The tooth ends with further education. Tooth histology or what are dentin and periodontium

TOOTH DEVELOPMENT

The main sources of tooth development are the epithelium of the oral mucosa (ectoderm) and ectomesenchyme. In humans, there are two generations of teeth: temporary (dairy) And permanent . Their development proceeds in the same way from the same sources, but at different times. The formation of primary teeth occurs at the end of the second month of embryogenesis. In this case, the process of tooth development occurs in several stages. There are 4 periods in it:

I. The period of formation of tooth germs.

II. The period of formation and differentiation of tooth germs.

III. The period of histogenesis (tissue formation) of the tooth.

IY. Period of eruption and beginning of functioning

I.The period of formation of tooth germs.

The period of formation of tooth germs includes 2 stages.

Stage 1 – stage of formation of the dental plate. It begins in the 6th week of embryogenesis. At this time, the epithelium of the gum mucosa, due to the proliferation and migration of cells, begins to grow into the underlying mesenchyme all over the edge each of the developing jaws. As a result, a dental plate is formed (Fig. 1, 2).

Stage 2 – stage of tooth bud formation(Fig.2). At this stage, the cells of the dental plate multiply in the distal part and form epithelial formations at the end of the dental plate, shaped like a kidney or sometimes a ball - dental buds. The number of such buds corresponds to the number of teeth.

Rice. 1. Scheme of development of baby teeth

1 – lip; 2 – buccal-labial groove; 3 – edge of the lower jaw; 4 – dental plate; 5 – rudiments of baby teeth; 6 – enamel organ; 7 – dental papilla; 8 – neck of the enamel organ

II. The period of formation and differentiation of tooth germs

The second period is characterized by the formation enamel organ (dental cup). During this period, the mesenchymal cells lying under the dental bud begin to multiply intensively and create increased pressure here, and also induce, due to soluble inducers, the movement of the dental bud cells located above them. As a result, the lower cells of the tooth bud protrude inward, gradually forming a double-walled dental cup – enamel organ(Fig. 2). The epithelium of the enamel organ gradually differentiates into cells inner, intermediate and outer enamel epithelium. Mesenchyme, penetrating inside the glass, forms dental papilla, and from the mesenchyme surrounding the dental cup is formed dental sac. At first, the enamel organ is cap-shaped (cap stage), and as the lower cells move into the kidney, it becomes bell-shaped (bell stage).

Fig.2. Stages of tooth development

A – dental plate stage: 1 – gingival epithelium; 2 – mesenchyme; 3 – dental plate.

B – tooth bud stage: 1 – gingival epithelium; 2 – epithelium of the dental plate;

3 – tooth bud; 4 – mesenchyme.

B – stage of the enamel organ: 1 – internal cells of the enamel organ;

2 – intermediate cells of the enamel organ; 3 – outer enamel cells

organ; 4 – dental papilla; 5 – dental sac.

G – late stage (histogenesis):

I. 1 – pulp of the enamel organ; 2 – enameloblasts; 3 – outer enamel cells

organ; 4 – dentinoblasts; 5 – dental pulp; 6 – dental sac.

II. Area at the apex of the enamel organ

Cells inner enamel epithelium(concave part), in contact with the cells of the dental papilla, multiply intensively and become tall prismatic - later they serve as a source for the formation of, - the main cells of the enamel organ that produce enamel.

Between the cells of the central part of the enamel organ, fluid containing glycosaminoglycans and proteins begins to accumulate, resulting in intermediate cells move away from each other and acquire a stellate shape, held in the area of ​​their processes by desmosomes. These epithelial cells form pulp of the enamel organ, (stellate reticulum), which for some time carries out the trophism of enameloblasts, and later gives rise to the cuticle.

Cells outer enamel epithelium, on the contrary, are flattened. Over a larger area of ​​the enamel organ they degenerate. The inner enamel epithelium joins the outer enamel epithelium at the lower edge of the enamel organ, in an area called cervical loop. The cells of this zone, after the formation of the crown, will give rise to epithelial (Hertwig's)) root sheath, which will cause the formation of a tooth root. Inductive influences emanating from the root sheath determine the number of developing tooth roots.

The second period for primary teeth is completely completed by the end of the 4th month of embryogenesis.

III period – period of histogenesis (tissue formation) of the tooth.

This period of tooth development is the longest: it begins at the end of the 4th month of intrauterine development and ends after birth. The first signs of tooth tissue formation are noted at the final stages of the “bell” stage, when the tooth germ is already taking the shape of the crown of the future tooth (Fig. 2).

Forms most early from the hard tissues of the tooth. dentine through a process called dentinogenesis.

Adjacent to the internal cells of the enamel organ (future enameloblasts), the connective tissue cells of the dental papilla, under the inductive influence of these cells, first turn into predentinoblasts - elongated or pear-shaped cells with basophilic cytoplasm, located in several rows. Predentinoblasts later differentiate into odontoblasts, which are arranged in one row like epithelium (Fig. 3). The basement membrane under the enameloblasts plays the role of a differentiation factor. The odontoblast nucleus moves to the basal part of the cell (the end facing the dental papilla); synthesis organelles develop: granular ER, Golgi complex located above the nucleus, processes are formed directed towards the enameloblasts, and the cells begin to secrete the intercellular substance of dentin - collagen fibers and ground substance (Fig. 4).

Fig.3.

The formation of the fibers themselves occurs outside the cells. First, immature precollagen fibers are formed, arranged radially - radial corf fibers. Between them lie the processes of dentinoblasts. They are part of the main substance of young, non-calcified dentin - predentina. When the predentin layer reaches a certain thickness, it is pushed to the periphery by newly formed predentin layers - thus forming mantle dentin(with Korff fibers), located under the enameloblasts. In new layers, collagen fibers run tangentially (parallel to the surface of the dental papilla) - this tangential fibers Abner- thus, it is formed peripulpal dentin(with Ebner fibers).

Fig.4. Scheme of the structure of odontoblast

1 – dentin;

2 – odontoblast process;

3 – predentin;

4 – mitochondria;

5 – Golgi complex;

6 – state power station;

7 – core.

In addition to fibers and ground substance, odontoblasts synthesize the enzyme alkaline phosphatase. This enzyme breaks down blood glycerophosphates to form phosphoric acid. As a result of the connection of the latter with calcium ions, hydroxyapatite crystals are formed, which are released between collagen fibrils in the form of matrix vesicles surrounded by a membrane. Hydroxyapatite crystals increase in size. Mineralization (calcification) of dentin gradually occurs.

Calcification of dentin occurs only at the end of the 5th month of embryonic development. The processes of dentinoblasts do not undergo mineralization, as a result of which a system of radial dentinal tubules is formed in the dentin, running from the inner surface of the dentin to the outer. Predentin And interglobular dentin also do not undergo calcification.

