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Teeth are derived from the oral mucosa of the embryo. From the epithelium of the mucous membrane, enamel organs develop, and from the mesenchyme under the epithelium - dentin, pulp, cement, and the hard and soft tissues surrounding the tooth (periodontium).

There are 3 stages in the development of teeth: Stage I - the formation of teeth and their primordia; Stage II - differentiation of tooth germs; Stage III - tooth formation.

Stage I: at the 6-7th week of embryonic development, a thickening of the epithelium occurs on the upper and lower surfaces of the oral cavity - dental plate (lamina dentalis)growing into the underlying mesenchyme. On the surface of the dental plate facing the lip or cheek, as a result of the further development of the epithelium, flask-shaped protrusions are formed, which then turn into enamel organs (organum enamelum) milk teeth. In each dental plate, 10 protrusions are formed, corresponding to the number of milk teeth. At the 10th week of embryonic development, the mesenchyme grows into the enamel organs, protruding into their walls, which is the rudiment dental papilla (papilla dentalis)... By the end of the 3rd month of development, the enamel organs are partially separated from the dental plate, remaining in connection with it by means of epithelial strands - enamel organ neck (fig. 1). In the circumference of the enamel organ, as a result of the compaction of the surrounding mesenchyme, a dental bag (sacculus dentalis), which at the base of the tooth germ merges with the dental papilla (Fig. 2).

Figure: 1. Development of the enamel organ. (Plastic reconstruction): 1 - oral epithelium; 2 - dental plate; 3 - enamel organ; 4 - the rudiment of the dental papilla; 5 - the neck of the enamel organ

Figure: 2.

1 - dental plate; 2 - rudiments of teeth; 3 - enamel organs; 4 - the lower jaw; 5 - dental plate in the lower jaw; 6 - a layer of outer enamel cells; 7 - pulp of the enamel organ; 8 - a layer of internal enamel cells; 9 - dental bag; 10 - dental papilla

Stage II: both the rudiments of the teeth and the surrounding tissues change. There is a division of homogeneous cells of the enamel organ into separate layers. In the center of the enamel organ, a pulp is formed, and along the periphery - outer enamel cell layer and inner enamel cell layer, giving rise to ameloblast cells involved in the formation of enamel. At the edge of the enamel organ, internal enamel cells pass into outer enamel cells... Part of the pulp cells adjacent to the ameloblast layer becomes intermediate layer enamel organ.

Simultaneously with the transformation of the enamel organ, the process of differentiation of the dental papilla occurs: it increases and grows deeper into the enamel organ. Vessels and nerves fit to the papilla. In addition, several rows of odontoblasts - dentin-forming cells - are formed on the surface of the papilla from mesenchymal cells (Fig. 3). By the end of the 3rd month, the necks of the enamel organs germinate with mesenchyme and dissolve. Dental rudiments, as a result, are finally separated from the dental plate, which, in turn, also grows with mesenchyme and loses its connection with the epithelium of the oral cavity. The posterior sections and free edges of the dental plates are preserved and grow, which are further transformed into the enamel organs of the permanent teeth. Around the tooth buds in the jaw mesenchyme appear bony barsforming the walls of the dental alveoli.

Figure: 3. Tooth rudiment at the stage of formation of hard tissues: 1 - processes of odontoblasts; 2 - predentin; 3 - odontoblasts; 4 - near-pulp dentin; 5 - transformation of mesenchymal cells into odontoblasts; 6 - overdone oblast; 7 - mesenchymal cell

Stage III begins at the end of the 4th month of the embryonic period. Dental tissues are formed: dentin, enamel and tooth pulp. Dentin formation occurs due to odontoblasts, which synthesize thin pre-collagen fibers (fig. 4). These fibers further form the outer, raincoat, and inner, peri-pulpal, layers of predin Odontoblasts are not part of dentin and dentin, but remain in the outer layers of the dental papilla (pulp). At the end of the 5th month of the prenatal period, the process begins calcifications of predin and the formation of the final dentin. However, complete calcification does not occur, and a layer of uncorrected peri-pulpal dentin remains inside the tooth (Fig. 5).

Figure: 4. Collagen fibers of preentin: 1 - dentinal tubule

Figure: five.

