Chemistry Reference
In-Depth Information
There are two means by which the cell can transfer constituents to the matrix. In the
first, the cell may actively pump cations through the membrane and into the surrounding
region (e.g., Simkiss 1986). Once out of the cell, the supersaturation level of the fluid is
established and maintained by ion diffusion over relatively large distances to the organic
matrix (Figure 5a). In the second approach, the cations may be concentrated within the
cell into cation-loaded vesicles, exported through the membrane and later broken down
by precursor compounds at the organic matrix (Figure 5b). The latter mechanism is used
for cartilage mineralization in the epiphyseal growth plate (Ali 1983). It is also probably
used by sea urchin larvae to introduce amorphous calcium carbonate into spicule-forming
vesicles, where it subsequently crystallizes into calcite (Beniash et al. 1999). Anion
movement is typically the result of passive diffusion in response to the electroneutrality
requirement and is ultimately driven by pH gradients created during cation transport
(Simkiss 1976; McConnaughey 1989b). In both approaches, the cell works actively to
supply cations to an external organic matrix for “on-site” nucleation and growth. This is
distinguished from the epicellular nucleation and growth that occur during biologically
induced mineralization.
Almost all structures that form by extracellular processes develop upon a pre-formed
matrix derived from secretory products of multicellular epithelial tissues. Watabe and
Kingsley (1989) suggest that these tissues have additional significance as extensive
Figure 5.
Illustrations of biologically controlled
extracellular mineralization showing that this process
is distinguished by nucleation outside of the cell. a.)
Cations are pumped across the cell membrane and
move by passive diffusion through extracellular
fluids to the site of mineralization. b.) Cations are
concentrated intracellularly as aqueous ions into a
vesicle that is subsequently secreted. Compartment
breakdown at site of mineralization releases cations
for biomineral formation.
