Ultrastructural study of the embryonic development of the anoplocephalid cestode Anoplocephaloides dentata, an intestinal parasite of Arvicolidae rodents. I. Egg envelope formation

Ultrastructural characteristics of egg envelope formation in the preoncosph eral and oncospheral stages of development of the anoplocephalid cestode, A noploeephaloides dentata, an intestinal parasite of Arvicolidae rodents, ar e described. In this species, the first embryonic envelope, a delicate caps ule, occurs only in the early embryos and disintegrates rapidly in the preo ncospheral phase of embryogenesis. In this stage, the developing embryos ar e surrounded by two primary envelopes of cellular origin: (1) an outer enve lope, formed by two macromeres and containing macromere nuclei; and (2) an inner envelope, formed by three mesomeres, the nuclei of which usually pers ist in the mature oncospheres. In the more advanced preoncospheral stage, a delicate oncospheral membrane is formed by a delamination of the innermost layer of inner envelope. The remaining part of the inner envelope undergoe s differentiation into three layers - an extraembryophoral cytoplasmic laye r, an electron-dense embryophore which rapidly thickens forming a rigid pyr iform apparatus, and an intraembryophoral cytoplasmic layer still containin g three nuclei of mesomeres, trapped at the base of the pyriform apparatus. The enlarged part of the pyriform apparatus forms some sort of cupule in w hich oncosphere is situated. Below the cupule first appear two horns which elongate rapidly and fuse together into an enlarged cone composed of hard, electron-dense embryophore material. The tip of the cone is branched into s everal processes. In addition to the above mentioned egg envelopes, the ant erior pole of the oncosphere is surrounded by the "hook region membrane", d raped over the hooks, which differentiates simultaneously with the oncosphe ral tegument. These results on the origin, differentiation and ultrastructu re of egg envelopes in A. dentata are compared with literature on the ultra structure of egg envelopes in other cestode groups and in particular in oth er anoplocephalids.