Only after the initial layers of dentin have been deposited along the periphery of the dental papilla, cells in the epithelial enamel organ differentiate and begin to produce enamel on top of the developing dentin. The process of enamel formation is called amelogenesis.

The deposition of the first layers of dentin induces the differentiation of cells of the inner enamel epithelium - enameloblasts (ameloblasts). With the onset of amelogenesis in the enameloblast, the nucleus moves (inverts) to the opposite pole of the cell (to the former apical pole, which has become functionally basal); the cells acquire a highly prismatic shape; Organelles of synthesis develop abundantly (granular endoplasmic reticulum, free ribosomes, Golgi complex) (Fig. 5,6). The organelles are located above the nucleus in the direction of the dentin. At this pole a process is formed ( Tom's process). The processes accumulate granules with electron-dense contents, which are released into the intercellular space and participate in the formation of the organic basis of the enamel. The enamel rudiments mineralize very quickly, which is facilitated by specific ( non-collagenous) enamel proteins – amelogenins(90% proteins) and enamelines, which are secreted by anameloblasts. An organic enamel matrix is ​​deposited on top of the newly formed dentin layer.

Enameloblasts are connected to each other by complexes of intercellular connections at two levels - in the region of the new apical and basal poles. The basement membrane on which they were previously located is destroyed after the deposition of predentin and during the differentiation of enameloblasts. After deposition of the first layer of initial (non-prismatic) enamel, enameloblasts move away from the dentin surface and form Thoms' process. The conditional boundary of the process and the cell body is considered to be the level of the apical complex of intercellular connections. The cytoplasm of the cell body contains mainly organelles of the synthetic apparatus, and the cytoplasm of the process contains secretory granules and small vesicles.

Rice. 5. Scheme of the stages of the enameloblast life cycle

1. stage of morphogenesis

2. stage of histodifferentiation

3. initial secretory stage (no Toms processes);

4. stage of active secretion (Toms' process);

5-6. maturation stage

7. reduction stage (protective stage)

Fig.6. Scheme of the structure of the enameloblast at the stage

active secretion

1 – core; 2 – granular endoplasmic reticulum;

3 – Golgi complex; 4 – Toms’ process; 5 – secretory granules with enamel components; 6 – enamel prisms; 7 – mitochondria.

After completion of enamel formation, secretory active enameloblasts are transformed into enameloblasts of the maturation stage: they provide maturation (secondary mineralization) of the enamel, which only after this acquires an exceptionally high mineral content and strength. Only after completing this important function do enameloblasts collapse and turn into reduced dental epithelium (secondary enamel cuticle), which performs a protective function.

Outer enamel epithelial cells When teeth erupt, they merge with the gum epithelium and are subsequently destroyed. The enamel is covered with a cuticle formed from the pulp of the enamel organ

From inner cells dental papilla develops tooth pulp, which contains blood vessels, nerves and provides nutrition to tooth tissues. The process of pulp differentiation parallels the development of dentin. Mesenchyme cells differentiate into fibroblasts, fibroblasts synthesize and secrete the ground substance, precollagen and collagen fibers, a network of blood vessels develops - thus the loose connective tissue of the dental pulp is formed.

In mesenchyme dental sac two layers are differentiated: the outer layer is denser and the inner layer is loose. From mesenchyme of the inner layer, in the root area, differentiate cementoblasts, which produce the intercellular substance of cement and participate in its mineralization by the same mechanism as in the mineralization of dentin. Cementoblasts turn into processous cementocytes.

Thus, as a result of differentiation of the enamel organ rudiment, the formation of the main dental tissues occurs: enamel, dentin, cement, pulp.

From the mesenchyme of the outer layer of the dental sac develops periodontal tooth.

Tooth root development

The development of roots, in contrast to the development of crowns, occurs later and coincides in time with the eruption of teeth.

After the formation of the tooth crown, before eruption, the zone of activity of the enamel organ moves to the region of the cervical loop, where the cells of the inner and outer enamel epithelium connect.

This two-layer epithelial cord of a cylindrical shape - the epithelial root sheath (Hertwig) - grows into the mesenchyme between the dental papilla and the dental sac, and gradually descends from the enamel organ to the base of the papilla and covers the elongating dental papilla.

The internal cells of the root sheath do not differentiate into anameloblasts, but induce differentiation of the peripheral cells of the papilla, which turn into odontoblasts of the tooth root.

Odontoblasts form root dentin, which is deposited along the edge of the root sheath.

The cells of the root sheath disintegrate into small anastomosing strands - epithelial remains (islands) of Malasse (can be a source of development of cysts and tumors).

As the vagina deteriorates, the mesenchymal cells of the dental sac come into contact with the dentin and differentiate into cementoblasts, which begin to deposit cementum on top of the root dentin.

The periodontium develops from the dental sac soon after the formation of the tooth root begins. The cells of the pouch divide and differentiate into fibroblasts, which begin to form collagen fibers and ground substance. Periodontal development includes the growth of its fibers from the side of the cementum and dental alveoli and becomes more intense immediately before tooth eruption.

Root dentin is characterized by a lower degree of mineralization, less strict orientation of collagen fibrils, and a lower rate of deposition. The final formation of root dentin is completed only after teething: in temporary teeth ~ after 1.5-2 years, and in permanent teeth – after 2-3 years from the start of teething

Teething – gradual appearance of tooth crowns above the surface of the alveolar process of the jaw and gums; ends with the appearance of the entire tooth crown (up to the neck) above the gum surface. Humans erupt teeth twice.

During the first eruption which begins at the 6th months and ends by 24-30 months During the life of a child, 20 temporary (baby) teeth appear.

Theories explaining the mechanisms of eruption:

– The theory of tooth root growth (the elongating root rests against the bottom of the alveolus; a force appears that pushes the tooth vertically;

– Theory of hydrostatic pressure

– Theory of bone tissue remodeling

– Periodontal traction theory(shortening of collagen bundles and contractile activity of fibroblasts)

Before eruption, the enamel is covered with reduced enamel epithelium (REE). Reduced enamel epithelium, in the form of several layers of flattened cells, is formed by enameloblasts that have completed the production of enamel, as well as cells of the intermediate layer, pulp and outer layer of the enamel organ

Changes in the tissue covering the erupting tooth.

As the tooth approaches the oral mucosa, regressive changes occur in the connective tissue separating the tooth from the epithelium of the mucous membrane. The process is accelerated due to ischemia caused by the pressure of the erupting tooth on the tissue. The reduced enamel epithelium, covering the tooth crown in the form of several layers of flattened cells (formed by enameloblasts that have completed the production of enamel, as well as cells of the intermediate layer, pulp and outer layer of the enamel organ), secretes lysosomal enzymes that promote the destruction of connective tissue. Approaching the epithelium lining the oral cavity, the cells of the reduced enamel epithelium divide and subsequently merge with it. The epithelium covering the crown of the tooth stretches and degenerates; Through the resulting hole, the tooth breaks through the tissue and rises above the gum - erupts. In this case, there is no bleeding, since the crown moves through the epithelium-lined canal.