1 - near-pulp dentin; 2 - matrix; 3 - salt globules; 4 - the border of calcification; 5 - predentin; 6 - raincoat dentin

At the beginning of the 5th month, ameloblasts at the top of the dental papilla form enamel. This process begins in the area of \u200b\u200bthe masticatory tubercles, from where the enamel formation spreads to the lateral surfaces of the crown. In the future, enamel calcification occurs, which ends only after teething. The development of the root of the tooth occurs in the postembryonic period, while in connection with the formation of the crown of the tooth, the upper part of the enamel organ is reduced, and the lower, on the contrary, proliferates and turns into root epithelial vagina (vagina radicularis epithelialis), consisting of two rows of enamel cells - inner and outer. The root epithelial sheath grows deeply into the underlying mesenchyme and covers its area, from which the tooth root will form (Fig. 6). Mesenchymal cells, enclosed in root epithelial sheaths, turn into odontoblasts, which form the dentin of the tooth root. As soon as the root dentin is formed, the root epithelial sheaths grow with mesenchyme, most of them are absorbed, as a result of which the mesenchymal cells of the dental sac begin to directly contact the root dentin and transform into cementoblasts, which deposit cement on the surface of the tooth root dentin. Part of the cells of the dental sac, surrounding the tooth root, gives rise to dense connective tissue - periodontium. The bundles of collagen fibers that form the periodontium are “soldered” into the cement with their inner ends, and their outer ends pass into the bony dental alveoli, thereby providing a tight fixation of the root to the surrounding tissues. In multi-rooted teeth, several root epithelial sheaths are formed and, accordingly, several roots. The pulp of the tooth develops from the mesenchyme of the dental papillae.

Figure: 6.

1 - root epithelial sheath; 2 - inner layer of cells; 3 - outer layer of cells; 4 - cementblasts; 5 - cement; 6 - periodontium; 7 - tooth pulp

Permanent teeth also arise from dental plates. At the 5th month of development, behind the rudiments of milk teeth, enamel organs of incisors, canines and small molars are formed. At the same time, the dental plates grow posteriorly, where the enamel organs of the large molars are laid along their edges. Further stages of formation are similar to those of milk teeth, and the rudiments of permanent teeth lie in the same dental alveolus together with the milk tooth (Fig. 7).

Figure: 7.

1 - the rudiment of a milk tooth; 2 - the rudiment of a permanent tooth; 3 - dental alveoli screed

Dysfunctional tooth development can lead to improper deposition of solids ( enamel hypoplasia, erosion pits on the tooth surface, calcification defects dentin), deviations in the number of teeth (complete or partial absence of teeth - adentia), the formation of additional teeth, the irregular shape of individual teeth, the incorrect position of the teeth in the jaw (dystopia).

Human Anatomy S.S. Mikhailov, A.V. Chukbar, A.G. Tsybulkin

Dental pouch (saccus dentalis, LNE)

the accumulation of mesenchymal cells around the dental organ, which is the shell of the tooth germ; cement is also formed from the earth.

1. Small Medical Encyclopedia. - M .: Medical encyclopedia. 1991-96 2. First aid. - M .: Great Russian Encyclopedia. 1994 3. Encyclopedic Dictionary of Medical Terms. - M .: Soviet encyclopedia. - 1982-1984.

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TEETH

Stages of tooth development.In the odontogenesis of deciduous and permanent teeth, several stages are distinguished, which smoothly pass one into the other. These are the formation of teeth, stages of the tooth kidney, dental cup, tooth bell, apposition and maturation of hard tooth tissues such as enamel, dentin and cement. At the 8th week, a dental plate is formed. She participates in the formation of the rudiments of milk and permanent teeth. On the 10th week, the rudiment of a milk tooth contains an enamel organ and a dental papilla. By this time, the outgrowth of the dental plate in the form of a kidney of a permanent tooth was formed. Developing ameloblasts form enamel, and odontoblasts from the peripheral mesenchyme of the dental papilla form dentin.

Dental kidney stagecharacterized by intensive multiplication of cells at the edge of the dental plate (1), the rounded mass of which actively grows into the adjacent mesenchyme. This epithelial cell mass (2), separated from the surrounding mesenchyme by the basement membrane, is called the tooth kidney - the rudiment of the enamel organ. Staining with hematoxylin and eosin.

