The ultrastructure of oncospheral hook formation in the anoplocephalid cestode Mosgovoyia ctenoides (Railliet, 1890) Beveridge, 1978, is described. The hook morphogenesis takes place inside the six symmetrically arranged hook-forming cells, the oncoblasts. They show characteristic large nuclei of semilunar shape, localized at one pole of the embryo. At the beginning of the hook formation, the "hook-forming centre" appears in the cytoplasmic part of each oncoblast. It consists of numerous free ribosomes and polyribosomes surrounded by several mitochondria and Golgi complexes. The hook-forming centre is involved in synthesis of an electron-dense, undifferentiated hook primordium, which undergoes progressive differentiation and elongation into the fully developed hook. A fully formed oncospheral hook consists of the three parts: blade, shank, and base. Each hook, at the site of its protrusion from the oncosphere, is surrounded by two electron-dense rings interconnected by a circular septate junction. The hook material consists of two or three layers that differ in electron density: (1) a moderately electron-dense core, (2) a middle layer of low electron density, and (3) a highly osmiophilic cortex. Wide bands of hook muscles are attached to the basal and collar parts of the hook. The hook blades project outside of the oncospheral body into a large cavity delimited by the hook region membrane attached at this pole directly to the oncospheral surface. In the fully developed oncosphere of M. ctenoides, the three pairs of oncospheral hooks and their muscles form a complex "hook muscle system", responsible for coordinated hook action. The differentiation and ultrastructure of oncospheral hooks in the oncospheres of M. ctenoides are compared to those described in other cestode species.
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