The occurrence of submicrovillar endotube (modified terminal web) and associated cytoskeletal structures in the intestinal epithelia of nematodes

Abstract
The intestines of 22 genera of nematodes from five different orders were examined for the presence of an endotube, the submicrovillar entity previously described for Haemonchus contortus , a member of the order Strongylida. The endotube can be obtained by blunt dissection as a complex with the microvilli essentially free of the rest of the cytoplasm. Representatives of all three suborders and eight families of the order Strongylida possessed an endotube but of the representatives of the four other parasitic orders and the one free-living group examined only one genus, Strongyloides (Rhabditida) possessed this structure. The thickness of the endotube ranged from about 80 nm in Metastrongylus up to 6 μm in Strongylus . In all samples the filamentous cores of the microvilli, whether formed into an axial bundle (as usual) or dispersed in a net (as in Dictyocaulus ), which extended 0.1-0.5 μm below the base of the microvilli terminated in the luminal surface of the endotube. The basal side of the endotube was usually associated with a layer of microfibrils. The depth and distribution of the microfibrillar layer determined the extent to which the endotube-brush-border complexes could be dissected free from other cytoplasmic components. There was electron microscopic evidence for an endotube-like entity not associated with the microvilli in the intestine of Syphacia (Ascarida). A survey of published electron micrographs of nematode intestines indicated that the true submicrovillar endotube occurred only in members of the order Strongylida and the genus Strongyloides (Rhabditida) in which the structure here described as an endotube has previously been described as terminal web.