A ïstract. The skin structure and the plasma cortisol leve of trout, Oncorhynchus m ykiss, were examined durii : 7 days of exposure to water of pH 5. By day-4 and -7 the thickness of the epidermis was significantly (/ <0.05) less in acid exposed fish than in controls, and cl generative cells were common in the upper epidermal I; ers. Many epidermal cells exhibited signs of necrosis, a 1 by day-7 many apoptotic cells were also present. Seci tory vesicles of high electron density were abundant h the filament cells of the 3-4 outermost layers of epid mis, and intercellular spaces had increased. Mitotic Il Lires occurred throughout the epidermis, with the exc uion of the outermost cell layer. Mucous cells became e »ngated after day-1, and later, newly differentiating n icous cells could be seen close to the skin surface, and n my mucocytes contained mucosomes of high electron d isity. Rodlet cells were occasionally seen. Chloride c Is appeared similar to those of control fish. Many leuc cytes, mainly macrophages and lymphocytes, had pene ated the epidermis via the highly undulating basal k nina, and at day-7, numerous apoptotic lymphocytes w re found. In the dermis, melanosomes became disp rsed in the cytoplasmic extensions of melanocytes w lich were present in the epidermis of all acid-exposed fi h. Iridocytes were rare after day-4, while fibroblasts w re abundant and secreted larçe amounts of collagen. A ter 1 day of exposure to acidified water, a significant (/ <0.05) elevation of the plasma cortisol level had occ rred, but this subsequently declined, and had returned u control values by day-7. The changes in skin struce, however, remained throughout the whole exposure period.

Vbstract. The ultrastructure of club cells and n eig h b o u r l y filament cells and leucocytes in the epiderm is of carp, vas studied under norm al conditions and after exposure o several stressors: acid water, heavy metals, organic nanure, brackish w ater and w ounding. The effects of the stressors were rem arkably similar. T he club cells in creased in size and contained m ore endoplasm ic reticuum and Golgi areas. In both control and stressed fish, most m itotic figures of the filament cells were found ad ja cent to club cells, as was d em o n strated after colchicine injection. W hereas in the controls apoptosis of filament cells was scarce and limited to the upper layer of the epithelium , in the stressed fish it was com m only seen in close proxim ity to the club cells but not in other mid-epiderm al parts of the epithelium. This indicates th at club cells influence the cellular kinetics of the filament cells. U nder stress conditions leucocytes infiltrated the epider mis. Som e were seen inside club cells. A pparently these leucocytes were taken up in phagosom es and su b seq u en t ly they show ed signs of necrotic degeneration. Leucocyte incorporation and degeneration in club cells were not observed in control fish. C ontrol of the cellular turnover of filament cells and the elim ination of leucocytes m ay represent new functions for club cells, which have m ainly been associated with the production of pherom ones.