Endocytosis, lysosomes, calcium storage and other features of digestive-gland cells in cephalaspidean gastropods (Euopisthobranchia)

The digestive gland was studied by light and electron microscopy in six species of cephalaspidean gastropods including herbivores and carnivores, representing the families Bullidae, Haminoeidae, Philinidae and Aglajidae. Digestive cells presented a similar morphology in all of them. These cells showed an apical border of microvilli and formed membrane pits collecting electron-dense material. At the cell apex, many small endocytic vesicles could be observed near structures resembling early endosomes. In some digestive cells, lysosomes seemed to be fusing with each other to form very large vacuoles. Arylsulphatase activity was detected in smaller electron-dense lysosomes and in the large vacuole-like lysosomes. Using the pyroantimonate method, calcium was detected in endocytic vesicles and in endosomes, but not in lysosomes. Basophilic cells could be identified by their spherical secretory vesicles with a glycoprotein content. Calcium concretions were observed within small vacuoles of basophilic cells in Bulla striata, Haminoea navicula and H. orbignyana, which have large shells. Concretions were not observed in the digestive gland of Philine quadripartita, Aglaja tricolorata and Philinopsis depicta, which have reduced internal shells. However, the results show that important reserves of soluble calcium can also be present in basophilic cell vacuoles of species with a reduced shell and without mineral concretions in the digestive gland. Mucus-secreting cells were found only in the digestive diverticula of the aglajids A. tricolorata and P. depicta. A correlation between morphological features of the digestive gland and trophic position was not evident in these cephalaspideans. The fundamental morphofunctional features of this organ seem to have been conserved during the evolution of most gastropod clades.