Special Histology and Embryology: Practical Course - V. K. Napkhanyuk 2001

Digestive System
Anterior Segment of the Digestive System
Teeth

Teeth (dentes) are hard structures of the Oral Cavity rooted in the alveolar processes of the upper and lower jaws. Their primary function is the mechanical Processing of food. Human teeth appear in two successive generations: deciduous teeth, which eventually shed, followed by permanent teeth.

Tooth Development

Human deciduous teeth begin to develop at the end of the 2nd month of the intrauterine period. At this time, the vestibular dental lamina arises from the oral cavity epithelium, forming the oral vestibule. From the floor of the vestibule, in the region where single-rooted teeth will form, a second ridge-like protrusion grows and transforms into the dental lamina. In the region destined for multi-rooted teeth, the dental lamina develops independently from the oral epithelium.

On the inner surface of the dental lamina, epithelial clusters—tooth buds (germen dentis)—first appear, which give rise to the enamel Organs (organum enamelum). Subsequently, mesenchyme begins to grow toward each bud in the form of a dental papilla (papilla dentis). The papilla invaginates the epithelial organ, causing it to resemble a double-walled goblet or cap that gradually separates from the dental lamina.

The Cells of the epithelial enamel organ differentiate into three types:

1) the inner enamel epithelium, which adjoins the dental papilla, becomes tall and assumes a prismatic character, eventually forming the enamel;

2) the outer enamel epithelium flattens during growth;

3) the intermediate enamel epithelium, whose cells become star-shaped due to fluid accumulation between them, forming the pulp of the enamel organ, which in turn gives rise to the enamel cuticle (cuticula enameli).

The mesenchyme surrounding the tooth germ and the dental papilla also undergoes significant modifications as the bud develops. It condenses to form the dental sac (sacculus dentis). By the end of the 3rd month, the enamel organ completely separates from the dental lamina.

At the beginning of the 4th month of embryonic development, dentin formation begins. Its source is dentinoblasts—cells of mesenchymal origin that are tall and prismatic, with clearly defined polar differentiation. The apical portion of these cells features processes through which organic substances forming the dentin matrix (predentin) are secreted. The amount of predentin gradually increases. Over time, as dentin calcification occurs, this zone becomes part of the mantle dentin.

At the end of the 5th month of embryonic development, the deposition of calcium salts and The formation of definitive dentin begin within the predentin of the tooth germ. The process of predentin decalcification does not affect the areas surrounding the apical processes of the dentinoblasts. This results in the formation of canals extending from the inner surface of the dentin to the outer surface. Furthermore, the areas of predentin bordering the enamel also remain uncalcified and are termed interglobular spaces.

The process of pulp differentiation occurs concurrently with The Development of dentin, within which fibroblasts produce the ground substance containing precollagen and Collagen fibers.

Enamel formation occurs somewhat later. Its initial rudiments appear as cuticular plates On the surface of ameloblasts facing the dentin in the region of the tooth crown. As the enamel develops, the ameloblasts decrease in size and move away from the dentin. By the completion of this process, approximately around the time of tooth eruption, the ameloblasts drastically shrink and regress, while the enamel remains covered only by a thin sheath—the cuticle—formed by the cells of the intermediate pulp layer. The outer cells of the enamel organ fuse with the gingival epithelium during tooth eruption and subsequently degenerate. From the moment enamel prisms appear, the dentin surface becomes uneven. Partial resorption of dentin evidently helps strengthen its bond with the enamel and enhances enamel calcification via released calcium salts.

Cementum develops later than enamel, shortly before tooth eruption, from the mesenchyme surrounding the tooth germ that forms the dental sac. In the inner layer of the dental sac within the ROOT region, cementoblasts differentiate from the mesenchyme, synthesizing and secreting collagen Proteins into the ground substance. As the ground substance develops, cementoblasts transform into process-bearing cementocytes. Cementocytes reside in lacunae and canaliculi extending from them. The outer layer of the dental sac transforms into the periodontal ligament (periodontium).