The stage of loss of milk teeth and their replacement with permanent ones. The formation of permanent teeth is formed in the 5th month of embryogenesis as a result of the growth of epithelial cords from the dental plates. Permanent teeth develop very slowly, located next to the milk teeth, separated from them by a bony septum. By the time the baby teeth change (6-7 years), osteoclasts begin to destroy the bone septa and roots of the baby teeth. As a result, baby teeth fall out and are replaced by rapidly growing permanent teeth.

During the eruption of permanent teeth, destruction and loss of temporary teeth occurs, which includes resorption of dental alveoli and tooth roots. As the permanent tooth begins its rapid vertical movement, it puts pressure on the alveolar bone surrounding the primary tooth. As a result of this pressure, in the connective tissue separating the crown of a permanent tooth from the alveoli of a temporary tooth, they differentiate osteoclasts(odontoclasts), which begin to destroy the bone septum separating the socket of the milk and permanent teeth, and the root of the temporary tooth.

Osteoclasts-odontoclasts are located on the surface of the tooth root in lacunae, and destroy the tissues of the tooth root - cement and dentin. The root pulp of a baby tooth is replaced by granulation tissue, rich in blood vessels and osteoclasts and promoting the resorption of the root from the inside and the formation of odontoclasts, which carry out the resorption of predentin and dentin from the pulp side. The processes of root resorption of a temporary tooth lead to the loss of connection between the tooth and the alveolar wall and the pushing of the crown into the oral cavity (usually under the influence of chewing forces).

Plan

PERIODS OF DEVELOPMENT OF MILK TEETH

^ PERIOD OF DENTAL EMPLOYMENTS

DIFFERENTIATION OF DENTAL RUDIA.

HISTOGENESIS OF THE TOOTH

Dentin formation (dentinogenesis)

Clinical significance of dentinogenesis disorders

Enamel formation (enamelogenesis)

Clinical significance of amelogenesis disorders

Formation of cement, development of periodontium and dental pulp

Tissue changes during tooth eruption

^

PERIODS OF DEVELOPMENT OF MILK TEETH

The continuous process of tooth development is divided into three main periods:

• 
  period of formation of tooth germs;
• 
  period of formation and differentiation of tooth germs;
• 
  the period of formation of dental tissues (histogenesis of dental tissues).

^

PERIOD OF DENTAL EMPLOYMENTS

Dental plate. At the 6th week of intrauterine development, the multilayered epithelium lining the oral cavity forms a thickening along the entire length of the upper and lower jaws due to the active proliferation of its cells. This thickening (primary epithelial cord) grows into the mesenchyme, almost immediately dividing into two plates - vestibular and dental. Vestibular plate characterized by rapid proliferation of cells and their immersion into the mesenchyme, followed by partial degeneration in the central areas, as a result of which a gap begins to form ( buccolabial groove), separating the cheeks and lips from the area where future teeth are located and delimiting the actual oral cavity of its vestibule.

^ Dental plate has the shape of an arc or horseshoe, located almost vertically with a slight tilt back. The mitotic activity of mesenchymal cells directly adjacent to the developing dental plate is also enhanced.

^ Formation of bookmarks of enamel organs . At the 8th week of embryonic development, round or oval protrusions (tooth buds) are formed in each jaw on the outer surface of the dental plate (facing the lip or cheek) along the lower edge at ten different points, corresponding to the location of future temporary teeth - the anlage of the enamel organs. These anlages are surrounded by clusters of mesenchymal cells, which carry signals that induce the formation of a dental plate by the oral epithelium, and subsequently the formation of enamel organs from the latter.

^ Formation of tooth germs . In the area of ​​the dental buds, epithelial cells proliferate along the free edge of the dental plate and begin to penetrate the mesenchyme. The growth of the enamel organ primordia occurs unevenly - the epithelium seems to overgrow condensed areas of mesenchyme. As a result, the developing epithelial enamel organ initially takes on the appearance of a “cap”, which covers an accumulation of mesenchymal cells – the dental papilla. The mesenchyme surrounding the enamel organ also condenses to form the dental sac (follicle). The latter subsequently gives rise to a number of tissues of the supporting apparatus of the tooth.

The enamel organ, dental papilla and dental sac together form the tooth germ.

^

DIFFERENTIATION OF DENTAL RUDIA.

As the enamel organ grows, it becomes more voluminous and elongates, acquiring the shape of a “bell,” and the dental papilla that fills its cavity lengthens. At this stage, the enamel organ consists of:

• 
  outer enamel cells (outer enamel epithelium);
• 
  inner enamel cells (inner enamel epithelium);
• 
  intermediate layer;
• 
  pulp of the enamel organ (stellate reticulum).

At this stage, the enamel organ is accompanied by:

• 
  enamel nodule and enamel cord;
• 
  dental papilla;
• 
  dental sac.

^

HISTOGENESIS OF THE TOOTH

Dentin formation (dentinogenesis)

Dentin formation begins in the final stages of the bell stage with the differentiation of peripheral cells of the dental papilla, which turn into odontoblasts, which begin to produce dentin. Deposition of the first dentin layers induces the differentiation of the inner cells of the enamel organ into secretory-active anameloblasts, which begin to produce enamel on top of the resulting dentin layer. At the same time, enameloblasts themselves previously differentiated under the influence of cells of the inner enamel epithelium. Such interactions, as well as the interactions of mesenchyme from the epithelium at earlier stages of tooth development, are examples of reciprocal (mutual) inductive influences.

In the prenatal period, the formation of hard tissues occurs only in the crown of the tooth, while the formation of its root occurs after birth, beginning shortly before eruption and being completely completed (for different temporary teeth) by 1.5 - 4 years.

^ Formation of dentin in the crown of a tooth

Dentin formation (detinogenesis) begins at the apex of the dental papilla. In teeth with several chewing cusps, dentin formation begins independently in each of the areas corresponding to the future tips of the cusps, spreading along the edges of the cusps until the fusion of adjacent centers of dentin formation. The dentin formed in this way forms the crown of the tooth and is called coronal.

Secretion and mineralization of dentin do not occur simultaneously: odontoblasts initially secrete organic base (matrix) dentin ( predentin), and subsequently it is calcified. Predentin on histological preparations appears as a thin strip of oxyphilic material located between the odontoblast layer and the inner enamel epithelium.

