Chemistry Reference
In-Depth Information
proteins is intercalated into the cell membrane, and has a role comparable to that of the
Asp-rich protein of the mollusk shell at the interface between the silk fibroin gel and the
crystal surface. The sea urchin tooth is a continuously growing structure, with its size and
mineral content increasing from the aboral plumula to the adoral incisal edge. All stages of
growth and mineralization are present in a single tooth at all times, making this a very
interesting system to study. The three dimensional reconstruction of the mineral
distribution made possible by microCT scanning (Stock et al. 2002) indicates that the
crystals in the plane of the inter-membrane space are quite large, while their thickness
remains small. The components of the inter-cell membrane compartments must enter via
transport from the cells through the cell membrane. Labeling with tetracycline shows that
calcite deposition takes place in all parts of the teeth, even in the oldest adoral zone.
Although it is far from proven at this point, it is reasonable to hypothesize that the
presumed membrane proteins provide the interactive sites for mineral crystal initiation
within the inter-membrane plate channel space in this matrix-mediated mineralization
system. In effect, the syncitium of cells provides the structural matrix. There are no defined
polymeric organic matrix structural compartment walls, as evident in the bivalve shells.
A quite different system and organization is seen in the structure of the avian eggshell,
one of the two systems in which the mineral phase in a vertebrate is calcium carbonate
derived rather than a calcium phosphate. The mineral phase deposition takes place within a
very well defined zone during the passage of the egg through the oviduct. In addition to
secreting albumen, the epithelial cells of the oviduct also secrete a variety of proteins and
proteoglycans that organize into distinct layers and tightly regulate the shell structure. As
shown schematically in Figure 4 (Gautron et al. 2001), the inner shell membrane is not
mineralized, but consists of type I and X collagens, several glycoproteins, and
Figure 4.
Mineral-matrix arrangement with the avian egg shell. (A) Scanning electron micrograph
across a fractured avian shell (from Gautron et al. 2001 with permission of The American Society for
Biochemistry & Molecular Biology). Internal shell membrane, SM; Mammillary layer , ML; Pallisade
layer, PL; Surface cuticle, [B][C]. (B) Schematic of the shell cross-section. The mammillary layer, C,
rests on a collagenous fiber net, D, the mammillary knobs are evident. These are the sites of mineral
nucleation to form the palisade layer, B, which is capped by a cuticle membrane, A (from Wu et al.
1992a with permission of Elsevier).
