An ultrastructural study of alimentary tract development in the cercariae of Diplostomum pseudospathaceum (Digenea: Diplostomidae).

Morphogenesis of the alimentary canal in developing Diplostomum pseudospathaceum cercariae has been studied using electron microscopy. The foregut primordium appears in early cercariae as a cellular cord. Later, the lumen develops within the foregut primordium, and its cells give rise to the cellular epithelium, limiting this lumen. During subsequent development, the lateral plasma membranes, separating the cells, disappear from the primary foregut epithelium as do nuclei and most of cellular organelles. These events seem to progress in several steps, and eventually, the foregut lining becomes the thin anucleate syncytial layer. In late cercariae, this layer becomes connected with nucleated cytons, producing secretory inclusions. Each of two caecal branches appears to arise from a row of large cuboid cells. The primordial gastrodermal cells are involved in synthetic and secretory activity and give rise to numerous secretory inclusions. These inclusions release their contents into the cavities which develop between adjacent primordial caecal cells. The intercellular cavities gradually increase in size and fuse to eventually form a single caecal lumen. In mature cercariae, the large caecal lumen packed with electron-dense secretory material is limited by a thin cellular gastrodermis, displaying no secretory features.