During dentinogenesis, it is first produced mantle dentin– outer layer up to 150 microns thick. Further education occurs peripulpal dentin, which makes up the bulk of this tissue and is located inward from the mantle dentin. The processes of formation of mantle and peripulpar dentin have both a number of patterns and a number of features.

^ Formation of mantle dentin. The first collagen, synthesized by odontoblasts and released by them into the extracellular space, has the form of thick fibrils, which are located in the ground substance directly under the basement membrane of the inner enamel epithelium. These fibrils are oriented prependicular to the basement membrane and form bundles called radial Corf fibers . Thick collagen fibers together with amorphous substance form an organic matrix mantle dentin, the layer of which reaches 100-150 microns.

^ Calcification of dentin begins at the end of the 5th month of intrauterine development and is carried out by odontoblasts through their processes. The formation of the organic matrix of dentin precedes its calcification, so its inner layer (predentin) always remains unmineralized. In the mantle dentin, matrix vesicles containing hydroxyapatite crystals appear between the collagen fibrils, surrounded by a membrane. These crystals grow rapidly and, breaking the membranes of the vesicles, grow in the form of crystal aggregates in various directions, merging with other clusters of crystals.

^ Formation of peripulpar dentin occurs after completion of the formation of mantle dentin and differs in some features. Collagen secreted by odontoblasts forms thinner and denser fibrils, which intertwine with each other and are located mainly perpendicular to the course of the dentinal tubules or parallel to the surface of the dental papilla. The fibrils arranged in this way form the so-called Ebner tangential fibers.

The main substance of peripulpal dentin is produced exclusively by odontoblasts, which by this time have already completely completed the formation of intercellular connections and thereby separate predentin from the differentiating dental pulp. The composition of the organic matrix of peripulpal dentin differs from that of mantle dentin due to the secretion by odontoblasts of a number of previously unproduced phospholipids, lipids and phosphoproteins. Calcification of peripulpar dentin occurs without the participation of matrix vesicles.

^ Mineralization of peripulpal dentin occurs by deposition of hydroxyapatite crystals on the surface and inside collagen fibers, as well as between them (without the participation of matrix vesicles) in the form of rounded masses - globules (calcospherites). The latter subsequently increase and merge with each other, forming a homogeneous calcified tissue. This type of calcification is clearly visible in the peripheral areas of peripulpar dentin near mantle dentin, where large globular masses do not completely merge, leaving hypomineralized areas called interglobular dentin . The size of the globules depends on the rate of dentin formation. An increase in the volume of interglobular dentin is characteristic of dentinogenesis disorders associated with calcification defects, for example, due to vitamin D deficiency, calcitonin deficiency, or exposure to elevated fluoride concentrations.

The duration of the period of activity of odontoblasts, which carry out the deposition and mineralization of dentin, is approximately 350 days in temporary teeth, and about 700 days in permanent teeth. These processes are characterized by a certain periodicity, thanks to which it is possible to detect so-called growth lines in dentin. Their appearance is due to small periodic changes in the direction of deposition of collagen fibers. Thus, with an average interval of 4 µm, daily growth lines are revealed; at a distance of about 20 µm more clearly defined Abner's growth lines indicating the existence of cyclical dentin deposition with a period of about 5 days (infradian rhythm). Mineralization of dentin also occurs rhythmically with a period of about 12 hours (ultradian rhythm), independent of the cyclicity of the production of the organic matrix.

^ Formation of peritubular dentin. At the beginning of dentin formation, the dentinal tubules have a significant lumen, which subsequently decreases. This occurs due to deposition from the inside on their walls peritubular dentin, which would be more correctly called intratubular dentin. Peritubular dentin differs from intertubular dentin by its higher content of hydroxyapatite. Its secretion is carried out by processes of odontoblasts located in the dentinal tubules. Mineralization of the secreted organic base of dentin is ensured by calcium transfer in three ways:

• 
  as part of matrix vesicles, which are located along the periphery of the cytoplasm of the processes and are released into the extracellular space;
• 
  by intratubular (dentinal) fluid;
• 
  in chemical connection with phospholipids of the process membrane.

Peritubular dentin is found in small quantities in the teeth of young people; it is absent in interglobular dentin.

^ Formation of dentin at the root of a tooth

The formation of dentin in the root of a tooth proceeds in essentially the same way as in the crown, but it occurs at later stages, starting before and ending after the eruption of the tooth. During the formation of the crown, most of the enamel organ involved in the formation of the crown has already undergone regressive changes. Its components have lost their characteristic differentiation and have become several layers of flattened cells, forming a reduced enamel epithelium that covers the crown of the tooth. The zone of activity of the enamel organ at this stage moves to the region of the cervical loop, where the cells of the inner outer epithelium connect. Hence, due to the proliferation of these cells, a two-layer epithelial cord of cylindrical shape grows into the mesenchyme between the dental papilla and the dental sac - epithelial (Hertwig) root sheath . This vagina gradually descends in the form of an elongating skirt from the epithelial organ to the base of the papilla. Unlike the internal epithelium of the enamel organ, the internal cells of the root sheath do not differentiate into anameloblasts and retain a cubic shape. As the epithelial root sheath encloses the elongating dental papilla, its internal cells induce the differentiation of peripheral papillary cells, which develop into dental root odontoblasts. The inwardly curved edge of the root sheath, called the epithelial diaphragm, encloses the epithelial opening. When the roots of multi-rooted teeth are formed, the initially existing root canal is divided into two or three narrower canals due to the edges of the epithelial diaphragm, which, in the form of two or three tongues, are directed towards each other and ultimately merge together.

After odontoblasts form root dentin along the edge of the epithelial sheath, connective tissue grows into the vaginal epithelium in its various parts. As a result, the root sheath breaks up into numerous small anastomosing cords called epithelial remnants (islets) of Malasse (see lecture “Structure of the periodontium”). While the areas of the epithelial sheath closest to the crown undergo decay, the apical areas continue to grow into the connective tissue, inducing odontoblast differentiation and determining the shape of the tooth root. The epithelial remains of Malasse, which, along with the material of the decayed root sheath, also include the remains of the dental plate, can play an important role in pathology, since they can serve as centers for the formation of cementicles and a source of the development of cysts and tumors ( see lecture “Structure of the periodontium”).

During root formation, the growing edge of the epithelial sheath may encounter a blood vessel or nerve along its path. In this case, it grows along the edges of these structures, and in the area of ​​their location, the peripheral cells of the dental papilla do not come into contact with the inner layer of the epithelial vagina. For this reason, they do not turn into odontoblasts and, in this area of ​​the root, there will be a dentin defect - accessory (lateral) root canal , connecting the pulp with the periodontal connective tissue surrounding the tooth. Such channels can serve as routes for the spread of infection. In some cases, individual internal cells of the epithelial root sheath, in contact with dentin, are able to differentiate into anameloblasts, which will produce small droplets of enamel associated with the root surface or located in the periodontium (“enamel pearls”) .

