Microscopical study of the crop and oesophagus of the carnivorous opisthobranch Philinopsis depicta (Cephalaspidea: Aglajidae)

Abstract

A histochemical and ultrastructural study of the crop and oesophagus was carried out for the first time in Aglajidae. In Philinopsis depicta, the hind region of the crop contains two large folds creating a channel between them, lined by a ciliated epithelium. This ciliated groove continues through the posterior oesophagus. In addition to these larger folds, both crop and posterior oesophagus present many smaller longitudinal ridges lined by a nonciliated epithelium formed by cells bearing microvilli embedded in a layer of extracellular material. Lysosomes and mitochondria are common in the supranuclear region of these cells, and in the basal region hemidesmosomes are frequent. Epithelial secretory cells contain many large vesicles with a low electron-density content rich in acid polysaccharides, but without detectable amounts of protein. The basal region of the secretory cells comprises the nucleus and several Golgi stacks formed by many flat cisternae with low electron-density content. Secretory cells were not observed in the ciliated epithelium of the channel between the two large folds. The connective tissue of the crop and posterior oesophagus contains many large vacuolar cells with a thin layer of cytoplasm around the single vacuole that occupies about 90 or more of the cell volume. Tubular invaginations of the cell membrane are frequent in these cells and some also contain large vesicles that can open to the central vacuole, suggesting an intense transport activity across the cytoplasm. These vacuolar cells could be related to the calcium cells reported in the connective tissue of other gastropods. Ultrastructural and histochemical studies of the organs of the digestive system in carnivorous and herbivorous cephalaspideans can lead to the discovery of important unknown features, like the vacuolar cells, which might be useful to establish correlations with molecular phylogenetic data or food sources. © The Author 2011. Published by Oxford University Press on behalf of The Malacological Society of London, all rights reserved.