Echinococcus multilocularis (Cestoda, Cyclophyllidea, Taeniidae): origin, differentiation and functional ultrastructure of the oncospheral tegument and hook region membrane.

Both the oncospheral tegument and the hook region membrane (HRM) of Echinococcus multilocularis hexacanths originate from a syncytial binucleate complex that appears in the early stage of morphogenesis and organogenesis of the hexacanth larva. The primordium of this binucleate complex forms a binucleate syncytial cap or "calotte" situated beneath the inner envelope at one pole of the developing embryo. During oncospheral differentiation, the binucleate perikaryon of the syncytial cap is sunk progressively deeper into the central part of the embryo, but remains always connected with the distal cytoplasm by a tendrillar cytoplasmic connection or bridge. Following migration or sinking of the binucleate perikaryon, numerous cytoplasmic vesicles appear in the distal cytoplasm. These vesicles fuse progressively together and form a single large cavity or lacuna. The walls of this cavity are becoming at this point the walls of two delaminated layers: (1) the distal anucleated cytoplasmic layer is transformed into the oncospheral tegument and (2) the proximal thin cytoplasmic layer is transformed into the "hook region membrane". This delamination of the initially compact layer of distal cytoplasm into two layers seems to be closely associated with differentiation of oncospheral hooks, the elongating blades of which protrude progressively into a newly formed cavity. The pressure of hook blades on the hook region membrane appears to facilitate its further separation from the basal layer of distal cytoplasm which is transformed into the peripheral layer of oncospheral tegument. In the mature oncosphere, the surface of this peripheral layer forms a regular brush border of cytoplasmic processes or microvilli and represents the true body covering of the hexacanth. The very thin cytoplasmic connection between the peripheral layer of tegument and binucleate perikaryon appears only very seldom in the ultrathin sections as a narrow cytoplasmic strand and has a plasma membrane that is reinforced by a single row of cortical microtubules. The HRM covers only one pole of the oncosphere and is attached to the oncosphere surface. The HRM is clearly visible in the mature oncosphere and is draped over the hook blades, the sharp points of which are protected by moderately electron-dense caps. Comparison of the above morphology with that of TEM study of the tegument of adult cestodes shows a great similarity as well as homology in the body covering of both larval and adult cestodes.