The primordia of permanent teeth form between the late 4th and early 5th months of intrauterine development. The permanent tooth buds also originate from the dental lamina and underlying mesenchyme, positioning themselves lingual/posterior to each deciduous tooth bud. By the time deciduous teeth erupt (6th–7th month), only the tooth crown is formed, and root formation is just beginning. The deciduous molars are replaced by permanent premolars.

The formation of permanent molars takes place between the 1st and 4th years of life. Initially, both teeth (deciduous and permanent) lie within a common alveolus. Subsequently, a bony septum develops between them.

The permanent tooth develops very slowly. During the shedding of deciduous teeth, osteoclasts resorb the bony septum and the root of the shedding tooth, while the permanent tooth undergoes accelerated development.

Slide 9. Early stage of tooth development (Fig. 45).

Low power magnification. Examine and sketch the tooth bud. Locate the stratified squamous epithelium of the oral cavity, from which the dental lamina extends toward the mesenchyme. In the lower part of the lamina, There is a double-walled, cup-shaped expansion with its floor facing the epithelium—the enamel organ. The outer side of the cup is lined by the outer enamel epithelium, the inner side by the inner enamel epithelium, and the enamel organ pulp is located between them. From the inner (concave) side, the dental papilla invaginates into the enamel organ.

Label on the drawing: 1) stratified squamous epithelium of the jaw margin; 2) embryonic Connective Tissue; 3) dental lamina; 4) outer enamel cells; 5) pulp of the enamel organ; 6) inner enamel cells; 7) connective tissue dental papilla; 8) Blood Vessels.

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Fig. 45. Early stage of tooth development. Hematoxylin-eosin staining. x 160:

1 — stratified squamous epithelium of the jaw margin; 2 — embryonic connective tissue; 3 — dental lamina; 4 — outer enamel cells; 5 — pulp of the enamel organ; 6 — inner enamel cells; 7 — connective tissue dental papilla; 8 — blood vessels

Slide 10. Late stage of tooth development (Fig. 46).

Low power magnification. Observe and sketch the preparation. At this magnification, tissue differentiation leading to the Formation of the primary dental Tissues is visible. The pointed crown of the forming tooth bud appears to project from the enamel organ, in which we can distinguish the outer enamel cells, inner enamel cells, and enamel organ pulp. The narrowed edge of the enamel organ, where the inner and outer enamel cells transition into one another, continues to grow deeper into the embryonic connective tissue, outlining the future deciduous tooth. Examining the layer of enamel cells above, one can see that they acquire a prismatic shape and transform into ameloblasts, whose apical ends produce enamel. Its initial layer in the preparation stands out with a dark red staining. The peripheral cells of the connective tissue dental papilla differentiate into dentinoblasts—prismatic cells that produce dentin. Like ameloblasts, they are arranged in a single layer. At the apex, this layer is typically seen in an oblique section, creating a false impression of stratification. A network of blood vessels forms within the connective tissue papilla. The entire tooth bud remains deeply embedded in the embryonic connective tissue.

Fig. 46. Late stage of tooth development. Hematoxylin and eosin staining. x 160:

1 — Stratified Epithelium of the jaw margin; 2 — developing bone; 3 — reducing dental lamina; 4 — outer enamel cells; 5 — pulp of the enamel organ; 6 — dental papilla pulp; 7 — blood vessels; 8 — odontoblasts; 9 — unmineralized dentin; 10 — dental fibers (Tomes' fibers); 11 — enamel; 12 — ameloblasts

High magnification. Select an area where ameloblasts and odontoblasts are sectioned vertically and examine them at this magnification. It can be seen that ameloblasts lie in a continuous layer, their nuclei are located at the base, and their apical ends form cuticular spikes that gradually transform into enamel. This layer is clearly distinguished by its bright coloration. On the other side, a layer of odontoblasts adjacent to the embryonic connective tissue of the papilla can be observed. Fine fibrils extend from them, around which the ground substance of dentin is deposited. Directly adjacent to the row of odontoblasts is a layer of unmineralized dentin, which has a lighter color. Closer to the top lies a layer of mineralized dentin, which is intensely stained and adjacent to the newly formed enamel.