Root dentin differs from coronal dentin in the chemical composition of some organic components, a lower degree of mineralization, the absence of a strict orientation of collagen fibers, and a lower rate of deposition.

The final formation of root dentin is completed only after teeth eruption, in temporary teeth after approximately 1.5-2 years, and in permanent teeth, on average, after 2-3 years from the start of eruption.

In general, dentin formation continues until the teeth acquire their final anatomical shape; such dentin is called primary, or physiological. The slower formation of dentin in a fully formed tooth (secondary dentin) continues throughout life and leads to a progressive reduction in the pulp chamber. Secondary dentin contains lower concentrations of glycosaminoglycans and is characterized by weaker mineralization than primary dentin. A distinct line of rest can be identified between primary and secondary dentin. Tertiary dentin, or reparative dentin, is deposited in specific areas in response to damage to the tooth. The rate of its deposition depends on the degree of damage: the more significant the damage, the higher it is (reaches 3.5 µm/day).

^

Clinical significance of dentinogenesis disorders

Disruption of dentinogenesis can occur during the formation of its organic matrix, during mineralization, or at both of these stages. Matrix abnormalities are characteristic of a hereditary disease called dentinogenesis imperfecta. In this disease, the structure of the enamel is not changed, but its connection with dentin is fragile, as a result of which the enamel breaks off. When calcification is disturbed, calcospherites are revealed that do not merge with each other, leaving very large zones of interglobular dentin.

^

Enamel formation (enamelogenesis)

Enamel is a secretory product of the epithelium, and its formation differs significantly from the development of all other hard tissues of the body, which are derivatives of mesenchyme. Amelogenesis occurs in three stages:

• 
  stage of secretion and primary mineralization of enamel;
• 
  stage of maturation (stage of secondary mineralization) of enamel;
• 
  stage of final maturation (stage of tertiary mineralization) of enamel

During the first of them is the stage of secretion and primary mineralization of enamel– enameloblasts secrete the organic basis of the enamel, which almost immediately undergoes primary mineralization. However, the enamel thus formed is a relatively soft tissue and contains a lot of organic matter. During the second stage of amelogenesis – the stage of maturation (secondary mineralization) of enamel it undergoes further calcification, which occurs not only as a result of the additional inclusion of mineral salts in its composition, but also by removing most of the organic matrix. The third stage of anamelogenesis is the stage of final maturation(tertiary mineralization) of enamel occurs after tooth eruption and is characterized by the completion of enamel mineralization mainly through the entry of ions from saliva.

Enameloblasts

Cells that form enamel - enameloblasts arise due to the transformation of pre-enameloblasts, which in turn differentiate from the cells of the inner enamel epithelium. The differentiation of enameloblasts at the beginning of amelogenesis is preceded by changes in the enamel organ, affecting all its layers. The cells of the outer enamel epithelium turn from cubic to flat. The general shape of the enamel organ also changes - its smooth outer surface becomes uneven, scalloped due to the pressing into it in many areas of the surrounding mesenchyme of the dental sac and capillary loops. In this case, the surface area of ​​​​contact between the mesenchyme and the outer epithelium increases, the capillaries growing from the side of the mesenchyme approach the inner enamel epithelium, and the pulp of the enamel organ separating them decreases in volume. These changes contribute to increased nutrition of the layer of differentiating enameloblasts from the side of the dental sac. This compensates for the cessation of the supply of metabolites to them from the dental papilla, which previously served as the main source of nutrition for preenameloblasts, and is now cut off from them due to the deposition of a dentin layer between them. At the same time, a change in polarity occurs in the epithelial cells of the internal enamel organ, as a result of which the basal and apical poles change their places. The Golgi complex and the centrioles of preenameloblasts, located at the pole facing the intermediate layer (previously apical), are displaced to the opposite pole of the cell (which now becomes apical). Mitochondria, which were initially diffusely scattered throughout the cytoplasm, are concentrated in the region formerly occupied by the Golgi complex and becoming the basal part of the cell.

Enameloblasts differentiate only 24-36 hours after the completion of functional maturation of the adjacent odontoblasts. The final signal for this process is the beginning of the formation of predetin, in particular, its collagen and (or) proteoglycans. This explains why amelogenesis always lags behind dentinogenesis. For the same reason, the first secretory-active anameloblasts are formed where dentin deposition begins - in the area of ​​​​the future cutting edge of the crown of the anterior or chewing cusps of the posterior ones. From here, the wave of enameloblast differentiation spreads towards the edge of the enamel organ to the cervical loop. The connection between the differentiation of enameloblasts and the formation of dentin serves as another example of mutual induction, since the induction of odontoblast development was carried out by the internal cells of the enamel organ.

The secretory-active odontoblast is a tall prismatic cell (length to width ratio up to 10:1) with highly differentiated cytoplasm. The apical part contains the large Golgi complex, cisterns of the granular endoplasmic reticulum, and mitochondria. Polarization is accompanied by a reorganization of the cytoskeleton and ends with the appearance of Toms' process in their apical part. Functionally, the differentiation of preenameloblasts into enameloblasts is accompanied by inhibition of the ability to synthesize glycosaminoglycans and type IV collagen (a component of the basement membrane) and the emergence of the ability to synthesize specific enamel proteins - enamelins And amelogenins .

Secretion and primary mineralization of enamel

Secretion of enamel by enameloblasts begins with the release of organic matter between dentin and the apical surface of enameloblasts in the form of a continuous layer 5-15 microns thick, in which calcification processes occur very quickly due to the deposition of hydroxyapatite crystals. In this case, a layer is formed initial enamel . Enamel deposition begins in the area of ​​the future cutting edge of the front teeth and the chewing tubercles of the rear teeth, spreading towards the neck.

A feature of enamel that distinguishes it from dentin, cement and bone is that its mineralization occurs very quickly after secretion - the time period separating these processes is only minutes. Therefore, when enamel is deposited, it has virtually no non-mineralized precursor (pre-enamel). Enamel mineralization is a two-step process involving mineralization and subsequent crystal growth.

Enameloblasts control the transport of inorganic ions from the capillaries of the dental sac to the enamel surface. An important role in the mineralization of enamel is played by proteins produced by enameloblasts, which perform a number of functions:

• 
  participate in the binding of Ca 2+ ions and regulation of their transport by secretory enameloblasts;
• 
  create initial sites of nucleation (initiation) during the formation of hydroxyapatite crystals;
• 
  promote the orientation of growing hydroxyapatite crystals;
• 
  form an environment that ensures the formation of large hydroxyapatite crystals and their dense placement in the enamel.

