C. Cementogenesis
1. Development of afibrillar coronal cementum (human teeth)
In human teeth, coronal cementum is formed on the cervical portion of the crown. Its presence is restricted to localized areas of reduced enamel epithelium degeneration. The enamel exposed as a result of epithelial degeneration provides a surface on which cementoblasts from the dental follicle are able to deposit cementum. In humans, coronal cementum serves no anchoring function. In histological sections it may appear as an "island" of cementum on the cervical enamel surface or as a "spur" of cementum continuous with radicular cementum and overlapping the cervical enamel. Generally, coronal cementum in humans is acellular and afibrillar, although cellular coronal cementum has been reported in association with impacted teeth.
In other mammalian species (e.g. horses, cows, sheep, rabbits) coronal cementum is mostly fibrillar and/or cellular, and serves an anchoring function. In these species, most of the enamel of the crown is covered by a well-developed layer of coronal cementum that provides the major tooth support.
Fig. 82: Diagram of unerupted human tooth. The enamel (E) of the crown is almost
entirely covered with a reduced enamel epithelial layer (REE) with the exception of patchy
areas near the cervical region where the enamel is exposed to the surrounding cells of the
dental follicle.
Fig. 83: The reduced enamel epithelium (REE) no longer covers a denuded enamel patch
in the cervical region. The denuded enamel surface provides a suitable site for the
deposition of coronal cementum. This diagram represents an area similar to area 3 in
Fig. 82.
Fig. 84: Diagram of coronal cementum (A) that has been deposited in an area of exposed cervical enamel. The inset shows the internal basal lamina and hemidesmosomes that attach the reduced enamel epithelium to the enamel. When the reduced enamel epithelium is replaced by junctional epithelium, the latter can migrate apically and attach to the coronal and radicular cementum in a similar fashion. Note that the radicular cementum has partially covered the apical portion of the coronal cementum.
Fig. 85 (From Schroeder,
H.E. and Listgarten, M.A., 1977): Diagram of the cervical region of a partially
erupted premolar. Small patches of afibrillar coronal cementum have formed over
areas of the enamel that were exposed to the connective tissue as a result of focal
degeneration of the reduced enamel epithelium.
