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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.