SKIN ULTRASTRUCTURE IN RELATION TO PROLACTIN AND MSH FUNCTION IN RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) EXPOSED TO ENVIRONMENTAL ACIDIFICATION

The present study describes the effects of 14 days exposure to acidified (pH 4.0) soft water in the absence of aluminium, an the ultrastructure of the skin in rainbow trout (Oncorhynchus mykiss). Compared to control fish, there was a moderate increase in the incidence of necrosis in the filament cells of the fish exposed to pH 4.0, but since the integrity of the tissue appeared to be maintained, most of the ultrastuctural changes observed may be considered to be adaptive. There was an increase in epidermal thickness, a higher frequency of electron-dense vesicles in filament cells, an increase in the undulation of the basal lamina, and the penetration of the epidermis by cytoplasmatic processes of melanocytes in acid-exposed specimens. An infiltration of leucocytes into the epidermis, and the appearance of serous mucous cells, was also evident. Whether these events were under the control of prolactin and/or alpha-MSH, was also investigated, but no indication for activation or inhibition of either prolactin or alpha-MSH producing cells was obtained. Since a previous study (Balm and Pottinger 1993) had demonstrated that plasma cortisol levels were also identical in control and low pH treated trout throughout a 14 day experimental period, it is concluded that under conditions of environmental acidification, the integument autonomically maintains the adjustments necessary for successful acclimation, presumably via paracrine regulatory circuits.