Oncospheral membrane

III. The inner envelope a syncytial layer showing much variation. Some workers divide this layer into two zones - zone I, a cytoplasmic layer and zone II, a gelatinous layer (Fig. 7.14). Part of this embryonic layer gives rise to the embryophore (Fig. 7.4) and also to the oncospheral membrane (Figs. 7.4, 7.11 and 7.14) (a very thin layer surrounding the oncosphere), which is often counted as a fourth layer. Additional layers, which may be further derived from the above basic envelopes have been reported in some species (e.g. H. nana 204), but it is beyond the scope of this text to discuss all the various modifications which can occur. Only those features which have a special physiological significance are discussed below. 

Test Indicates presence of Shell Outer envelope Embryophore Cytoplasm Oncospheral membrane filamentous material filament layer... 

Fig. 7.11. Comparison of the fine structure of the membranes of the eggs of Hymenolepis diminuta and H. nana. Note in H. nana (in contrast to H. diminuta) (a) the embryophore is thin and incomplete - a feature which may facilitate hatching in the gut of the definitive host (b) there is an additional polar filament layer between the oncospheral membrane and the basal lamina. Gc, Golgi complex ger, granular endoplasmic reticulum /, lipid bodies m, mitochondria n, nucleus v, vacuoles. (After Fairweather Threadgold, 1981a Holmes Fairweather, 1982.)...

This envelope is a syncytial layer involving two cells Ailing the space between the embryo and the egg shell it is responsible for the formation of the embryophore and oncospheral membrane (204). After the latter is formed, what is left of the inner envelope becomes the cytoplasmic and gelatinous layers-zones I and II, respectively, in H. diminuta (Fig. 7.14). The latter has the ability to swell readily in dilute salt solutions, a property which may facilitate the escape of the oncosphere during hatching (p. 191) (442). 

This layer appears to be unique to H. nana. It forms a layer between the oncospheral membrane and the oncosphere (Fig. 7.11). It is apparently formed by the delamination of the epithelial covering of the oncosphere into two layers, separated from each other by membranes - the outer polar filament layer and the inner embryonic epithelium. It has been suggested that the polar filaments in H. nana are reminiscent of the tendrils of the egg cases of elasmobranchs and may serve to delay expulsion of the oncosphere from the mammalian intestine by becoming entangled amongst the intestinal villi or mucous lining of the gut. This may further serve to bring the oncosphere into close contact with the gut wall for successful penetration to take place (204). 

Hatching of the cyclophyllidean egg involves two processes (a) the rupture of the egg shell (when present) and the passive digestion and/or disruption of the embryophore, (b) activation of the hexacanth embryo to become motile and rupture its enclosing oncospheral membrane. These processes frequently require different stimuli for their initiation. The hatching process has been reviewed by Lethbridge (442), and Ubelaker (888). 

STAGE 4B Emergence of oncosphere oncospheral membrane soon lost... 

Sodium hypochlorite can also be used to disintegrate the embryophore and provides a simple, rapid technique for obtaining oncospheres 430, 614). However, after this procedure, additional treatment with AIF (Table 7.9) is needed to free the oncospheres from their oncospheral membranes 824). Taeniid eggs have also been hatched by using sodium sulphide to disintegrate the embryophore (75). 

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