Studies on gastrointestinal peptides in the ascidian Styela clava

Abstract

Cells analogous to vertebrate endocrine cells have been described in the gut epithelium of the ascidian Styela clava. As well as some histochemical similarities, ultrastructural correlations have been demonstrated, particularly the presence of electron dense granules, clustered around and mainly below the nucleus, thinning out towards the apex. Like the endocrine cells of the vertebrate gut, the cells are often pyramidal, with a narrow apex which is occasionally observed to extend to the lumen of the gut.

In addition, strong secretin immunofluorescence was observed in the endocrine-like (E-L) cells of the mucous cap of the gastric ridges. Because of these observations, acid extracts of Styela gut were assayed for secretin in the rat and in the turkey. The Styela extracts as prepared were inactive but it is possible that this reflected faults in the extraction technique.

Development of a perfused Styela gut preparation, however, produced evidence to support the hypothesis that a CCK-like peptide is released into the circulation, presumably from the E-L cap cells, although CCK-like immunoreactivity is not demonstrable in these cells.

The observation that in addition to CCK, bombesin and physalaemin also induce enzyme release suggests that the pre-pancreatic zymogen cells contain a rich complement of receptors, corresponding to all the classes of PI stimulating receptors which have been found on vertebrate acinar cells. There is therefore the implication that these hormones or analogues may be present in Styela. As secretin was found not to act as a secretagogue in this system, the significance of its production is unclear. By analogy with vertebrate systems it may exert some control over the secretion of mucus by the cap cells.