Label the following on the figure: 1) stratified epithelium of the jaw margin; 2) developing bone; 3) reducing dental lamina; 4) outer enamel cells; 5) pulp of the enamel organ; 6) dental papilla pulp; 7) blood vessels; 8) odontoblasts; 9) unmineralized dentin; 10) dental fibers (Tomes' fibers); 12) enamel; 13) Tomes' processes; 14) ameloblasts.

Structure of teeth

Anatomically, each tooth consists of a crown, a neck, and a root.

A tooth is composed of Hard and Soft tissues. Hard tissues include enamel, dentin, and cementum. Soft tissue is represented by the dental pulp.

Enamel (enamelum) is the hardest tissue in The Human Body, covering the anatomical crown of the tooth.

Chemical composition: 96–97% of enamel consists of Inorganic Compounds (calcium phosphate salts in the form of hydroxyapatite crystals) that form the hard framework of the enamel. Calcium carbonate and fluoride are present in significantly smaller amounts. The organic component consists of proteins—Glycoproteins that form the fine fibrillar matrix of the enamel.

The Structural and functional unit of enamel is the enamel prism. It consists of a thin fibrillar network containing hydroxyapatite crystals shaped like elongated prisms. The prisms are arranged in bundles, follow a wavy course, and lie almost perpendicular to the dentin surface. A less mineralized cementing substance is located between the enamel prisms. Due to the S-shaped curvature of the enamel prisms, some prisms appear cut longitudinally and others transversely in a Cytology/practical/54.html">Longitudinal section of the enamel. This causes the alternation of light and dark lines (Schreger lines). Thin parallel Retzius lines are visible in longitudinal ground sections of enamel. Their appearance is related to the Periodicity of Growth, varying zonal mineralization of the prisms, and the formation of stress lines within the enamel structure.

Externally, the enamel is covered by a thin cuticle (cuticula enameli) formed by the remnants of ameloblasts. A thin layer of glycoproteins—the enamel pellicle—lies on top of the cuticle.

Dentin (dentinum) is a hard tooth tissue that forms its bulk and structural foundation. Dentin is located in the root, crown, and neck of the tooth.

In its structure, dentin is an aggregation of collagen fiber bundles embedded in a ground substance. It is radially pierced by dentinal tubules (canaliculi) containing cellular processes of odontoblasts, whose Cell bodies reside in the pulp. Circumpulpal dentin is located closer to the pulp, while mantle dentin is located peripherally.

Circumpulpal dentin is distinguished by the tangential orientation of its collagen fibers (Ebner's fibers) and a high density of dentinal tubules.

Mantle dentin is characterized by a radial arrangement of collagen fibers (Korf's fibers) and a smaller number of dentinal tubules.

At the dentin-pulp border lies predentin, which consists of unmineralized collagen fibers and ground substance bordered by globules of mineralized dentin. Unmineralized areas also occur in the peripheral layers of dentin and are termed interglobular spaces or interglobular dentin. In the root dentin at the cementum border, the mineralized globules are smaller, and the interglobular spaces form the granular layer of Tomes.

Cementum (cementum) is a hard tissue covering the dentin of the tooth neck and root.

In its chemical composition, cementum resembles bone. It contains about 30% organic and 70% inorganic matter, with calcium phosphate and carbonate salts predominating.

Acellular (primary) cementum and cellular (secondary) cementum are distinguished.

Acellular cementum (cementum noncellulare) is primarily located in the upper part of the root. It contains neither cells nor their processes. It consists of collagen fibers and an amorphous cementing substance lying between them. The collagen fibers have longitudinal and radial directions. The radial fibers continue directly into the periodontal ligament and further enter the alveolar bone as Sharpey's (penetrating) fibers. On the inner side, they merge with the radial collagen fibers of the dentin.

Cellular cementum (cementum cellulare) contains cementocytes and numerous irregularly oriented collagen fibers.

