentin

forms around it. The transformation of

papilla to pulp only occurs after the

formation of primary dentin, the innermost

layer of

dentin

matrix, encloses the pulp cavity.

It is the area of the proliferating future papilla

that causes the oral epithelium to invaginate and form

the enamel organ in the earliest stages of tooth

Dental Papilla

development. These enlarge to enclose the dental papilla on the center portion of the

developing tooth.

The development of the dental pulp begins at about the eighth week of embryonic

life. Soon thereafter the more posterior tooth organs begin

differentiating. The dental papilla is a well-vascularized and

organized network of vessels, which appear by the time dentin formation, begins.

Capillaries crowd among the odontoblasts in the period of active dentinogenesis.

The cells of the dental papilla appear as undifferentiated mesenchymal cells.

These cells will differentiate into stellate shaped fibroblasts. After which, the

odontoblast then differentiates from the peripheral cells of the dental papilla. As this

occurs, it is no longer called dental papilla; instead, it is now designated as the pulp

organ. Fibroblasts and mesenchymal cells will have a decrease in concentration during

the transition of papilla into pulp. And there will be an increase in collagen fibers.

Fibroblasts came from the undifferentiated mesenchymal cell of the papilla. Some of the

original mesenchymal cells remain

in mature pulpal tissue as

undifferentiated cells. These will

form a reservoir of cells, which can

be used in a later time to replace

odontoblasts.

Nerves and blood vessels in

the dental papilla begin to form the

primitive dental pulp.

Once nerve fibers start to go near the cap stage of the developing tooth, and grow toward

the dental follicle. The nerves will then, develop around the tooth bud and enter the

dental papilla when dentin formation has already begun. These nerves never proliferate

the enamel organ.

Blood vessels is derived from the dental follicle and

will enter the dental papilla during cap stage. The

Dentin

1= dentin

2=predentin

3= odontoblastic zone

4= cell-rich zone

5= blood vessels (nerves, and veins are not seen here)

number of blood vessels reaches a maximum at the beginning of the crown stage, and the

dental papilla eventually forms in the pulp of a tooth.

Bone ossification

Ossification means bone formation. Bone is a hard, dense, calcified connective

tissue that forms most of the skeleton of most vertebrae. It can be formed by two ways:

Intramembranous ossification

Endochondral ossification

For both processes, bone tissue that appears first is primary, or immature bone. It is

a temporary tissue and will soon be replaced by lamellar, or secondary bone.

Remodeling of bones does not only occur in growing bones, but also throughout adult

life, although its rate of change is slower.

INTRAMEMBRANOUS OSSIFICATION

Intramembranous

ossification takes place within

condensation of connective

tissues, such as mesenchymal

tissues. Formation of flat

bones is derived from this

process. Examples of flat

bones are the bones of the

skull, such as the parietal

bone, temporal bone, frontal

bone, the mandible, maxilla, and occipital bone.

Mesenchymal cells

differentiate into osteoblasts.

These clusters of osteoblasts form an ossification center that secretes organic

extracellular matrix, called osteoid.

These mesenchymal cells usually group together near or around the blood vessels,

and differentiate into osteogenic cells, which deposit bone matrix. These aggregates of

bony matrix are called bone spicules. The spicules will trap osteoblasts in a lacuna, and

will eventually differentiate into osteocytes.

As the bony spicules continue to grow, they fuse with adjacent spicules to form the

trabeculae, forming the spongy bone. The appearance of the trabeculae is the first sign of

bone formation. Trabeculae is the anastomosing bony spicules in cancellous or spongy

bone which form a meshwork of intercommunicating spaces that are filled with bone

marrow. These trabeculae will

connect to form the compact

bone.

Intramembranous

ossification begins at about the

eighth week in the human

embryo.

ENDOCHONDRAL

OSSIFICATION

Unlike,

intramembranous

ossification, cartilage is

present during

endochondral

ossification. It is also

an essential process

during the

rudimentary formation of long bones, the growth of the length of long bones, and the

natural healing of bone fractures.

PRIMARY CENTER OF OSSIFICATION

The first site of ossification occurs in the primary center of ossification, located in the

middle of the diaphysis. The first that will happen is the formation of the periosteum.

The perichondrium becomes the periosteum. This periosteum contains a layer of

undifferentiated cells, called osteoprogenitor cells, that will later transform into

osteoblasts. Formation of the bone collar. The osteoblasts will secrete osteoid against the

shaft of the cartilage model, which will serve as support for the new bone. Calcification of

matrix. Chondrocytes in the primary center of ossification begin to grow. Then the

calcification of the matrix occurs and apoptosis of the hypertrophic chondrocytes occur.

This will create cavities within the bone. Invasion of periosteal bud. Blood vessels will

sprout from the Osperichondrium before the chondrocytes undergo apoptosis. These

will form the periosteal bud and invade the cavity left by the chondrocytes. These blood

vessels carry hemopoietic cells, which will later on form the bone marrow, and

osteoprogenitor cells inside the cavity. Formation of trabeculae. Osteoblasts use the

calcified matrix as a scaffold and begin to secrete osteoid, forming the bone trabecula.

Osteoclasts, formed from macrophages, break down spongy bone to form the medullary

cavity.

SECONDARY OSSIFICATION CENTER

Secondary ossification appears

in each end, epiphysis, of long

bones. The cartilage between the

primary and secondary

ossification center is called the

epiphyseal plate, and continues to

form new cartilage, which is

replaced by bone, which results in

an increase in length of the bone.

The point of union of the primary and secondary ossification centers is called the

epiphyseal line.

During endochondral ossification, five distinct zones can be seen:

1. Zone of resting cartilage. This

zone contains normal, resting

hyaline cartilage.

2. Zone of proliferation.

Chondrocytes in this zone

undergo rapid mitosis, forming

distinctive looking stacks.

3. Zone of maturation Chondrocytes

undergo hypertrophy (become

enlarged).

4. Zone of calcification.

Chondrocytes are either dying or

dead, leaving cavities that will

later become invaded by bone-

forming cells, osteoblasts.

5. Zone of ossification.

Osteoprogenitor cells invade the

area and differentiate into

osteoblasts, which elaborate

matrix that becomes calcified on the surface of calcified cartilage.