Chemistry Reference
In-Depth Information
OTOLITHS
The inner ear in humans is comprised from a bony labyrinth which forms a complex
cavity. This cavity is filled with a membranous labyrinth, a fluid filled assembly of
interconnected sacs and channels. The fluid filling the membranous labyrinth is called the
endolymph. The central part of the labyrinth is the vestibule which has two membranous
chambers, the utricle and saccule. Each of these membranes contains patches of epithelial
hair cells projecting into the gelatinous endolymph. These hairs support masses of
aragonitic calcium carbonate, the otoliths, which function in both hearing and
gravitational orientation. The patches of epithelial hair cells are associated with the
vestibular nerves in a complex called the macula. The otoliths of mouse and teleost fish
are similar and have been studied in biochemical detail. The principal structural
glycoprotein of the matrix in vestibular endolymph is otoconin-90 (Oc90). The Oc90 is a
potentially highly glycosylated protein of 453 amino acid residues, with two domains of
strong homology to secretory phospholipase A
2
. The aragonitic otoconia form on the
Oc90-rich matrix gel. However, the Oc90 mRNA is not expressed in the sensory macular
epithelia related to the otoconia, but is produced elsewhere in the non-sensory epithelia
(Wang et al. 1998). Borelli et al. (2001) compared the proteins of the otolith with the
endolymph protein. Although the total organic content of the mineral was about 0.2% of
the total weight, the mineral related proteins were 23% collagen, 29% proteoglycan and
48% other proteins. The endolymph contained all of the protein components of the
mineralized matrix, plus others, in larger amounts. Thus, the proteins accumulated in the
mineral were selectively retained. The distribution of proteins in the gel-like endolymph
was also non-uniform in terms of total concentrations, but not in relative content. Alcian
blue staining, which reveals anionic components, suggests that the proteins and
proteoglycans do form an anionic environment from which the calcium carbonate does
crystallize. Glycogen may also be present (Pisam et al. 2002). Much more work remains
to define the mineral deposition processes to form the otoliths. The saccular arrangement
of the compartments in which the otoliths form, with the endolymph constituents
delivered by epithelial cells not directly associated with the macula is reminiscent of the
process by which the avian eggshell develops. Perhaps it would be of interest to compare
the components of eggshell and otolith systems to determine any similarities that might
relate to the formation of carbonate rather than phosphate containing mineral phases.
A mechanistic model
Each of the systems discussed here have very specific attributes and control points
for the assembly of its specific biomineralized tissue, yet they also have many points in
common. In a sense one may consider that all use a similar overall strategy but each case
demands the use of different local tactics to achieve structures with different properties.
Figure 16 summarizes the overall scheme we propose for the mineralization of bone and
dentin. However, in different mineralizing systems in the vertebrate and invertebrate
worlds, the variety of tactics which lead to the enormous diversity of biominerals in
different species, and within the same animal, can be thought of as merely the result of
substituting different components, at different concentrations, along each of the arrows.
Thus chitin in the mollusc shell could substitute for the collagen in bone. The whole train
of events is orchestrated by the cells that produce or transport the components that
comprise the mineralized tissue. As complex as Figure 16 may appear, there are a host of
additional factors that have been ignored. At any given point in the lifetime of an animal
the cells are forced to respond to numerous biochemical inputs including nutrients,
growth factors, and hormones, and to mechanical stresses. Bone, dentin and cementum
are living tissues, always responding to these factors and always changing in subtle ways
to meet their functional responsibilities. Only enamel mineral is fixed in amount and