The dental pulp (pulpa dentis) is located in the pulp chamber of the crown and in the root canals. It consists of loose connective tissue in which three layers are distinguished:

1) the peripheral layer, consisting of several rows of multi-process, pear-shaped cells—odontoblasts; in addition, immature collagen fibers are found in the peripheral layer;

2) the intermediate layer, which contains immature collagen fibers and small cells capable of differentiation to replace worn-out odontoblasts;

3) the central layer, consisting of loose cells, fibers, and blood vessels. The cellular composition of this layer is rich in adventitial cells, macrophages, and fibroblasts. Both collagen and argyrophilic fibers are present.

The periodontium is a Dense connective tissue that secures the tooth within the socket of the alveolar process of the upper or lower jaw. It is formed by thick bundles of collagen fibers that have an S-shape and hold the tooth in a suspended state.

Slide 11. Longitudinal ground section of a human incisor (Fig. 47).

First, examine the slide with the naked eye to identify the crown, neck, and root of the tooth. The pulp chamber and root canal are filled with air instead of pulp, as the soft tissues are not preserved in a ground section.

Low magnification. In the main substance of the dentin (the slide is unstained), radial dentinal tubules are clearly visible, anastomosing with each other and branching at the enamel-dentin junction. A granular layer extends along the entire boundary between the dentin, enamel, and cementum. This is a collection of cavities at the sites of unmineralized dentin areas, which are filled with air in the ground section.

Fig. 47. Longitudinal ground section of a human incisor. x 15:

I — crown; II — neck; III — root; 1 — enamel; 2 — striae of Retzius; 3 — bands of Schreger; 4 — dentin; 5 — pulp cavity; 6 — dentinal tubules; 7 — interglobular spaces; 8 — acellular cementum; 9 — cellular cementum; 10 — Tomes' granular layer

The enamel consists of enamel prisms running radially with a slight twist, which makes the contours of the prisms appear to shimmer on the slide (microphoto 26).

Bundles of dentinal tubules penetrating from the dentin into the enamel are visible in the crown. Most of the root cementum is acellular (acellular cementum); only at the root apex is there an area of cellular cementum containing bone lacunae that house cells, which appear black on the slide because they are filled with air.

High magnification. At this magnification, the dentinal tubules and interglobular spaces, which together form the granular layer, can be examined in greater detail. Isolated bone lacunae containing cells are visible within the cementum. They have a spider-like shape. Sharpey's fibers are visible in the Superficial layer of the cementum; these are thick collagen bundles connecting the cementum to the periodontal connective tissue.

Label the following on the diagram: 1) crown; 2) neck; 3) root; 4) enamel; 5) striae of Retzius; 6) bands of Schreger; 7) dentin; 8) pulp cavity; 9) dentinal tubules; 10) interglobular spaces; 11) acellular cementum; 12) cellular cementum; 13) Tomes' layer.

Slide 12. Decalcified tooth (Fig. 48).

Unlike the ground section, this slide preserves not only the hard dental tissues (dentin and cementum) but also the soft tissue (pulp). In addition, the peridental tissues, namely the periodontium and the alveolar bone wall, can be observed.

Low magnification. Study and sketch the slide.

The dental pulp is formed by a specialized loose connective tissue rich in cellular elements, with a dense ground substance that exhibits distinct basophilia.

Identify the layers of the pulp: the outer layer, consisting of odontoblasts; the intermediate layer, which is cell-poor; and the central layer, which is cell-rich. The dental pulp contains numerous blood vessels and nerve fibers grouped into nerve bundles.

Dentin. Numerous dentinal tubules (canaliculi) are clearly visible, through which the processes of odontoblasts (Tomes' fibers) pass (microphoto 27).

Cementum. It can be cellular (at the root apex) and acellular. Cellular cementum contains cementocytes—branched cells resembling bone cells.

Periodontium (apparatus). It comprises cementum, the periodontal ligament, and the alveolar bone wall.

Periodontal ligament (periodontium). The slide clearly demonstrates that this is dense regular connective tissue, the bulk of which consists of collagen fibers. Each fiber is embedded with one end into the cementum (some may enter the peripheral layers of mantle dentin) and with the other into the Bone tissue of the alveolus. Cells, primarily fibroblasts, are located between the fibers. The collagen fibers, combining into bundles, create a highly complex architectonics of the periodontium, which plays a major role in ensuring the physiological mobility of the tooth.

