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EM studies on the coronet cells of the freshwater stickleback showed that the cells were characterised by abundant SER, perinuclear fibrils, microtubules, free ribosomes and mitochondria. Acid phosphatase-positive dense bodies were present, but there was no alkaline phosphatase activity nor were there any glycogen granules in the coronet cells. On the surface of the coronet cell and its globules was a layer of Ruthenium red-stainable material which almost completely disappeared when the fish were kept in 70% seawater for 1 week. Quantitative EM of fish kept in freshwater or 70% seawater for 1 week showed that the SER tubules of seawater-maintained fish had a wider lumen than those of freshwater-maintained fish. The volume occupied by the Golgi apparatus also increased but there was no change in the volume of the cytoplasm occupied by mitochondria. Both the morphology of the coronet cells and their response to 70% seawater support the hypothesis that the saccus vasculosus is an osmo(iono)regulatory structure. [ABSTRACT FROM AUTHOR]

The relative volume of the RPD region of the pituitary gland of Gasterosteus aculeatus form leiurus was greater in animals collected in the spring than in winter. A morphometric analysis of the prolactin cells from spring animals showed only slight changes in cell ultrastructure compared with winter animals. However, in spring the prolactin cells apparently formed and released more secretory granules. Three distinct sites for the release of secretory granules are described. A preliminary study of the response of prolactin cells from spring animals to 70% sea water did not reveal any large scale changes in cell ultrastructure. The significance of this finding is discussed. It is concluded that although the freshwater stickleback cannot be regarded as 'physiologically hypophysectomised:' with regard to prolactin secretion in the winter, it still shows a seasonal change in prolactin cell ultrastructure. [ABSTRACT FROM AUTHOR]

Two types of chloride cells were recognized in the gill epithelium of the fresh-water eel. Following transfer of the eel to sea water the number of strongly acidophilic cells (type A cells) increased markedly during the first week of adaptation. Simultaneously the cell size increased and cell inclusions such as mitochondria and smooth endoplasmic reticulum were also developed. On the other hand, the number of weakly acidophilic cells (type B cells) increased only during the initial 3 days of adaptation and thereafter decreased. This decrease in number of type B cells showed a mirror image of the increase in number of type A cells. Considering the changes in number and fine structures of type B cells, it is likely that the type B cell is a transitional stage of the typical chloride cell (type A cell). When seawater adapted eels were transferred to fresh water, the chloride cell degenerated slowly during 10 weeks. The involvement of the chloride cell in salt secretion of the gill of the seawater eel is discussed. [ABSTRACT FROM AUTHOR]