|
|
|
|
Ultrastructural characteristics of egg envelope formation in the preoncospheral and oncospheral stages of development of the anoplocephalid cestode, Anoplocephaloides dentata, an intestinal parasite of Arvicolidae rodents, are described. In this species, the first embryonic envelope, a delicate capsule, occurs only in the early embryos and disintegrates rapidly in the preoncospheral phase of embryogenesis. In this stage, the developing embryos are surrounded by two primary envelopes of cellular origin: (1) an outer envelope, formed by two macromeres and containing macromere nuclei; and (2) an inner envelope, formed by three mesomeres, the nuclei of which usually persist 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 undergoes differentiation into three layers – an extraembryophoral cytoplasmic layer, an electron-dense embryophore which rapidly thickens forming a rigid pyriform apparatus, and an intraembryophoral cytoplasmic layer still containing 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 which 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 several processes. In addition to the above mentioned egg envelopes, the anterior pole of the oncosphere is surrounded by the “hook region membrane”, draped over the hooks, which differentiates simultaneously with the oncospheral tegument. These results on the origin, differentiation and ultrastructure of egg envelopes in A. dentata are compared with literature on the ultrastructure of egg envelopes in other cestode groups and in particular in other anoplocephalids.