Rodlet cells in the epidermis of fish exposed to stressors.

Rodlet cells (RC) were detected in the epidermis of carp (Cyprinus carpio) and trout (Oncorhynchus mykiss) exposed to stressors (e.g. acid water, heavy metals, thermal elevation, polluted water of the Rhine river, water loaded with organic manure, and distilled water), as well as after wounding. These cells were not found in the epidermis of fish kept under control conditions, thus suggesting that the appearance of RC in the epidermis is stressor-related. Immature RC, resembling mucous cells, were found in deeper epidermal zones while mature RC were found close to and at the skin surface. The latter contained several rodlets embraced within a capsule-like structure composed of filaments. At the skin surface, RC opened the filamentous capsule at the cell apex and a cytoplasmic tuft, containing the rodlets, protruded outwards into the surrounding water. Structures which appeared to be free rodlets were also observed in the water, near the ridges of the pavement cells. Cytochemical and immunocytochemical study revealed that the rodlets contain alkaline phosphatase at their periphery and peroxidase activity at their cores. These enzymes have been previously reported in mucous cells of stressed fish. All together, the location of RC, their structure, cytochemistry and direction of migration suggest that these cells form part of the non-specific defence mechanism of the skin, and possibly other epithelial tissues.