Cup stage.The enamel organ is clearly visible on the specimen (1). The dental kidney, which forms the enamel organ, takes the shape of a bowl, which retains its connection with the rest of the dental plate using a thin epithelial cord - the neck of the enamel organ (2). In the enamel organ, cells of the internal enamel epithelium are visible (3). The cells of the central part of the enamel organ acquire a stellate shape. This part of the enamel organ is called the pulp (4). Part of the pulp cells, adjacent directly to the layer of the inner enamel epithelium, forms an intermediate layer of the enamel organ, consisting of 2-3 rows of cubic cells. Blood vessels enter it. The mesenchyme of the dental papilla (5) condenses into a dense cell mass, which repeats the curvature of the bowl of the enamel organ and grows into it. The enamel organ and the dental papilla are separated by a basement membrane, in the place of which the dentin-enamel joint will subsequently pass (6). The mesenchyme surrounding the tooth germ forms the tooth pouch (7). Staining with hematoxylin and eosin.

Tooth bell stage.IN

the dental papilla (1) shows a peripheral layer of correctly positioned odontoblasts (2) of a pear-shaped form, the long process of which is facing the enamel organ (3). These cells form a narrow strip of non-mineralized preentin, outside of it there is some

a large amount of mature mineralized dentin (4). On the side facing the dentin layer, a thin strip of enamel (5) is visible, outside of which ameloblasts are localized (6). Staining with hematoxylin and eosin.

Milk tooth bud(3 month old fetus). The enamel organ is connected to the dental plate by means of a thin epithelial cord - the neck of the enamel organ. Around the enamel organ, a dental pouch is formed, merging at the base of the tooth germ with the mesenchyme of the dental papilla. In the enamel organ, internal cylindrical enamel cells (ameloblasts participating in the formation of enamel) are visible. Along the edge of the enamel organ, the internal enamel cells pass into the external ones lying on the surface of the enamel organ and having a flattened shape. The cells of the central part of the enamel organ acquire a stellate shape. This part of the enamel organ is called the pulp. Part of the pulp cells, adjacent directly to the layer of enameloblasts, forms an intermediate layer of the enamel organ, consisting of 2-3 rows of cubic cells. The dental papilla grows in size and grows even deeper into the enamel organ. Blood vessels enter it. On the surface of the dental papilla, odontoblasts, cells with a dark basophilic cytoplasm, are differentiated from mesenchymal cells, located in several rows. This layer is separated from the ameloblasts by a thin basement membrane. In the circumference of the tooth germ, the crossbeams of the bone tissue of the dental alveoli are formed.

Secreting ameloblast-prismatic cell, has a hexagonal shape across. In the basal part of the cell, facing the pulp of the enamel organ, there is an oval nucleus with a nucleolus and a compact accumulation of mitochondria. Glycogen granules are located between mitochondria. The Golgi complex is located in another part of the cell. Its flat cisterns are oriented parallel to the long axis of the cell and cover a part of the cytoplasm, which contains secretory granules, vesicles, and cisterns (both smooth and granular) of the endoplasmic reticulum. The apical part of the cell contains small vesicles that can fuse with the cell membrane. Together with them in this part of the cell, large

secretory granules. They contain material to be removed from the cell by exocytosis. Part of the cytoplasm, located outside of the Golgi complex, is filled with elongated cisterns of the granular endoplasmic reticulum, forming branches and anastomoses. Clusters of free ribosomes are common. Immediately below the plasmolemma, there is a compact terminal network consisting of thin actin microfilaments. Apical to the terminal network, the ameloblast forms a process (processus enameloblasti).

Odontoblast- a tall cylindrical cell with a nucleus located in the basal part. The main part of the cytoplasm is occupied by a well-developed granular endoplasmic reticulum. A pronounced Golgi complex is located in the central part of the cell. Electron-dense granules are grouped near it. Mitochondria are evenly distributed throughout the cell. The cytoskeleton in the apical part of the cell forms a terminal network to which the stem of the dentin process is connected (processus dentinoblasti).The shoot contains a moderate amount of granules, filaments, microtubules and vesicles fused with the plasmolemma.

