Morphological and cytochemical studies on the skin of rainbow trout, Salmo gairdneri, Richardson

Abstract

A series of studies concerning the structure and ultrastructure of the skin of rainbow trout was carried out, especially associated with growth and sexual maturation. An increase in the thickness of the dermis and epidermis and a decrease in the number of mucous cells were noted in the males, together with an infiltration of mononucleated cells in both sexes during sexual maturation. The structure of the gills and thymus was also examined.

In the ultrastructural studies special attention was paid to lymphocyte-like and macrophage-like cells, which were interpreted as infiltrating elements. Presumed macrophages, with or without melanin granules, were seen crossing the basement membrane, which reinforced the idea of a dermal-epidermal traffic.

Phosphotungstic-acid staining was applied to ultrathin sections of the epidermis to evaluate the distribution of complex carbohydrates. Among other localizations, positive staining was observed in mucous cells and in the membrane of large vesicular bodies in macrophages and filament-containing cells. The possibility of such structures being lysosomes is discussed.

Immunoglobulin-containing cells were located in the skin through immunocytochemical methods. PAP and immunogold labelling, in semithin and ultrathin sections. Rabbit serum anti-trout Ig was used and the details of its production are given.

In the epidermis, lymphocyte-like cells showed a degree of intracytoplasmic labelling, but this was not sufficient to draw conclusions as to whether these cells were antibody-producing or not. Some mucous cells stained positively.

To evaluate how bath vaccination influenced the production of antibody in the skin, a group of juvenile fish were vaccinated inti-Vibrio anguillarum. Immunocytochemical studies were undertaken, using an immuno-sandwich technique, to locate specific antibody anti-Vibrio anguillarum.

Finally, a discussion is presented of the involvement of the skin in specific immune reactions. It seems to be a tissue fully equipped for antigen-trapping, where mucous cells could be responsible for the transport, or even the synthesis, of antibody into the skin surface, to be incorporated into the mucus.

The lack of protection shown by the skin of sexually mature males of certain salmonid species is considered and thought possibly due to the decrease in mucus production under hormonal influence and thereby a decrease in available immunoglobulins at the body surface.