The periodontium is rich in blood Vessels and nerves. The vessels (Arteries, and especially Veins) have a thin tunica media and a strongly developed tunica adventitia formed by loose connective tissue.

Fig. 48. Decalcified tooth. Hematoxylin and eosin staining. x 200:

1 — dentin (a — dentinal tubules); 2 — predentin; 3 — dental pulp (b — odontoblasts; c — blood vessel)

Electron photomicrograph 26. Enamel prisms of a tooth. x 44,800:

1 — cross-sections of enamel prisms; 2 — longitudinal sections of enamel prisms; 3 — densely packed crystals in enamel prisms (after Travis and Glimcher)

Electron photomicrograph 27. Human dentinal tubules. X 30,000:

1 — odontoblast process; 2 — lumen of the dentinal tubule; 3 — Sheath of the dentinal tubule; 4 — cuticle of the sheath; 5 — intertubular matrix

Alveoli. Their walls are formed by fine-fibered lamellar bone tissue consisting of two layers of equal thickness. The layer bordering the periodontal ligament is thin and non-osteonal. It is represented by tightly apposed bone lamellae. The next layer is denser and formed by osteons, with interstitial lamellae located between them. In the region of the alveolar fundus, the wall is formed by thick bone trabeculae containing osteons.

Indicate on the figure: 1) dentin: a) dentinal tubules; 2) predentin; 3) dental pulp: b) dentinoblasts; c) blood vessel.

Review Questions

1. Sources of tooth development.

2. Development of enamel.

3. Features of dentin development.

4. Development of cementum.

5. Initiation and development of permanent teeth.

6. Structure of enamel.

7. Dentin: structural features.

8. Localization and structure of cementum.

9. STRUCTURE OF THE pulp.

10. Periodontal ligament and its structure.

Situational Problems

1. In the developing enamel organ of a tooth, three cell types can be distinguished: internal, external, and intermediate. Which of these will participate in enamel formation? What will they be called?

2. During the development of deciduous teeth (in the histogenesis period), dentin appears first. Which cells participate in its formation? From which embryonic primordium do they develop?

3. The process of deciduous tooth development continues into the postembryonic period. Which part of the tooth is formed during this time?

4. Cementum formation occurs during the period of tooth root development. Which cells take part in its development? From which embryonic source do they originate?

5. In children aged 6–8 years, tooth replacement occurs: deciduous teeth are replaced by permanent ones. Which embryonic primordia serve as the source for the formation of permanent teeth?

6. In the second month of intrauterine development, tooth germs—dental buds—begin to form in the oral cavity. From which germ layer do they develop? In the formation of which tooth structure do they participate?

7. A dental pulp extirpation has been performed. Will this impair The activity of odontoblasts? How does this affect METABOLISM in dentin and enamel?

8. Two tooth micropreparations are provided. One is prepared from the crown region, and the other from the tooth root. How can they be distinguished?

Sample Examination Questions

1. General morphofunctional Organization of teeth. Concept of hard and soft dental tissues.

2. Enamel prisms and interprismatic substance. MODERN CONCEPTS OF enamel structure. Enamel bands and lines.

3. Enamel tufts, enamel lamellae, enamel spindles. Features of enamel metabolism.

4. Structural Features of the superficial enamel layers. Cuticle, pellicle, and their role in metabolic processes. Sources of their development.

5. Comparative characteristics of tooth dentin layers.

6. Features of dentin mineralization. Interglobular dentin. Dentin Nutrition.

7. Dentin response to injury. Secondary dentin. Transparent dentin.

8. Similarities and differences in The structure of dentin and Bone Tissues.

9. Tooth cementum. Comparative characteristics of Different types of cementum.

10. Features of nutrition and regeneration of tooth cementum. Remodeling of cementum under altered functional load.

11. Similarities and differences in the structure of cementum and bone tissue.

12. Dental pulp. Comparative characteristics of its individual layers.

13. Reactive Properties of the dental pulp. Denticles.

14. Features of Blood supply and innervation of dental tissues.

15. Supporting apparatus of the tooth. Periodontium. Histological features. Functional significance.