As The Dental Cap Grows And The Indentation Deepens The Tooth Takes On The Appearance Of A Bell Bell Stage Fig. 15.31 C .

The shape of the face is determined not only by expansion of the paranasal sinuses but also by growth of the mandible and maxilla to accommodate the teeth. Teeth themselves arise from an epithelial-mesenchymal interaction between overlying oral epithelium and underlying mesenchyme derived from neural crest cells. By the sixth week of development, the basal layer of the epithelial lining of the oral cavity forms a C-shaped structure, the dental lamina, along the length of the upper and lower jaws. This lamina subsequently gives rise to a number of dental buds (Fig. 15.31 A), 10 in each jaw, which form the primordia of the ectodermal components of the teeth. Soon the deep surface of the buds invaginates, resulting in the cap stage of tooth development (Fig. 15.31 B). Such a cap consists of an outer layer, the outer dental epithelium, an inner layer, the inner dental epithelium, and a central core of loosely woven tissue, the stellate reticulum. The mesenchyme, which originates in the neural crest in the indentation, forms the dental papilla (Fig. 15.31 B).

As the dental cap grows and the indentation deepens, the tooth takes on the appearance of a bell (bell stage) (Fig. 15.31 C). Mesenchyme cells of the papilla adjacent to the inner dental layer differentiate into odontoblasts, which later produce dentin. With thickening of the dentin layer, odontoblasts retreat into the dental papilla, leaving a thin cytoplasmic process (dental process) behind in the dentin (Fig. 15.31 D). The odontoblast layer persists throughout the life of the tooth and continuously provides predentin. The remaining cells of the dental papilla form the pulp of the tooth (Fig. 15.31 D).

Dental lamina

Dental lamina

Budding OdontoblastEnamel Prisms
Figure 15.31 Formation of the tooth at successive stages of development. A. Bud stage; 8 weeks. B. Cap stage; 1 0 weeks. C. Bell stage; 3 months. D. 6 months.

In the meantime, epithelial cells of the inner dental epithelium differentiate into ameloblasts (enamel formers). These cells produce long enamel prisms that are deposited over the dentin (Fig. 15.31D). Furthermore, a cluster of these cells in the inner dental epithelium forms the enamel knot that regulates early tooth development (Fig. 15.315).

Enamel is first laid down at the apex of the tooth and from here spreads toward the neck. When the enamel thickens, the ameloblasts retreat into the stellate reticulum. Here they regress, temporarily leaving a thin membrane (dental cuticle) on the surface of the enamel. After eruption of the tooth, this membrane gradually sloughs off.

Formation of the root of the tooth begins when the dental epithelial layers penetrate into the underlying mesenchyme and form the epithelial root sheath (Fig. 15.31 D). Cells of the dental papilla lay down a layer of dentin continuous with that of the crown (Fig. 15.32). As more and more dentin is deposited, the pulp chamber narrows and finally forms a canal containing blood vessels and nerves of the tooth.

Mesenchymal cells on the outside of the tooth and in contact with dentin of the root differentiate into cementoblasts (Fig. 15.32 A). These cells produce a thin layer of specialized bone, the cementum. Outside of the cement layer, mesenchyme gives rise to the periodontal ligament (Fig. 15.32), which holds the tooth firmly in position and functions as a shock absorber.

With further lengthening of the root, the crown is gradually pushed through the overlying tissue layers into the oral cavity (Fig. 15.325). The eruption of deciduous or milk teeth occurs 6 to 24 months after birth.

Buds for the permanent teeth, which lie on the lingual aspect of the milk teeth, are formed during the third month of development. These buds remain dormant until approximately the sixth year of postnatal life (Fig. 15.33). Then

Bone Indentation Teeth

Enamel

- Cementoblasts

Cementum

Enamel

Dentin

- Cementoblasts

Cementum

Dentin

Cementoblasts Shape

Periodontal ligament

Bone of socket

Figure 15.32 The tooth just before birth (A) and after eruption (B).

Periodontal ligament

Bone of socket

Figure 15.32 The tooth just before birth (A) and after eruption (B).

Milk Teeth Scan
Figure 15.33 Replacement of deciduous teeth by permanent teeth in a child of 8 or 9 years.

they begin to grow, pushing against the underside of the milk teeth and aiding in the shedding of them. As a permanent tooth grows, the root of the overlying deciduous tooth is resorbed by osteoclasts.

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Responses

  • Medhanie
    How dental papillary shapes?
    8 years ago

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