Immunocytochemical localization of secretin-like cells in the digestive tract of the ascidian Halocynthia papillosa.

The digestive tract of the ascidian Halocynthia papillosa was studied by means of both immunofluorescent and immunocytochemical methods. In the esophageal epithelium there are immunoreactive secretin-like (S2) cells, which are at the same time serotonin-containing cells. In the gastric and intestinal epithelia, immunoreactive (S1) cells are present which contain exclusively secretin-like substances. On the basis of the comparison between the gastric morphology of H. papillosa and that of other ascidians (styelids) it is possible to reach some conclusions of an evolutionary nature.