Cellular diversity in the epidermis of Raja clavata (Chondrichthyes)

The epidermal layer of the skin of the cartilaginous fish Raja clavata take n from a suitable place on the ventral side of the head was examined by lig ht and electron microscopy. Cells of the basal layer divide and differentia te as epithelial cells of the mid-epidermis, characterized by numerous elec tron-dense, membrane-bound vesicles; their content is assumed to be mucus. Cells at the superficial layer of the epidermis lose these vesicles but hav e a palisade of mucous vesicles below the apical membrane. A modified glyco calyx coats the outer leaflet of the apical membrane. In addition to epithelial cells, there are epidermal cells differentiated a s two kinds of mucous goblet cells, sacciform cells similar to those found in teleost fish epidermis, holocrine cells with some resemblance to the clu b cells of teleosts but differing in their cytology, and chemosensory cells similar to those found in the oral epithelium of Raja. Merkel cells were n ot found. Cells that might be interpreted as ionocytes were seen but their nature was not confirmed. The epidermis is penetrated by numerous nerve fib res; swellings contain mitochondria and glycogen granules. The nerve fibres also contain 50-70 nm vesicles with an indistinct core. Cells intrusive to the epidermis include melanocytes, lymphocytes, macrophages, and several t ypes of granular leucocyte. The cytology of the various epidermal elements is generally similar to that known in teleost fishes.