Microscopical and functional aspects of calcium-transport and deposition in terrestrial isopods

Terrestrial isopods (Crustacea) are excellent model organisms to study epithelial calcium-transport and the regulation of biomineralization processes. They molt frequently and resorb cuticular CaCO3 before the molt to prevent excessive loss of Ca2+ ions when the old cuticle is shed. The resorbed mineral is stored in CaCO3 deposits within the ecdysial gap of the first four anterior sternites. After the molt, the deposits are quickly resorbed to mineralise the posterior part of the new cuticle. The deposits contain numerous small spherules composed of an organic matrix and amorphous CaCO3, which has a high solubility and, therefore, facilitates quick mobilization of Ca2+ and HCO3- ions. During the formation and resorption of the deposits large amounts of Ca2+, HCO3- and H+ are transported across the anterior sternal epithelial cells. Within the last years, various light and electron microscopical techniques have been used to characterize the CaCO3 deposits and the cellular mechanisms involved in biomineralization. The work on the CaCO3 deposits includes studies on the ultrastructure of the deposits, the sequence of events during deposit formation and dissolution, and the mineral composition of the sternal deposits. The differentiation of the anterior sternal epithelial cells and the mechanisms of epithelial ion transport required for the mineralization and demineralisation of the deposits was studied using various analytical light and electron microscopical techniques including polarized light microscopy, immunocytochemistry, electron microprobe analysis, electron energy loss spectroscopy and electron spectroscopic imaging. Comparative analysis of deposit morphology and the differentiation of the sternal epithelia provide information on the evolution of CaCO3 deposit formation in relation to the degree of adaptation to terrestrial environments.