Intracellular calcium phosphate deposits contribute to transcellular calcium transport within the hepatopancreas of Porcellio scaber

Abstract

Like in most Crustacea, the cuticle of terrestrial isopods is hardened by a calcareous mineral phase. This rigid cuticle is frequently shed during a process called moulting. To reduce calcium loss, Porcellio scaber eats the shed cuticle, the exuviae, and absorb the calcium from it through large tubular diverticula of the intestine, called the mid gut glands or hepatopancreas. After moulting the absorbed calcium should be transported immediately into the hemolymph from which it is used to rapidly mineralize the new cuticle. This suggests that the hepatopancreas epithelium transports calcium from the lumen to the hemolymph. We used TEM, energy-filtered TEM and electron-probe X-ray microanalysis to analyse the distribution of elevated calcium within the hepatopancreas cells of P. scaber. We used animals in the postmoult stage that have eaten their exuviae and, as a control, those that have not ingested the exuviae. To minimize calcium loss within the samples, we used high pressure frozen and freeze substituted samples and propane-1–3-diol as floatation medium for thin-sectioning. The results reveal intracellular dense deposits containing calcium, phosphorus and oxygen at the apical microvillus membrane, within the cytoplasm, attached to vesicles and to the basolateral membrane, as well as extracellular between cells and the basal lamina. Control animals were devoid of these deposits. The results indicate that calcium from the exuviae is absorbed and transported across the epithelium into the hemolymph. We propose that during transport, intracellular calcium is bound to phosphate avoiding toxic effects of high concentrations of ionized calcium.