Ultrastructure of oncospheral hook formation in the nematotaeniid cestode, Nematotaenia dispar (Goeze, 1782).

Ultrastructural characteristics of oncospheral hook morphogenesis in the nematotaeniid cestode, Nematotaenia dispar, are described. The primordia of embryonic hooks appear in the advanced phase of the pre-oncosphere in 6 specialised hook-forming cells or oncoblasts. Each hook primordium, situated near an invaginated part of the nucleus, is surrounded by numerous free ribosomes, mitochondria and extended Golgi regions. Simultaneously with the hook primordium elongation and transformation into a blade, handle and base, the hook material differentiates into an electron-dense cortex and a less dense, inner, crystal-like core. The exit of the blade of the mature hook, protruding from the oncosphere, is surrounded by a circular, septate desmosome and 2 rigid, dense rings on either side. The pattern of oncospheral hook morphogenesis in N. dispar is compared with that of 2 previously examined cyclophyllidean cestodes, Inermicapsifer madagascariensis and Catenotaenia pusilla.