HISTOGENESIS OF THE TOOTH TISSUESBrillante, CharmagneBusto, TrebligCambe, EstephanieDe Leon, JanineDe Los Santos, Andrea LauraMacainag, Mary Louisse ChristineDOH 121- DBA
Dentinogenesis Dentinogenesis is the formation of dentin by the odontoblasts. It begins at latebell stage. The presence of preameloblast will induce the peripheral cells of the dentalpapilla to differentiate. This peripheral cells are star-shaped, have rounded nuclei andhave small cytoplasmic volume. Their nuclei gradually migrate toward the cell pole. Theywill now change in shape and become short columnar cells. They move closer together atthe periphery of the papilla. This is now the preodontoblasts, which are columnar cellswhich exhibit also short cytoplasmic processes on their distal poles. The membranapreformativa, (basal lamina) the basement membrane betweenthe preameloblast and dental papilla, will thicken. The outer side of the wavy basallamina follows the cytoplasmicmembrance of the preameloblast, while the inner sidelines against the fibrillar material. The wavy basal lamina will determine the latercontour of the dentino-enamel junction. The Terminal Bar Apparatus, formed at the distal pole, keeps the individualpreodontoblasts in contact with one another and seal off the intercellular spaces. As itmoves towards the center, towards the pulp, it will become a highly specialized cell,Odontoblasts, now a slender columnar cell with thick cytoplasmic processes calledodontoblastic 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 7µmThe first dentin formed is at the incisal or cusp area of the tooth that progresses in arootward 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 fibronectin o 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 dentinmatrix may either be contributed by acid mucopolysaccharide (noncollagen elementssuch as phosphoprotein and other glycoaminoglycan like chondroitin sulfate) from thedental papilla which becomes progressively smaller with continued dentin formation oralternatively, 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 dentinwhich, 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 avariety of enzyme (including alkaline phosphatise) and other molecules that lead to theformation of the first mineral crystals of hydroxyapatite within the vesicles. The crystalsthen break out of the vesicles and subsequent mineralization of the remainder of thedentin occurs without the presence of matrix vesicles. Similar matrix vesicles have beenimplicated in the initial mineralization of bone and calcified cartilage. Once the initial thin layer of mantle dentin has formed collagen fibrils that isbeing formed will be oriented parallel to the dentino-enamel junction. This is theformation of the circumpulpal dentin. When the predentin reaches a thickness of about10-20µm it attains a state of maturity that will allow it to mineralize. The fullydifferentiated odontoblast continue moving pulpward, trailing out an odontoblastprocess around which the odontoblast continues to secrete the predentin associatedwith 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 thepulp, the pale predentin layer with mineralized dentin beyond. Note the mineralisationfront with calcospherites between predentin and dentin. There is a trace of enamel at topright.
Higher power of dentinogenesis. Dentin with tubules at right; note themineralization front with calcospherites. Observe the odontoblasts with processespassing 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 lightpurple dentin on the left. Notice the ameloblast layer immediately to the right of theenamel.
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 fewdifferences. 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 odontoblasticprocesss. 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 Tomes B: Hertwig’s epithelial root sheath C: Hyaline layerInterglobular dentin There are two distinct patterns of dentin that can occur: a linear or aspherical (calcospherite) pattern. *In calcospherites, the crystallites are arranged in a radial pattern and,despite complete mineralization of dentin, this pattern still be discerned usingpolarized light. Failure of calcospherites to fuse may result in the appearance ofinterglobular 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 odontoblast A. 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 dentin It 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 dentin Secondary dentin is formed by the same odontoblast that formed the primarydentin, and is laid down as a continuation of the primary dentin after root formation. It isformed the same way as primary dentin but at a much slower pace. Secondary dentin iseasily distinguished from primary dentin due to its changed in direction and also by thepresence of the demarcation line between the secondary dentin and primary dentin.Tertiary dentin It is a dentin that is deposited at specific sites in response to injury or trauma. Itsformation depends on the degree of the injury; the more severe the injury, the morerapid the rate of dentin deposition. Because of the rapid deposition tubular patterns aredistorted. *tertiary dentin is poor in collagen and enriched in noncollagenous matrixproteins such as sialoprotein and osteopontin
Incremental linesThe rate of dentin formation varies, producing incremental lines. These are thefollowing: 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 FormationRoot formation occurs after the crown has completely formed and shaped. Therefore,tooth begins to form from crown to root. It involves interactions between the Enamelorgan, 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 andinternal enamel epithelia begins to grow down into the dental sac forming a doublelayered epithelial root sheath (Hertwig’s epithelial root sheath). Epithelial root sheathproliferates apically to shape the future root except at the basal portion of the pulpwhich 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 sheathwill differentiate into cementoblast that deposit cementum on the developing root, tothe fibroblast of the developing periodontal ligament and possibly to the osteoblasts ofthe developing alveolar bone.Formation of Periodontal Ligament Formation of the periodontal ligament occurs after the cells of the Hertwig’sepithelial 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 externalsurface of the newly formed root dentin. Other cells of the dental follicle willdifferentiate into fibroblast. Fibroblast will make the fibers and ground substances ofthe periodontal ligamnet by secreting collagen. The fibers will then be embedded in thesurface of newly developed adjacent cementum and alveolar bone. The attachment ofthe periodontal ligament fibers in the cementum and alveolar bone holds the toothsecurely in the socket . As the tooth errupts , the periodontal ligament fibers arereoriented. The different orientations are alveolar crest group, oblique fiber group,apical fiber group,horizontal fiber group and interradicular fiber group. The orientationof 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
4. Horizontal fiber group5. Alveolar crest groupCementogenesis Cementogenesis is the formation of primary (acellular) cementum and thesecondary (cellular) cementum. The process begins at the cervical loop and extendsapically as the root grows downwards. It begins shortly after the fragmentation ofHertwig’s epithelial root sheath. Figure 2 below shows the cervical root area with theHertwig’s epithelial root sheath and its extended diaphragm that will out line the rootformation.
