Ultrastructural features of the epidermis in turbellaria from the Lake Baikal Geocentrophora wagini (Lecithoepitheliata, Plathelminthes)]

The epidermis of Geocentrophora wagini was studied using transmission electron microscopy. The turbellarian body was entirely covered by cilia, whose density was higher on the ventral surface compared with the dorsal one. In all regions examined, the epidermis was made up of a one-layered insunk epithelium. The basal matrix, underlying the epidermis, was a well developed basement membrane (BM) with bilayered structure, overlying the muscle network of circular and longitudinal fibers. The double plasma membranes, extending from the apical surface of epidermis to BM, were linked by specialized cell junctions. This suggested that epidermis had a cellular rather than a cyncytial arrangement. Each insunk epidermal cell was made of two unequal parts: a comparatively thin surface plate attached to BM by hemiadherens junctions, and a massive nucleated portion located below the body wall musculature in the parenchyma. A thin cytoplasmic bridge connected the epidermal plate with the nucleated cell body. The epidermal plates were joined by belt-like junctions along their adjacent surfaces. Inconspicuous zonula adherens (ZA) had a most apical position, and prominent septate junction was arrayed proximally to this zonula. Except ZA, cell boundaries in epidermis were frequently flanked by rows of light tubules and vesicles. In the basal half of the epithelial sheet, they were occassionally accompanied by single cisternae of rough endoplasmic reticulum (RER). The ultrastructure of the insunk cell body and that of the surface plate showed a considerable similarity. The common features were distinctive profiles of RER and GA, the presence of epitheliosomes, light tubules and vesicles, centrioles and fibrous granules. Thus, ultrastructural features allow a rather reliable identification of epidermal cells in the parenchyma, despite the absence of any visible morphological association between cell body and its epidermal plate.