Enamel proteins are non-collagenous, which also distinguishes enamel from other calcified human tissues. The main proteins during its secretion are amelogenins , making up 90% of the proteins secreted by enameloblasts. Amelogenins are hydrophobic proteins. They contain a large amount proline, glutamine And histidine and are formed due to the cleavage of a secreted large glycoprotein molecule. Amelogenins are mobile and not associated with crystals. It is believed that they are modified and migrate along the enamel, participating in the regulation of crystal growth in length, width and thickness. To continue the growth of crystals after formation, some of the proteins must be removed. This is achieved in two ways:

• 
  due to the pressure created by the growing crystals, amelogenins are forced out of the space between the crystals towards the enameloblasts;
• 
  Some of the proteins remaining between the rapidly growing crystals are cleaved to low molecular weight substances due to the action of proteolytic enzymes secreted by anameloblasts.

The second group of proteins found in enamel are enamelines , which bind to hydroxyapatite crystals and are characterized by a high content glutamine, aspartic acid And serine. Enamelins are probably not an independent secretory product, but the result of polymerization of the digestion products of amelogenins.

In the initial enamel, small hydroxyapatite crystals are arranged randomly (mainly perpendicular to the dentin surface) and interdigitate with dentin crystals. According to some authors, denine crystals are nucleation (initiation) sites for the formation of crystals in enamel.

After deposition of the first layer of initial (non-prismatic) enamel, enameloblasts move away from the dentin surface and form shoots Toms , which serves as a sign of the complete completion of their functional differentiation. Although the cytoplasm of the enameloblast directly passes into the cytoplasm of the process, their conditional boundary is considered to be the level of the apical complex of intercellular junctions. The cytoplasm of the cell body contains mainly organelles of the synthetic apparatus, and the cytoplasm of the process contains secretory granules and small vesicles.

Subsequent portions of the resulting enamel fill the intercellular spaces between the Toms processes. This enamel is secreted by the peripheral portions of enameloblasts at the base of their processes at the level of the apical junctional complexes. In the future, it will turn into interprismatic enamel. As a result, a cellular structure appears in the form of a honeycomb, the walls of which are formed by future interprismatic enamel, and inside each cell there is a Toms’ process. Once formed, such a cellular structure will determine the nature of the enamel structure, including the shape, size and orientation of the enamel prisms that will be formed by Toms' processes and fill the holes in the cells. Thus, interprismatic enamel has an initial organizing influence on the structure of the entire resulting enamel.

There is disagreement on the issue of the mechanisms of formation of enamel prisms and the fate of Toms' process. The most common idea is that secretory-active anameloblasts, together with their processes, are constantly pushed back by the newly formed enamel to its periphery. The displacement occurs at an angle to the dentinal-enamel boundary. According to other views, the process remains in place and is compressed by the growing prism. In this case, during enamelogenesis, the part of the process that is more distant from the cell body continuously dies, and the part located near the cell body grows.

With an arched configuration of enamel prisms, each of them is formed by more than one enameloblast; in fact, four cells take part in its formation, with one of them forming the “head” of the prism, and the other three together forming the “tail” (interprismatic enamel). In turn, each enameloblast participates in the formation of four prisms: it forms the “head” of one prism and the “tails” of the other four.

The orientation of crystals in the resulting prisms differs from that in the interprismatic areas. In prisms, especially in its central sections, most of the crystals are located parallel to their axis, and in the peripheral sections they deviate from it. In the interprismatic areas, the crystals lie at right angles to the crystals in the central part of the prism.

The growth of enamel prisms occurs cyclically, as a result of which, on each of them, with an interval of 4 microns, transverse striations are detected, corresponding to the 24-frequency rhythm of secretion and mineralization of enamel. During the formation of enamel, a slower (about a week) rhythm of its deposition is also noted, which is manifested by the appearance of enamel growth lines (Retzius lines). On longitudinal sections they are visible as brown lines running obliquely from the surface of the enamel to the dentin-enamel boundary, on transverse sections they are visible as concentric circles corresponding to the fronts of enamel deposition. These lines are associated with the periodicity of calcification (according to other sources, the formation of an organic matrix) of the enamel. According to the latest data, the appearance of Retzius lines is associated with periodic bending of the enamel prisms due to compression of the Thoms processes, combined with an increase in the secretory surface forming the interprismatic enamel.

Enamel proteins are found in all areas of newly formed enamel, however, as it matures, their highest concentration remains in the peripheral layer of enamel prisms, traditionally called shell. This is due to the fact that hydroxyapatite crystals in the shells are located at different angles, as a result of which they are not packed tightly, and the proteins filling the spaces between them are not completely removed. Thus, the shells are not independent formations, but only peripheral sections of the enamel prisms themselves with a less ordered arrangement of crystals and an increased content of proteins.

The formation of enamel in the form of enamel prisms begins at the initial enamel (near the dentin surface) and is pumped up at the outer surface of the enamel, where a layer is formed ultimate enamels . In its structure, the final enamel is similar to the initial one and also does not contain prisms.

During amelogenesis, the cells of the outer enamel epithelium, the pulp of the enamel organ and the intermediate layers lose their individual morphological characteristics and form a single layer of multilayered epithelium adjacent to the enameloblasts.

^ Maturation (secondary mineralization) of enamel

Enamel formed by secretory enamaloblasts and subjected to primary mineralization , is immature . It consists of 70% mineral salts and 30% organic matrix. This enamel has the consistency of cartilage and is unable to perform its function. It persists after decalcification and is therefore clearly visible on histological preparations. The only area of ​​more mineralized enamel is its innermost layer. Its thickness is several micrometers (initial enamel).

Mature enamel 95% is formed by mineral salts and 1.2% by organic substances. Almost all of it consists of densely spaced crystals of hydroxyapatite. The organic (protein) matrix of enamel has the form of a three-dimensional network of fibrillar structures about 8 nm thick, connected to each other and to hydroxyapatite crystals. During decalcification, the enamel almost completely dissolves and, therefore, on histological sections, empty spaces correspond to its location.

In progress maturation (secondary mineralization ) enamels , occurring upon completion of its secretion and primary mineralization, the content of mineral salts in it increases significantly, which leads to a sharp increase in its hardness. This is accomplished by the influx and inclusion of mineral salts into the enamel while simultaneously removing organic compounds (mainly proteins) and water from it. The maturation of enamel, as well as its secretion, begins along the cutting edge of the front teeth and on the chewing cusps of the rear teeth, spreading towards the neck of the tooth.

As a result of the maturation process, the highest level of enamel mineralization is achieved in its surface layer, and in the direction of the dentin-enamel boundary it decreases down to the innermost layer of the initial enamel, which is also characterized by an increased mineral content.

