b. Formation of the reduced enamel epithelium
Following completion of the secretory phase of amelogenesis, the ameloblasts undergo a
functional and structural change as they enter the maturation stage of amelogenesis.
During this phase they remove proteins as well as water from the maturing enamel, allowing
the latter to achieve complete mineralization. Once the enamel is completely mineralized,
the ameloblasts shrink from columnar to cuboidal or flattened cells, but remain a part of
the reduced enamel epithelium that
forms a more or less continuous lining over the completed enamel.
The reduced enamel epithelium consists of two major
cellular layers: (1) the reduced ameloblasts that are in contact with the enamel, but are
no longer able to undergo cell division, and (2) the external cells of the reduced enamel
epithelium that consist mostly of stratum intermedium cells, but possibly cellular
remnants of the stellate reticulum and outer enamel epithelium. The external cells of the
reduced enamel epithelium consist of undifferentiated epithelial cells able to divide and
multiply. It is the external cells of the reduced enamel epithelium that will
eventually give rise to the junctional epithelium.
Fig. 39 (From Schroeder,
H.E. and Listgarten, M.A., 1977): Unerupted tooth after completion of
amelogenesis. The crown is located within a bony housing (B). The enamel is
covered by a thin reduced enamel epithelium (REE). CT, connective tissue; D, dentin; ES,
enamel space; OE, oral epithelium. The areas marked x and y are shown at higher
magnification in the following figures.
Fig. 40 (From Schroeder,
H.E. and Listgarten, M.A., 1977): Magnified view of the section in Fig. 39
marked "x". The enamel surface is lined with a layer of reduced enamel
epithelium (REE) that consists of cuboidal reduced ameloblasts (RA) and external cells of
the reduced enamel epithelium (EC/REE). The follicular connective tissue (CT) next to the
epithelium is very vascular, with many dilated blood vessels (BV). ES, enamel space.
Fig. 41 (From Schroeder,
H.E. and Listgarten, M.A., 1977): Magnified view of the section in Fig. 39
marked "y". The reduced enamel epithelium (REE) adjacent to the enamel space
(ES) ends at the cementoenamel junction, near the bottom of the figure. CT, follicular
connective tissue; D, dentin.
Fig. 42: Section of reduced enamel epithelium adjacent to the enamel space (ES). In
this section the reduced ameloblasts (RA) are still elongated.
Following their differentiation into ameloblasts, these cells are no longer able to
enter mitosis and divide. This is in contrast to the external cells of the reduced
enamel epithelium (EC/REE) that are largely undifferentiated and will be major
contributors to the formation of the junctional epithelium. BV, blood vessel; CT/DF,
connective tissue of the dental follicle.
Fig.
43 (From Schroeder,
H.E. and Listgarten, M.A., 1977): Transmission electron micrographs of the
junction of enamel and ameloblasts (AB) at different stages of amelogenesis. A:
Secretory stage. Arrowheads indicate secretory vesicles emptying their content of enamel
matrix proteins on the surface of the partially mineralized enamel (EM). B: Late
maturation stage. The reduced ameloblast (RA) has established an attachment (primary epithelial attachment) to the
enamel that is mediated by hemidesmosomes (HD) and an internal
basal lamina (IBL/REE). The arrowhead points to a membranous infolding typically
observed in resorbing cells. C: Post-amelogenesis. The
reduced ameloblasts (RA) remain attached to the enamel by a basal lamina (BL/REE) and
hemidesmosomes (HD). ES, enamel space.
