Cellular responses of the skin of carp (Cyprinus carpio) exposed to acidified water

The skin of carp was examined after exposure to acidified water. Degenerative cells were common in the upper epidermal layers. During the first days most of these cells exhibited signs of necrosis. Later on the incidence of necrosis decreased and that of apoptosis increased. In the acid-exposed fish, the upper filament cells and pavement cells produced secretory vesicles of high electron density, some of which showed peroxidase activity. This enzyme activity was also present in the glycocalyx covering these cells, and in the cytoplasm of apoptotic cells. Mitotic figures and newly differentiating mucous cells were common in the outer epidermal layers. Mucous cells became elongated and produced mucosomes of high electron density. Mucosomes with peroxidase activity were also found. Club cells increased in number. Chloride cells and solitary chemo-sensory cells, not seen in the controls, appeared in the upper epithelial layer. The skin was invaded by many leucocytes and by pigment-containing cytoplasmic extensions of melanocytes. Some leucocytes apparently penetrated into the club cells. These structural observations reflect the complexity of the physiological response of the skin to acid water.