The stage of apposition and maturation. AND- differentiation of preameloblasts and odontoblasts. Preameloblasts differentiate from cells of the inner enamel epithelium. The superficial cells of the dental papilla differentiate into odontoblasts. B- odontoblasts produce and secrete components for the dentin matrix in the space between the basement membrane and the secretory surface of odontoblasts. The basement membrane between preameloblasts and odontoblasts disintegrates. This allows preameloblasts to come into contact with newly formed predentin and differentiate into ameloblasts. IN- dentine-enamel connection. Ameloblasts form the enamel matrix on the side facing the preentin. As the regular and rhythmic process of enamel and dentin matrix formation proceeds, the secreting surfaces of ameloblasts and odontoblasts are removed from the dentinoenamel junction. Unlike ameloblasts, odontoblasts retain their processes in the extracellular matrix, first in predentin and then in mineralized dentin.

Single root tooth.The main volume of the tooth is occupied by dentin - one of the types of bone tissue. The root of the tooth is fixed in the dental alveolus of the bone, surrounded by a periodontium, which is attached to the root dentin with the help of cement. The crown is covered with enamel. The dentin underneath continues into the root of the tooth. In the central part of the tooth, in the pulp cavity, there is the pulp of the tooth - the pulp. The pulp cavity at the root apex opens with one or more dental openings. Dentin contains thin tubules that run from the pulp cavity to the tooth surface. In these tubules in a living tooth are the processes of odontoblasts. Their bodies are located in the pulp at the border with dentin.

Look question 104

106. Tooth development. Tooth histogenesis. Odontoblasts and their role in dentin formation. Cloak and peri-pulp dentin. Predentin.
PERIOD OF HISTOGENESIS.
During the period of histogenesis, the products of cell secretion initially form a kind of "building structure", which subsequently undergoes calcification. The final stage of odontogenesis is reached when the tooth tissues are consistently and completely mineralized.

For the proliferation and differentiation of cells that provide the formation of these parts of the

ba, inductive effects are needed between the ectodermal cells of the enamel organ and the mesenchymal cells of the dental papilla and dental sac. These inter-tissue interactions and communication between cells are provided by the basement membrane.

First, the crown of the tooth is formed, then its root.
Odontoblast (dentinoblast) formation and dentinogenesis

Odontoblasts of mesenchymal origin also undergo repolarization, which is manifested in the movement of their nuclei from the center to the position farthest from the basement membrane. These cells adjoin the basement membrane in a mirrored orientation relative to the preeameloblasts. Odontoblasts begin to show secretory activity and form an organic matrix of dentin - predentinon the side facing the basement membrane (Fig. 54, 55). Thus, dentin is formed before the enamel matrix.

Odontoblasts have a well-defined secretory apparatus, characteristic of collagen-producing cells. With the onset of secretion, a process begins to form in the apical part of the cells - toms fiber(Fig. 56, a). Desmosome-like contacts and tight junctions are formed between odontoblasts, which, as it were, separate the dentinal and pulpal compartments of the forming tooth.

Most of the proteins secreted by odontoblasts are similar to those secreted in the bone. The main organic components of dentin are type I collagen, as well as glycoproteins, proteoglycans, glycosaminoglycans.

However, predentin contains dentin phosphoprotein and dentin sialoprotein. Dentin phosphoprotein binds large amounts of calcium. This protein to a certain extent initiates mineralization and "controls" the size and shape of minerals.

In the process of dentinogenesis, the bodies of odontoblasts are pushed aside by the formed dentin from the layer of enameloblasts, and the process of the odontoblast is elongated. The latter is initially located in the predentin, and as it calcifies, in the dentin, inside the forming dentin tube (Fig. 57). The "case" of the tubule becomes highly mineralized peritubular dentin.Calcified dentin located between the dentinal tubules is intertubularm.

In the course of dentinogenesis, the matrix of the outer layer of the mantle dentin is first produced, then the matrix of the peri-pulpal dentin. The first collagen synthesized by odontoblasts forms thick fibrils and bundles of fibrils - radial fibers of Corfe.Together with the amorphous substance, they form the organic matrix of the mantle dentin. The peri-pulpal dentin matrix develops later. Collagen secreted by odontoblasts during this period forms thinner fibrils that intertwine with each other, are located parallel to the surface of the dental papilla and form ebner's tangential fibers.At the same time, the matrix of the cloakroom dentin is pushed to the periphery.

Dentin calcification begins at the end of the 5th month of intrauterine development and is carried out with the participation of odontoblasts. The formation of the organic matrix of dentin is ahead of its calcification, therefore, pre-dentin remains always hypomineralized.

