Locations, ultrastructural morphology, and putative functions of the branchial podocytes of the fresh water crab Potamon niloticus - Savigy (Crustacea, Decapoda, Potamonidae)

The gills of the African fresh water crab, Potamon niloticus, were investigated by transmission electron microscopy to verify the presence, outline the location(s) of, and describe the ultrastructural attributes of the branchial podocytes (BPs). Topographically, the cells were diffusely distributed in the gills. They were found at the arterial ends of the intralamellar spaces (ILSs), in the efferent hemolymphatic vessel (EHV), in the gill shaft, and in the marginal and central ILSs. In the EHV and the terminal ends of the ILSs, the BPs occurred in cohesive clusters of from three to eight large cells which were affixed to the vessel wall by small fibrocytic cells and bands of myofibrils. In the clusters, the BPs attached directly across interspersed junctional complexes (separated by wide intercellular spaces) and indirectly over a common basement membrane. Abundant heteromorphic, variably electron-dense vacuoles were scattered in the cytoplasm, apparently displacing the nuclei peripherally. The plasmalemma of the BPs were amplified into feet processes (pedicels) which inserted onto a basement membrane. The feet were joined by a thin unit membrane (diaphragm), leaving subcisternal spaces which contained flocculent to granular electron-dense material. The general ultrastructural morphology of the BPs of Potamon was similar to that of other crustaceans. However, atypically, a labyrinth of intercellular spaces (reckoned to be filtratory channels) was observed in the cell clusters. An ultrafiltration role was attested by the characteristic specializations of the plasmalemma and a phagocytic one inferred from the conspicuous intracytoplasmic vacuolation, presence of phagosomes, and overt necrosis and desquamation of the outlying cells of the clusters. The topographic location of the BPs in the EHV and at the terminal ends of the ILSs was perceived to be a strategic arrangement for promoting the detoxification and destruction of harmful materials and invasive agents which pass through the gills, an organ that presents an extensive surface area that interfaces with the ambient medium.