(figure 1) (figure 2) Fragmentation of root sheath permits penetration of the connective tissue cells of thefollicle so that they come to lie between the remnants of the root sheath and the surfaceof the newly formed root. Figure 3 below shows the fragmentation/disintegration ofHertwig’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 differentiateinto cement-forming cells or cementoblast. Present in these cells are numerousmitochondria, 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 firstformed cement matrix. When sufficient organic matrix has been formed it becomesmineralized. As matrix formation proceeds, the cement-forming cells can beincorporated within the developing cement where they become cementocytes, or mayremain on the surface of the forming cement as more rounded cells lacking processes.Two types of cement are then recognized, cellular and acellularcementum. Cementocytesare characterized by processes radiating towards the periodontal ligament and theircytoplasm shows a drastic reduction in the number of organelles when compared tocementoblast. After eruption of the tooth the fibers of the periodontal ligament lie oblique to theroot 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 cementumand periodontal ligament. (figure 6) Once incorporated within the cellular cement they become fully mineralized andindistinguishable from the few other fibers of cement matrix. Acellular cement servesthe 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 fewcollagen content derived from Sharpeyfibres. (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. Dental Papilla 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 ofdentin matrix, encloses the pulp cavity. It is the area of the proliferating future papillathat causes the oral epithelium to invaginate and formthe enamel organ in the earliest stages of toothdevelopment. These enlarge to enclose the dental papillaon the center portion of the developing tooth. The development of the dental pulp begins atabout the eighth week of embryonic life. Soon thereafter Dentinthe more posterior tooth organs begin differentiating.The dental papilla is a well-vascularized and organized network of vessels, which appearby the time dentin formation, begins. Capillaries crowd among the odontoblasts in theperiod 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 nolonger called dental papilla; instead, it is now designated as the pulp organ. Fibroblastsand mesenchymal cells will have a decrease in concentration during the transition ofpapilla into pulp. And there will be an increase in collagen fibers. Fibroblasts came fromthe undifferentiated mesenchymal cell of the papilla. Some of the original mesenchymalcells 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 1= dentin 4= cell-rich zone dental papilla when dentin formation 2=predentin 5= blood vessels (nerves, and veins are not seen has already begun. These nerves never 3= odontoblastic zone here) proliferate the enamel organ. Blood vessels is derived from the dental follicle and will enter the dental papilla during cap stage. The 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 connectivetissue 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 isa 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 adultlife, 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 parietalbone, temporal bone, frontal bone, the mandible, maxilla, and occipital bone. Mesenchymal cellsdifferentiate into osteoblasts.These clusters of osteoblastsform an ossification centerthat secretes organicextracellular matrix, calledosteoid. These mesenchymal cellsusually group together nearor around the blood vessels, and differentiate into osteogenic cells, which deposit bonematrix. These aggregates of bony matrix are called bone spicules. The spicules will traposteoblasts in a lacuna, and will eventually differentiate into osteocytes.
As the bony spicules continue to grow, they fuse with adjacent spicules to form thetrabeculae, forming the spongy bone. The appearance of the trabeculae is the first sign ofbone 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,intramembranousossification, cartilage ispresent duringendochondralossification. It is alsoan essential processduring therudimentary formationof long bones, thegrowth of the length oflong bones, and thenatural healing of bonefractures.
PRIMARY CENTER OF OSSIFICATION The first site of ossification occurs in the primary center of ossification, located in themiddle of the diaphysis. The first that will happen is the formation of the periosteum.The perichondrium becomes the periosteum. This periosteum contains a layer ofundifferentiated cells, called osteoprogenitorcells, that will later transform intoosteoblasts. Formation of the bone collar. The osteoblasts will secrete osteoid against theshaft of the cartilage model, which will serve as support for the new bone. Calcification ofmatrix.Chondrocytes in the primary center of ossification begin to grow. Then thecalcification of the matrix occurs and apoptosis of the hypertrophic chondrocytes occur.This will create cavities within the bone. Invasion of periosteal bud.Blood vessels willsprout from the Osperichondrium before the chondrocytes undergo apoptosis. Thesewill form the periosteal bud and invade the cavity left by the chondrocytes. These bloodvessels carry hemopoietic cells, which will later on form the bone marrow, andosteoprogenitor cells inside the cavity.Formation of trabeculae.Osteoblasts use thecalcified matrix as a scaffold and begin to secrete osteoid, forming the bone trabecula.Osteoclasts, formed from macrophages, break down spongy bone to form the medullarycavity.SECONDARY OSSIFICATION CENTER Secondary ossification appearsin each end, epiphysis, of longbones. The cartilage between theprimary and secondaryossification center is called theepiphyseal plate, and continues toform new cartilage, which isreplaced by bone, which results inan increase in length of the bone.The point of union of the primaryand 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.