Secondary mineralization of enamel is ensured due to the active activity of enameloblasts ( enameloblasts stage of maturation ), which are formed as a result of structural and functional transformations secretion stage enameloblasts (secretory-active enameloblasts) (check!) who have completed their activities. The last product of the synthesis of secretory-active enameloblasts is a material that forms a structure similar to the basement membrane. This material is deposited on the surface of the enamel and serves as an attachment site for hemidesmosomes of enameloblasts. (primary cuticle of enamel, or Nasmyth's shell) . Upon completion of enamel secretion, enameloblasts undergo a short transition phase, during which they shorten, lose Thoms' processes, and are included in the process of enamel maturation. Excess organelles involved in secretion processes undergo autophagy and are digested by lysosomal enzymes. Some enamaleblasts die by apoptosis and are phagocytosed by neighboring cells.

The cyclical nature of the enamel maturation process is reflected in the morphological characteristics of enameloblasts. Among the latter, two types of cells are found that are capable of mutual transformations.

Enameloblasts type 1 characterized by the appearance of a striated edge on the apical surface. Their basal (remote from the enamel) complexes of intercellular junctions have significant permeability, and their apical (adjacent to the enameloblasts) have a high density. It has been established that these cells participate predominantly in the active transport of inorganic ions, which are transported through their cytoplasm and released on the apical surface. They have a very high concentration of calcium-binding proteins. Absorption of breakdown products of enamel proteins also occurs through the striated edge.

Enameloblasts of the second type have a smooth apical surface. Their basal junction complexes are impermeable, while their apical complexes are highly permeable. These cells take a major part in removing organic substances and water from the enamel. The molecules of these substances easily penetrate into the intercellular space at the apical ends of cells and are then transported by vesicles formed on their lateral surfaces.

After enamel maturation is completed, the layer of enameloblasts and the adjacent epithelial layer (formed by the outer enamel epithelium, collapsed pulp and the intermediate layer of the enamel organ) together form reduced dental epithelium (secondary enamel cuticle), which covers the enamel and plays a protective role, especially significant before tooth eruption.

^ Final maturation (tertiary mineralization) of enamel

The maturation of enamel, associated with an increase in the content of mineral substances in it, is not completely completed in the formed crown of an unerupted tooth. The final maturation of enamel occurs after tooth eruption, especially intensively during the first year that the crown is in the oral cavity. The main source of inorganic substances entering the enamel is saliva, although some of them can come from the dentin. In this regard, the mineral composition of saliva, including the presence in it of the required amount of ions, calcium, and fluorine phosphorus, is of particular importance for the complete mineralization of enamel during this period. The latter are included in the hydroxyapatite crystals of the enamel and increase its acid resistance. Subsequently, throughout life, enamel participates in ion exchange, undergoing processes of demineralization (removal of minerals) and remineralization (intake of minerals), balanced under physiological conditions.

^

Clinical significance of amelogenesis disorders

Enamaloblasts are sensitive to external influences, which lead to deviations in the normal course of amelogenesis. Even small impacts can manifest themselves as morphologically noticeable changes in the composition of the enamel and its quantity. More significant lesions can lead to profound disturbances in enamelogenesis and even death of enameloblasts.

If the impact of a damaging factor occurs during the period of enamel secretion, then the amount of enamel formed (the thickness of its layer) in this area decreases. This violation is called hypoplasia enamel, or its underdevelopment.

If the impact occurs during the period of enamel maturation, its mineralization is disrupted to a greater or lesser extent. This condition is called hypocalcification enamels. At the same time, enamel with a reduced content of mineral substances is easily subject to decalcification and caries.

Hypoplasia and hypocalcification of enamel can affect one, several teeth, or all teeth. In these cases, the causes of the disorder are local, systemic or hereditary in nature, respectively. The most common systemic factors are endocrinopathies, diseases accompanied by febrile conditions, nutritional disorders and the toxic effects of certain substances.

Local enamel hypoplasia may affect one tooth or part of it. It is usually caused by local disorders, such as trauma, osteomyelitis. In a permanent tooth, it can be caused by a periapical infection of the corresponding primary tooth.

Systemic enamel hypoplasia develops in various infectious diseases and metabolic disorders, covering several teeth in which enamel formation occurred during the disease. Upon recovery, the normal process of amelogenesis resumes. As a result, stripes of hypoplastic enamel alternating with normal enamel are clinically visible on the teeth. If the normal development of enamel is interrupted several times due to metabolic disorders, multiple enamel hypoplasia occurs.

Enamel defects can be caused by taking tetracycline antibiotics. Tetracyclines are incorporated into calcifying tissues, leading to enamel hypoplasia and brown pigmentation. The degree of enamel damage depends on the dosage of the antibiotic and the duration of its use.

Hereditary (congenital) enamel hypoplasia, or amalogenesis imperfecta , affects all teeth (both temporary and permanent), in which the entire crown is affected. Since the thickness of the enamel decreases sharply, the teeth have a yellow-brown color. Amalogenesis imperfecta can be combined with dentinogenesis imperfecta.

Local enamel hypocalcification , as a rule, is caused by local disturbances. Systemic hypocalcification covers all teeth in which the action of a damaging factor occurred during the period of enamel maturation. The most common example of such a disorder would be abnormal calcification of enamel when the fluoride content in drinking water increases (5 or more times its concentration in fluoridated water), leading to the development of a disease called fluorosis. It is characterized by the formation of so-called “moth-eaten” enamel, in which multiple areas of hypomineralization are found.

Congenital hypocalcification of enamel – a hereditary disease in which irregularities are detected in all teeth. Immediately after eruption, the crown has a normal shape, but the enamel is soft, dull in color, and quickly wears off or separates in layers.

^

Formation of cement, development of periodontium and dental pulp

Formation of cement (cementogenesis)

During tooth root formation, dentin is deposited in the inner surface of the epithelial (Hertwig's) root sheath, which separates the dental papilla from the dental sac. During dentinogenesis, the root sheath breaks up into separate fragments (epithelial remnants of Malasse), as a result of which poorly differentiated connective tissue cells of the dental sac come into contact with dentin and differentiate into cementoblasts - cells that form cement. Cementoblasts are cubic cells with a high content of mitochondria, a large Golgi complex, and a well-developed hydroelectric power station.

Cementoblasts begin to produce an organic matrix (cementoid), which consists of collagen fibers and ground substance. Cementoid is deposited on top of the root dentin and around the fiber bundles of the developing periodontium. According to some information, however, the deposition of cementoid does not occur directly on the surface of the mantle dentin, but on top of a special highly mineralized structureless layer ( Hopewell-Smith hyaline layer) 10 µm thick, covering the root dentin and formed, presumably, by the cells of the epithelial root sheath before its disintegration. This layer probably contributes to the strong attachment of cementum to dentin and periodontal ligament fibers to cementum.

