Ultrastructural observations on effects of different concentrations of calcium and thyroxine in vitro on larval epidermal cells of Rana catesbeiana tadpoles.

During anuran metamorphosis dramatic changes in morphogenesis and differentiation of epidermis occur under the influence of thyroid hormones. Modification of ionic calcium concentration also markedly alters the pattern of proliferation and differentiation in amphibian epidermal cells in vitro. The present study was designed to determine the direct effect of low (0.05 mM) and high (0.5 mM) calcium (Ca2+) in the absence or presence of thyroxine (10(-7) M) on epidermal cells of the body and tail tissue in vitro. When tail fin and body skin explants were maintained in low (0.05 mM) calcium for 48 h, normal ultrastructural morphology and integrity of the cells was observed in both the tissue types. When tissues were exposed to high levels of calcium (0.5 mM) in culture medium, tail epidermis showed stratification, and skein cells exhibited apoptosis, both in the presence or absence of thyroid hormones. Under high calcium conditions, the body epidermis showed keratinization of apical cells, apoptosis of skein cells, and increased desmosome formation. These results suggest that (1) optimal Ca2+ concentration for larval epidermal cells is quite low (0.05 mM), (2) high Ca2+ leads to keratinization only in body epidermis, and (3) apoptosis occurred in skein cells of both the tissues at high Ca2+ concentrations (0.5 mM). The present study therefore suggests that the extracellular calcium concentration regulates the process of cell death and differentiation in Rana catesbeiana larval epidermis, and this effect may be similar to the effect of calcium on mammalian epidermal cells.