Developmental implications of differential effects of calcium in tail and body skin of Anuran tadpoles

The effects of external Ca(++) on metamorphosis of Rana catesbeiana tadpoles were assessed. Treatment of tadpoles with Ca(++) (0.05 mM) during early prometamorphic stages induced precocious metamorphic events such as tail regression, shortening of the intestine, forelimb emergence, and keratinization of body epidermis within 23 days of treatment compared to control tadpoles still in mid-prometamorphic stages. These effects of Ca(++) are probably mediated by the thyroid gland, as indicated by histological features of the gland at the light and electron microscopic levels. Calcium levels of tail and body skin were measured at various stages of development by atomic absorption spectrophotometry. In control and experimental groups, body skin had significantly higher Ca(++) concentrations than tail skin. There were no statistically significant effects of developmental stage on Ca(++) levels of tail or body skin. Experimental Ca(++) treatment significantly increased Ca(++) concentration in tail but not body skin. Ultrastructure studies and gel electrophoresis indicated that calcium induced keratinization of body skin, but not tail epidermis. Ca(++)-treated tail epidermis showed various autolysing figures in apoptotic cells. In summary, calcium treatment accelerated metamorphosis and induced the following region-dependent cellular events: keratinization of body skin-a characteristic of adult epidermis-and programmed cell death in the tail. Whatever signal elicited by calcium in this experimentally induced accelerated metamorphosis is probably mediated via the thyroid gland.