Premolt calcium secretion in midgut posterior caeca of the crustacean Orchestia: Ultrastructure of the epithelium.

Both structural and functional changes are observed within the posterior caeca (PC) of Orchestia during the molt cycle. During the intermolt period, there are two segments which are structurally different: a distal segment lined by type I epithelial cells and a proximal segment lined by type II cells. During molting, the PC cells are active in calcium turnover. Calcium is secreted and stored as calcareous concretions in the caecal lumen during the preexuvial period; then during the postexuvial period it is reabsorbed to mineralize the new cuticle. During the preexuvial period, cellular type III differentiates along the whole length of the PC in poster-anterior sequence and functions in ionic calcium secretion, from the basal part to the cellular apex. During the postexuvial period, this cellular type turns into cellular type IV engaged in calcium reabsorption from successive generations of spherites, from the cellular apex to the basal part. The role played by the caecal epithelium during both formation and reabsorption of the concretions was investigated by experiments in which caeca were transplanted to host pericardial cavities or were blocked by causing an abdominal hernia. The main structural characteristic features of cellular type III are as follows: an extracellular network of channels extends from basal to apical ends; microvilli are long and often apically dilated; multivacuolar complexes are localized in extracellular channels and within dilated tips of microvilli before secretion into caecum lumen; bundles of microtubules are oriented in parallel around the luminal orifices of the extracellular network; ribosomes are abundant in cytoplasm. Cellular type III develops progressively from the distal end of the caecum to the proximal one as the preexuvial period advances and concretions form in the caecum lumen.