[Embryonic development of the cestode Mosgovoyia ctenoides (Anoplocephalidae)]

In this study the cleavage divisions and the ultrastructural analysis of early embryos as well as cellular organisation of infective oncosphere of the anoplocephalid cestode Mosgovoyia ctenoides are described. The early cleavage is unequal and results in the formation of three types of blastomeres: 2 large macromeres containing large electron dense granules, 3 medium-size mesomeres and several small micromeres. In the early stage of oncospheral morphogenesis, formation of three following primary embryonic envelopes takes place: (1) the capsule replaced by thick, rigid outer coat originated form the uterine material secretion, (2) the outer envelope and (3) the inner envelope. The capsule is formed from the vitellocyte material. Two macromeres contribute to the formation of the outer envelope and three mesomeres take part in the formation of the inner envelope. The inner envelope undergoes differentiation into three sublayers: (1) a thick extraembryophoral cytoplasmic layer, (2) an electron-dense embryophore, as a stiff pyriform apparatus, and (3) a thin intraembryophoral cytoplasmic layer containing mesomere nuclei. The oncosphere is located in the extended cupule-like part of the pyriform apparatus. Four egg envelopes surround the mature infective oncosphere of M. ctenoides: (1) a thick outer coat, (2) the outer envelope, (3) the inner envelope with a characteristic pyriform apparatus and (4) the oncospheral membrane. Hook morphogenesis takes place inside six symmetrically arranged oncoblasts, each of which shows a characteristic large nucleus of semi-lunar shape. At the beginning the "hook-forming center" appears in the cytoplasmic part of each oncoblast. It consists of numerous free ribosomes, polyribosomes, mitochondria and Golgi complexes. The hook-forming center is involved in synthesis of a hook primordium, which undergoes differentiation and elongation into the fully developed hook. Mature hook consists of three parts: (1) blade, (2) shank, (3) base, and at the site of its protrusion from the oncosphere, is surrounded by a circular septate junction. Wide bands of hook muscles are attached to the basal and collar parts of the hook. The hook blades project outside the oncospheral body into a large cavity that is delimited by the hook region membrane. In the fully developed oncosphere of M. ctenoides three pairs of oncospheral hooks together with specialized hook muscles form a complex of "hook muscle system", responsible for coordinated hook action. The surface of the infective oncosphere is covered by a thin cytoplasmic layer of oncospheral tegument connected with the so-called "binucleate subtegumental cell", situated deeper in the oncospheral body. Below the cytoplasmic layer are situated wide bands of the somatic musculature responsible for oncospheral body movements. Five major types of oncospheral cells have been distinguished in the infective oncosphere: (1) a binucleate subtegumental cell, (2) a binucleate penetration gland, (3) two nerve cells, (4) numerous somatic cells, and (5) six germinative cells. During development of the oncosphere, changes in the concentration of glycogen and number of lipid droplets were observed. In the early embryos glycogen particles were most abundant in the macromere cytoplasm, whereas in micromeres concentration of glycogen was observed to be lower. In the course of the differentiation of the oncospheral envelopes glycogen was progressively distributed to other parts of the developing embryo. Simultaneously, a great increase in the number of lipid droplets was detected. However, during the preoncospheral phase of development a progressive reduction of lipid droplets was observed. This may indicate that lipids play a role of the energy source for developing oncosphere.