It is believed that in mantle dentin, calcification is carried out with the participation of matrix vesicles. Matrix vesicles are tiny rounded structures ranging in size from 30 nm to 1 μm, surrounded by a membrane identical to the plasmolemma. These formations are involved in the initiation of calcification. Various assumptions have been made regarding the nature of the bubbles. Most likely, they bud off from the plasmolemma of cell processes. Matrix vesicles store calcium and contain lipids. They are characterized by high activity of alkaline phosphatase. (Phosphatase hydrolyzes the phosphoric acid ester enzymatically to form orthophosphate, which can react with the calcium collected in the vesicles to form a precipitate.)

Hydroxyapatite crystals grow and rupture vesicle membranes. Aggregates of crystals grow in different directions and merge. The calcification process is associated with the association of a mineral substance with collagen fibrils located near the processes of odontoblasts.

Apparently, in the peri-pulpal dentin, calcification is carried out by odontoblasts without the participation of matrix vesicles. The composition of the organic matrix of the peri-pulpal dentin is somewhat different from that of the mantle dentin. Odontoblasts activate the secretion of phospholipids, phosphoproteins, which are secreted into preentin and diffuse to the dentinal side, where they form granular material. In the peri-pulp dentin, hydroxyapatite crystals on the surface and between collagen fibers are deposited in the form of rounded masses - globules or calcospherites. The globules further increase in size and merge, forming a homogeneous calcified tissue.

In the peripheral areas of the peri-pulpal dentin near the mantle dentin, large globular masses do not completely merge, leaving areas of hypomineralized interglobular dentin between them. Dentinal tubules pass through interglobular dentin without interrupting or changing their course. This type of calcification is well traced in the crown of the tooth at the border near the pulp and mantle dentin. In the area of \u200b\u200bthe tooth root, areas of interglobular dentin form a granular Toms layer (see Fig. 36). An increase in interglobular dentin is seen as a sign of insufficient calcification.

Peritubular dentin is more correctly called intratubular, since it is formed inside the tubule with the participation of odontoblast processes. Peritubular dentin reduces the initial diameter of the dentinal tube lumen over time. Mineralization of the secreted organic base is provided mainly by the transfer of calcium within the matrix vesicles, which are located along the periphery of the cytoplasm of the processes and are released into the extracellular space. Peritubular dentin differs from intertubular dentin by a higher content of hydroxyapatite.

107. Tooth development. Stages of histogenesis. Enamel formation. Enameloblasts. The emergence of enamel prisms. Calcification of the enamel.
Enameloblast formation and amelogenesis

After the differentiation of odontoblasts from the outer cells of the dental papilla and the formation of preentin by them, the basement membrane between preeameloblasts and odontoblasts disintegrates. This creates conditions for close contact of pre-enameloblasts with newly formed predentin and induces them to further differentiate into enameloblasts,providing the formation of enamel (see Fig. 54).

The first stage of amelogenesis is the formation of an organic enamel matrix secretory-active enameloblasts(fig. 58). Second

stage - maturation of the enamel matrix - consists in the removal of organic material and the active inclusion of minerals in the maturing enamel enameloblasts at the stage of maturation.These cells differentiate from secretory-active enameloblasts and function mainly as a transport epithelium, carrying out the movement of substances both inside the maturing enamel and out of it.

The first secretory-active enameloblasts are formed from the cells of the inner enamel epithelium in the area of \u200b\u200bthe crown apex (at the site of the primary deposition of predin). Further, the wave of differentiation spreads towards the edge of the enamel organ.

The diameter of the enameloblast is about 4 microns, the height is 40 microns. In the cross section, the cells are hexanal. After repolarization of pre-enameloblasts, at the stage of enameloblasts, a pyramidal process is formed at the apical pole of each cell (not to be confused with Toms fiber in odontoblasts!) (See Fig. 56, b).

The Toms process is the secretory surface of each cell and faces the area of \u200b\u200bthe dentinoenamel junction (Fig. 59). The nucleus and accumulations of mitochondria are localized in the basal part of the cells. The cytoplasm contains a developed endoplasmic reticulum, the Golgi complex, and electron-dense secretory granules (see Fig. 56, b). There are connecting complexes in the basal and apical parts of the cells. Actin filaments, which are part of the connective complexes, facilitate the movement of secretory enameloblasts, preserve and maintain the orientation of cells gradually moving away from the dentinoenamel border to the periphery. Deposition of the enamel matrix defines the Toms process.

