Morphology of intestinal epithelial cells in the freshwater elasmobranch Potamotrygon mag dalenae was studied by electron microscopy. Columnar cells lining the internal surface of the intestine as a single layer in P. magdalenae are characterized by microvilli densely arranged on the free surface, intermicrovillous invaginations, numerous vesicles beneath the microvilli and granules of varying sizes. These morphological characteristics suggest that columnar cells with microvilii function as absorptive cells. Interdigitations of the lateral plasma membranes , which are not found in teleosts, occur at junctions of adjacent cells in the P. magdalenae intestine. It is shown that these interdigitations exist not only in marine, but also in freshwater elasmobranchs.

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We identified the presence in the Aleutian skate, Bathyraja aleutica, of two classes of immunoglobulins (Ig), a high molecular weight Ig analogous to mammalian IgM and a low molecular weight Ig, similarly to the spiny rasp skate, Raja kenojei, (Kobayashi, K. et al., Mol. Immunol. 1984. 21: 397), using an immunological cross-reaction with the specific antisera to the spiny rasp skate Ig components. The antigenic similarity of the heavy chains of the Ig of the Aleutian skate and those of the spiny rasp skate was less than that between their light chains. Two types of Ig-producing cells, one producing the high molecular weight and the other forming the low molecular weight Ig, were present in the spleen of embryos and adults in the Aleutian skate at a ratio of 5-6:4-5. The number of these Ig-producing cells increased with advancing development of the embryos but was 1/20 to 1/50 of those of adults. Cells, each of which were capable of forming both classes of Ig, were found in the spleen of embryos but not in that of adults. These results suggested that the spleen of the Aleutian skate is the primary lymphoid organ for B lymphocyte differentiation and proliferation, possibly equivalent to the bursa of birds.

Summary. Splenic white pulp of the Aleutian skate (Bathyraja aleutica), an elasmobranch, was investigated using light and transmission electron microscopy. The major cellular constituent was plasma cells, of the typical Marshalko type, characterized by well developed rough-endoplasmic reticulum and a Golgi complex. The morphology of the rough-endoplasmic reticulum was variable, being lamellar in some cells and spherical in others. Plasma cells with distended cisternae of rough-endoplasmic reticulum and cells with Russel bodies were often observed. Only a small number of lymphocytes were encountered. These findings indicate that the splenic white pulp is the major site for immunoglobulin production in this fish, thus confirming our previous immunocytochemical observation. Globules presumably containing immunoglobulin were found consistently associated with the Golgi complex. The secretion mechanism of immunoglobulin by plasma cells is discussed in connection with the globules.