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The ultrastructural localization of glucagon in the presence of Scyliorhinus canicula was investigated. We used a post-embedding immunoelectron microscopy method on pancreatic samples fixed in glutaraldehyde and osmicated before embedding. Contrasting with uranyl acetate and lead citrate was also performed after immunolabelling, but best results were obtained with uranyl acetate only. Glucagon-like immunoreactivity was located in round granules (300-600 nm) surrounded by a limiting membrane. The matrix varied in electron density and exhibited a dense core surrounded by a less dense mantle. The granules were seen in two different cell types, which differed in the electron density of their cytoplasm. Glucagon-immunoreactive cells were the largest pancreatic cells types and were often localized near somatostatin-containing cells.

The comparative distribution of somatostatin- and glucagon-like-containing cells in the histomorphologically different gastric mucosae of the cartilaginous fishes Heptranchias perlo, Raja asterias, Scyliorhinus canicula, Squatina aculeata, and Torpedo marmorata was immunocytochemically studied to demonstrate a possible interrelationship between these endocrine cells and/or other endocrine or nonendocrine cells. In the gastric mucosa, these open-type glucagon and somatostatin immunoreactive cells show a double localization with different morphology and interrelationships. At the bottom of gastric pits, which corresponds to a proliferative zone, spindle or pear-shaped immunopositive cells appear rather numerously and are often in close proximity to each other. In gastric glands, triangular or oval immunopositive cells never in contact with each other were detected; their numeric ratio seems to be rather constant even if their numeric frequency and distribution vary according to the histomorphological aspect of selachian gastric glands. Glucagon immunoreactive cells seem to be more related to pepsinogenic cells, while somatostatin immunoreactive cells seem to be more ubiquitous. Both cell types can present basal cytoplasmic processes. From our results we can suggest a possible regulative role exerted by these two peptides on gastric secretion and cell proliferation.

The ontogeny and distribution of glucagon-like cells were studied in the gastrointestinal tract of embryos, neonates, and adults of the cartilaginous fish Scyliorhinus stellaris (L.) by immunocytochemistry. The results indicate that they appear early during embryonic development, and, in some portion of the gastrointestinal tract, even before the mucosa morphological differentiation. Immunoreactive glucagon-like cells were observed both in gastric and intestinal epithelium, being present in the pyloric portion only at a particular period of its differentiation. Some differences were observed between the embryonic and adult distributive pattern. They were more numerous in proliferative zone and sometimes were situated near other endocrine epithelial cells. These findings together with available information on trophic effects of some gastrointestinal hormonal peptides suggest a possible regulatory role of this peptide on the growth and differentiation of the gastrointestinal tract.