CELLULAR-RESPONSES OF THE SKIN AND CHANGES IN PLASMA-CORTISOL LEVELS OF TROUT (ONCORHYNCHUS-MYKISS) EXPOSED TO ACIDIFIED WATER

The skin structure and the plasma cortisol levels of trout, Oncorhynch us mykiss, were examined during 7 days of exposure to water of pH 5. B y day-4 and -7, the thickness of the epidermis was significantly (P < 0.05) less in acid exposed fish than in controls, and degenerative cel ls were common in the upper epidermal layers. Many epidermal cells exh ibited signs of necrosis, and by day-7 many apoptotic cells were also present. Secretory vesicles of high electron density were abundant in the filament cells of the 3-4 outermost layers of epidermis, and inter cellular spaces had increased. Mitotic figures occurred throughout the epidermis, with the exception of the outermost cell layer. Mucous cel ls became elongated after day-1, and later, newly differentiating muco us cells could be seen close to the skin surface, and many mucocytes c ontained mucosomes of high electron density. Rodlet cells were occasio nally seen. Chloride cells appeared similar to those of control fish. Many leucocytes, mainly macrophages and lymphocytes, had penetrated th e epidermis via the highly undulating basal lamina, and at day-7, nume rous apoptotic lymphocytes were found. In the dermis, melanosomes beca me dispersed in the cytoplasmic extensions of melanocytes which were p resent in the epidermis of all acid-exposed fish. Iridocytes were rare after day-4, while fibroblasts were abundant and secreted large amoun ts of collagen. After 1 day of exposure to acidified water, a signific ant (P < 0.05) elevation of the plasma cortisol level had occurred, bu t this subsequently declined, and had returned to control values by da y-7. The changes in skin structure, however, remained throughout the w hole exposure period.