Ultrastructure of the calcium-sequestering gastrodermal cell in the hydroid Hydractinia symbiolongicarpus (Cnidaria, Hydrozoa).

Large, free-floating crystals of calcium carbonate occur in vacuoles of gastrodermal cells of the hydroid Hydractinia symbiolongicarpus. Here, morphological details about the process by which these cells accumulate and sequester calcium are provided by a cytochemical method designed to demonstrate calcium at the ultrastructural level. Electron-dense material presumably indicative of the presence of calcium was EGTA-sensitive and was shown by parallel electron energy loss spectroscopy (EELS) and energy spectroscopic imaging (ESI) to contain calcium. Calcium occurred in only one cell type, the endodermally derived gastrodermal cell. In these cells, the electron-dense material appeared first as a fine precipitate in the cytosol and nucleus and later as larger deposits and aggregates in the vacuole. During the life cycle, gastrodermal cells of the uninduced planula and the planula during metamorphic induction sequestered calcium. In primary polyps and polyps from established colonies, gastrodermal cells sequestered calcium, but the endodermal secretory cells did not. Our observations support the hypothesis that gastrodermal cells function as a physiological sink for calcium that enters the organism in conjunction with calcium-requiring processes such as motility, secretion, and metamorphosis.