The second phase of cement formation involves the mineralization of the cementoid by the deposition of hydroxyapatite crystals into it. Crystals are deposited first in matrix vesicles, followed by mineralization of collagen fibrils of cement. Cementum deposition is a rhythmic process in which the formation of a new cementoid layer is combined with the calcification of a previously formed layer. The outer surface of the cementoid is covered with cementoblasts. Between them, connective tissue fibers of the periodontium, consisting of numerous collagen fibers, called Sharpey's fibrils, are woven into the cement.

As cement forms, cementoblasts either move to its periphery or become immured in it, settling in lacunae and turning into cementocytes . The first to form is cementum, which does not contain cells ( acellular , or primary ), it is slowly deposited as the tooth erupts, covering 2/3 of the surface of its root closest to the crown.

After tooth eruption, cementum containing cells ( cellular , or secondary ). Cell cement is located in the apical 1/3 of the root. Its formation occurs faster than acellular cement; in terms of the degree of mineralization, it is inferior to it. The matrix of cellular cement contains internal (intrinsic) collagen fibers formed by cementoblasts, and external (external) fibers penetrating into it from the periodontium. External fibers penetrate the cement at an angle to its surface, and their own fibers are located along the surface of the root, weaving a network of external fibers. The formation of secondary cement is a continuous process, as a result of which the cement layer thickens with age. Secondary cement is involved in the adaptation of the supporting apparatus of the tooth to changing loads and in reparative processes.

^ Periodontal development

The periodontium develops from the dental sac soon after the formation of the tooth root begins. The cells of the pouch proliferate and differentiate into fibroblasts, which begin to form collagen fibers and ground substance. Already at the earliest stages of periodontal development, its cells are located at an angle to the tooth surface, as a result of which the resulting fibers also acquire an oblique course. According to some reports, the development of periodontal fibers occurs from two sources - from the cementum and from the alveolar bone. The growth of fibers from the first source begins earlier and occurs rather slowly, with only some fibers reaching the middle of the periodontal space. The fibers growing from the side of the alveolar bone are thick, branched and, in terms of their growth rate, are significantly ahead of the fibers growing from the cement; they meet with them and form a plexus.

Before the tooth erupts, its cemento-enamel boundary is located significantly deeper than the crest of the developing dental alveolus, then, as the root forms and the tooth erupts, it reaches the same level, and in a fully erupted tooth it becomes higher than the crest of the alveolus. In this case, the fibers of the developing periodontium associated with the ridge, following the movement of the root, are first located obliquely (at an acute angle to the alveolar wall), then occupy a horizontal position (at a right angle to the alveolar wall) and ultimately again take an oblique direction (at an obtuse angle). angle to the alveolar wall). The main groups of periodontal fibers are formed in a certain sequence.

The thickness of the periodontal fiber bundles increases only after the tooth erupts and begins to function. Subsequently, throughout life, there is a constant restructuring of the periodontium in accordance with changing load conditions.

^ Dental pulp development

The pulp develops from the dental papilla, formed by ectomesenchyme. The papilla initially consists of branched mesenchymal cells separated by large spaces. The process of differentiation of the papilla mesenchyme begins in the region of its apex, from where it further spreads to the base. The vessels begin to grow into the papilla even before the appearance of the first odontoblasts; nerve fibers, however, grow into the papilla relatively late - with the beginning of dentin formation.

The cells of the peripheral layer of the papilla, adjacent to the inner enamel epithelium, turn into preodontoblasts. And later - odontoblasts, which begin to form dentin. The course of odontoblast differentiation is described above. In the central areas of the pulp, the mesenchyme gradually differentiates into loose, unformed connective tissue. Most of the mesenchymal cells turn into fibroblasts, which begin to secrete components of the intercellular substance. The latter accumulates collagen types I and III. Despite the progressive increase in collagen content in the developing pulp, the ratio between collagen types I and III remains unchanged, and type III collagen is present in the pulp in an unusually high concentration for connective tissue. Collagen is first detected in the form of isolated fibrils, lying without strict orientation; later the fibrils form fibers that fold into bundles. As the pulp matures, its glycosamyoglycan content decreases.

At the same time, active proliferation of blood vessels occurs in the connective tissue of the pulp. Larger arterioles and venules are located in the center of the developing dental pulp; an extensive capillary network develops at the periphery, including both fenestrated capillaries and capillaries with a continuous vascular wall. The development of blood vessels is combined with the proliferation of nerve fibers and the formation of their networks.

^

Tissue changes during tooth eruption

Once crown formation is complete, the developing tooth undergoes small movements in conjunction with jaw growth. During the process of eruption, the tooth travels a considerable distance in the jaw. Moreover, its migration is accompanied by changes, the main of which are:

• 
  tooth root development;
• 
  periodontal development;
• 
  alveolar bone remodeling;
• 
  changes in the tissues covering the erupting tooth.

Tooth root development associated with the ingrowth of the epithelial root sheath into the mesenchyme of the dental papilla, extending from the cervical loop of the enamel organ. Vaginal cells induce the development of root odontoblasts, which produce its dentin. As the sheath is destroyed, the mesenchymal cells of the dental sac differentiate into cementoblasts, which begin to deposit cementum on top of the root dentin.

^ Periodontal development includes the growth of its fibers from the cementum and dental alveoli and becomes more intense immediately before tooth eruption.

Alveolar bone remodeling combines rapid deposition of bone tissue in some areas with its active resorption in others. The localization of changes in the alveolar bone and their severity varies at different times and is not the same in different teeth. When a tooth root is formed, it reaches the bottom of the bone cell and causes resorption of bone tissue, resulting in freeing up space for the final formation of the root end. Bone deposition usually manifests itself as the formation of bony trabeculae separated by wide spaces.

In multi-rooted teeth, bone deposition occurs most intensively in the area of ​​the future interradicular septum. In premolars and molars, such areas are the bottom and distal wall of the socket (which indicates their additional medial displacement during axial movement during eruption). In incisors, areas of increased deposition of bone beams are the bottom and lingual surface of the socket (which indicates their subsequent displacement towards the lips during eruption). Bone deposition occurs in those areas of the bone socket from which the tooth is displaced, and resection occurs in those areas towards which the tooth migrates. Resorption of bone tissue frees up space for the growing tooth and reduces resistance to its movement.

LITERATURE

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   Danilevsky N.F., Lenontiev V.K., Nesin A.F., Rakhniy Zh.I. Diseases of the oral mucosa Publisher: OJSC "Dentistry" -: 2007- 271 p.: Ch. 1. Oral cavity - concept, features of structure, function and processes; Ch. 2 Histological structure of the oral mucosa