Fine structure of the corpuscles of stannius of the trout, Salmo gairdneri: Structural changes in response to increased environmental salinity and calcium ions

These studies confirm that there are two cell types, C₁ and C₂, in the corpuscles of Stannius (CS) of the trout, Salmo gairdneri, both of which have the appearance of protein-or peptide-secreting cells. C₁ cells have large secretory granules, up to 1.0 μm in diameter, while the C₂ cells have small granules up to 0.5 μm. In seawater (SW)-adapted trout the C₂ cell granules are significantly larger than those in freshwater (FW)-adapted trout. In fresh water the C₁ cells are relatively inactive and the C₂ cells are mainly sparsely granulated and actively secreting. The converse situation is found in seawater-adapted trout in which the C₁ cells are very active and the C₂ cells are relatively inactive and contain stored granules. During adaptation to SW from FW some C₁ cells develop extensive rough endoplasmic reticulum and Golgi but other C₁ cells degenerate and disintegrate apparently releasing the entire cell contents, including granules, into the capillary lumena. Protein may be released from these granules in capillaries by digestive enzymes from lysosome-like vesicles. In both adapted FW and SW corpuscles the active cells appear to release their granules by exocytosis. The appearance of C₁ cells of fish transferred from FW to calcium-enriched fresh water is similar to that of fish transferred to dilute SW, suggesting that these cells respond to high levels of external calcium, while the C₂ cells are active in media of low ionic and osmotic strength.