Critical Reviews in Oral Biology & Medicine, Vol 9, 128-161, Copyright © 1998 by International & American Associations for Dental Research
Cellular and chemical events during enamel maturation
C. E. Smith
Faculty of Dentistry, and Department of Anatomy & Cell Biology, McGill University, Montreal, Quebec, Canada.
This review focuses on the process of enamel maturation, a series of events
associated with slow, progressive growth in the width and thickness of
apatitic crystals. This developmental step causes gradual physical
hardening and transformation of soft, newly formed enamel into one of the
most durable mineralized tissues produced biologically. Enamel is the
secretory product of specialized epithelial cells, the ameloblasts, which
make this covering on the crowns of teeth in two steps. First, they roughly
"map out" the location and limits (overall thickness) of the entire
extracellular layer as a protein-rich, acellular, and avascular matrix
filled with thin, ribbon-like crystals of carbonated hydroxyapatite. These
initial crystals are organized spatially into rod and interrod territories
as they form, and rod crystals are lengthened by Tomes' processes in tandem
with appositional movement of ameloblasts away from the dentin surface.
Once the full thickness of enamel has been formed, ameloblasts initiate a
series of repetitive morphological changes at the enamel surface in which
tight junctions and deep membrane infoldings periodically appear
(ruffle-ended), then disappear for short intervals (smooth-ended), from the
apical ends of the cells. As this happens, the enamel covered by these
cells changes rhythmically in net pH from mildly acidic (ruffle-ended) to
near-physiologic (smooth-ended) as mineral crystals slowly expand into the
"spaces" (volume) formerly occupied by matrix proteins and water. Matrix
proteins are processed and degraded by proteinases throughout amelogenesis,
but they undergo more rapid destruction once ameloblast modulation begins.
Ruffle-ended ameloblasts appear to function primarily as a regulatory and
transport epithelium for controlling the movement of calcium and other ions
such as bicarbonate into enamel to maintain buffering capacity and driving
forces optimized for surface crystal growth. The reason ruffle-ended
ameloblasts become smooth-ended periodically is unknown, although this
event seems to be crucial for sustaining long-term crystal growth.