Early enamel matrix is \u200b\u200ban ectodermal product consisting mainly of non-collagen proteins and a small amount of calcium hydroxyapatite crystals. In the developing enamel, the main proteins are:

1) amelogenins (hydrophobic proteins, mobile and not associated with crystals);

2) enamelins (enamel proteinases that provide the degeneration of amelogenins in maturing enamel);

3) ameloblastins (produced by enameloblasts from the early secretory stage to the late stage of maturation, regulate and direct the process of mineralization);

4) taftelins (acidic proteins, localized mainly in the area of \u200b\u200bthe dentin-enamel junction and are involved in the formation of enamel crystals).

As the enamel matures, the content of proteins in it decreases, which is associated with the displacement of amelogenins from the intercrystalline spaces and

cleavage of part of the proteins by proteolytic enzymes. More mature enamel contains only enamelins and taftelins.

Enameloblasts at the stage of maturation are shortened, they lose Toms' processes and some organelles. Some of the enameloblasts die due to apoptosis.

Among the maturation stage enameloblasts, cells of 2 types are found, capable of mutual transformations. Maturation stage 1 enameloblastscharacterized by the appearance of a striated edge on their apical surface (see Fig. 58). These cells are involved in the active transport of inorganic ions, which are transported through

their cytoplasm and are secreted on the apical surface. Type 1 enameloblasts contain high concentration of calcium-binding proteins.

Ripening stage 2 enameloblastshave a smooth apical surface (see Fig. 58). These cells are involved in removing organic matter and water from the enamel.

Thus, during the maturation of the enamel matrix, enameloblasts actively "pump" calcium into the already partially mineralized matrix and at the same time "withdraw" a certain amount of organic matter.

Final maturation of the enamel - tertiary mineralization- occurs after the eruption of the tooth. In this case, the main source of inorganic substances entering the enamel is saliva.

The structure of the enamel depends on the time of its formation.

So-called the initial and final enamel has a prismatic structure.The initial enamel includes the inner layer at the dentine-enamel border, which does not contain prisms, since during its formation the Toms processes in enameloblasts have not yet formed.

The final enamel is formed at the final stages of enamel secretion, when the enameloblasts undergo degenerative changes and the Toms process disappears.

Prism formation mechanismsare not entirely clear.

After the deposition of the first layer of prismless initial enamel (between the dentin and the apical surface of the cell), the enameloblasts move away from the surface of the dentin and form Toms processes.

108. Development of the tooth root. Cement formation. Cementoblasts and their importance in the formation of cement.
Development of root dentin and cementum

The cells of the dental papilla, as a result of the inductive effect of the epithelial vagina, differentiate into root dentinoblasts, which produce dentin (Fig. 63).

Then the epithelial vagina breaks down into separate fragments (epithelial remains of Malasse, found in the periodontium), and the cells of the inner layer of the dental sac come into contact with dentin, differentiating into cementoblasts (Fig. 64).

These large cubic cells synthesize the proteins of the cement matrix (pre-cement, cementoid). The cementoid is deposited over root dentin or over the highly mineralized Hopewell Smith hyaline layer. (According to some reports, this amorphous layer is formed by the epithelial cells of the root sheath before it decays.)

Cementoid mineralization occurs through the deposition of hydroxyapatite crystals in it. In this case, cementoblasts are displaced to the periphery or walled up in it, turning into cementocytes (Fig. 65).

Cement that does not contain cells embedded in it is called acellular, or primary.

109. Development of the tooth root. Formation of the root epithelial sheath. Role of the root sheath in root formation in single-rooted and multi-rooted teeth
The enamel organ not only participates in the formation of enamel, but also plays an important role in the formation of the roots of future teeth. Development of the tooth root occurs in the postembryonic period shortly before eruption and continues after the eruption of the tooth. Teething begins when the root is 25-50% formed.

The structure that determines the development of the tooth root is the cervical (cervical) loop. It consists of 2 rows of cells: the inner epithelium and the outer epithelium of the enamel organ. The cervical loop grows, going deeper into the mesenchyme of the dental sac and moving away from the newly formed tooth crown.

The edges of the enamel organ begin to grow vigorously, penetrating into the underlying mesenchyme, forming an epithelial (Hertwig's) root sheath. This formation in the form of an elongating skirt descends from the enamel organ to the base of the dental papilla. The root sheath consists of two rows of cells of the enamel organ (external and internal), which are in close contact.

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