Cortisol induces stress-related changes in the skin of rainbow trout (Oncorhynchus mykiss).

The ultrastructure of the skin of rainbow trout, Oncorhynchus mykiss, was studied over a 7-day period after a single meal containing cortisol. The fish experienced increased plasma cortisol levels for 1 day. Pavement cells contained significantly more vesicles of high electron density, which were also numerous in deeper filament cells, and displayed peroxidase activity. Mitotic cells were common after 4 and 7 days. Increased apoptosis, taken to indicate accelerated ageing, was detected in both pavement and mucous cells. Newly differentiated mucous cells were found close to skin surface, and many mucous cells contained mucosomes of high electron density. The basal lamina became highly folded. The low numbers of leukocytes present in the skin did not change noticeably, but substantially more lymphocytes were apoptotic. The melanosomes in the pigment initially dispersed and subsequently reaggregated in the cell bodies of these cells. The reaggregation was accompanied by apoptosis of melanocyte extensions. The results demonstrate the ability of the hormone to regulate several of the effects observed in the skin of fish challenged by stressors. Other phenomena generally observed in stressed fish, such as pavement cell necrosis and massive leucocyte infiltration, were not found after cortisol treatment. The latter observation indicates that regulatory factors in addition to cortisol must be operative during stress.