77. Development of The Teeth

  1. Introduction: the teeth develop from ectoderm and mesoderm: the enamel develops from ectoderm of the oral cavity, and all other tissues come from the associated mesenchyme. Not all teeth develop at the same time. The first tooth buds are seen in the anterior mandibular region, later in the anterior maxillary region, then posteriorly in both jaws. Development is in continuous stages
    1. THE DENTAL LAMINA AND BUD STAGE: the dental laminae are seen early in week 6 as U-shaped thickenings or buds of the oral epithelium (surface ectoderm)
      1. Localized proliferation of cells in the dental laminae forms round or oval swellings, the tooth buds, which grow into the mesenchyme
      2. The tooth buds develop into the deciduous or milk teeth (shed during childhood). There are 10 tooth buds in each jaw, one for each tooth
      3. The tooth buds for the permanent teeth, with deciduous predecessors, are seen in the 10-week fetus, developing from deeper continuations of the dental lamin They lie on the tongue or lingual side of the deciduous buds
      4. Tooth buds for the permanent teeth appear at different ages during the fetal period except for the second and third permanent molars, which appear after birth, at about 4 months and 5 years, respectively
      5. The permanent molars with no deciduous predecessors develop as buds from backward extensions of the dental laminae
    2. CAP STAGE OF DEVELOPMENT: the deep surface of each ectodermal tooth bud becomes invaginated by mesenchyme called the dental papilla, which gives rise to the dentin and dental pulp. The ectodermal, cap-shaped covering over the papilla is called an enamel organ since it will produce the future enamel of the tooth
      1. The outer cellular layer of the ectodermal enamel organ is called the outer enamel epithelium; the inner layer lining the "cap" is the inner enamel epithelium
        1. The cell region between the above layers forms the core or bulk of the cap and is called the stellate or enamel reticulum
      2. As the enamel organ and dental papilla form, the surrounding mesenchyme condenses as the dental sac, which later forms the cementum and periodontal ligament
    3. THE BELL STAGE: with invagination of the enamel organ, the tooth assumes a bell shape
      1. The mesenchymal cells in the dental papilla, adjacent to the inner enamel epithelium, differentiate into odontoblasts, which produce predentin, and deposit it adjacent to the inner enamel epithelium. The predentin later calcifies to form dentin
      2. As the dentin thickens, the odontoblasts regress toward the center of the dental papilla but odontoblastic processes remain embedded in the dentin and are called Tomes' dentinal fibers or processes
      3. Cells of the inner enamel epithelium near the dentin form ameloblasts, which produce enamel in the form of prisms or rods over the dentin layer, thus help form the outer layer of the tooth or the crown. As enamel increases, the ameloblasts regress
        1. Thus, both enamel and dentin help create the crown, which begins formation at the cusp or tip of the tooth and progresses, in development, to the future root
      4. The root begins after the enamel and dentin are well along in development
        1. The inner and outer enamel epithelia come together in the neck region and form an epithelial fold, the epithelial root sheath, which grows into the mesenchyme and begins the formation of the root
        2. The odontoblasts near the sheath form the dentin (continuous with that of the crown). As the dentin increases, the pulp cavity gets smaller and becomes a narrow canal for the vessels and nerves to enter the root
      5. The inner cells of the dental sac form cementoblasts which produce cementum, which is deposited over the root dentin and meets the enamel at the neck of the tooth
      6. As the teeth develop, the jaws ossify and the outer cells of the dental sac also become active in bone formation. Each tooth is soon surrounded by bone, except at its crown, and is held in its bony socket or alveolus by the periodontal ligament

development of the teeth: image #1