Oncosphere penetration

Larval cestodes in mammals make intimate contact with host tissues at two sites (Fig. 11.7) (a) during the early phase of infection, when the recently hatched oncosphere penetrates the intestine and (b) at the final encystment... 

Oncospheres of many cestode species penetrate and develop into metacestodes within cysts (cysticerci, hydatid, multilocular) in the soft tissues of their rodent, ruminant or human hosts. Thus, species such as Echinococcus granulosus, E. multilocularis, Eaenia multiceps, T. ovis, E. saginata and E solium are of economic and medical importance. Such soft tissue invasion elicits a host immune response to resist the helminths. However, although some cysts may be destroyed as evidenced by involution or calcification, the host response is often too little - too late to eliminate the invaders. The susceptibility of the host to invasion is often due to successful evasive strategies... 

Taeniid cestodes have a prey-predator life cycle involving two mammalian hosts. The definitive host is a carnivore or omnivore, which harbours the adult tapeworm parasite in the small intestine. Mature infective eggs are released with the faeces and, when these are ingested by a suitable species of intermediate host, the oncosphere contained within the egg is liberated through the influence of intestinal secretions, particularly bile. The activated parasite penetrates the mucosa of the small intestine... 

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). 

It is important to note that hatching in H. nana is somewhat different from that of H. diminuta (204) and this may be related to the fact that hatching not only takes place in the insect intermediate host but also in the intestine of the definitive rodent host. It has been pointed out earlier (p. 183) that the embryophore in H. nana (in contrast to that of H. diminuta) is thin and discontinuous (Fig. 7.11) and this may make it more readily vulnerable to the host s enzyme action and hence facilitates intestinal hatching. The presence of polar filaments in this species may serve to delay expulsion of the hatched oncosphere from the rodent intestine by becoming entangled in the intestinal mucus and villi. This process would also serve to bring the oncosphere into close contact with the gut wall and hence facilitate successful penetration (204). 

The process of hatching in the Cyclophyllidea, which results in the release of the oncosphere, has been discussed in Chapter 7. The basic structure of a mature oncosphere (Fig. 8.16), which will not be discussed in detail here, does not vary substantially between species. It is composed essentially of (a) a thin covering epithelium with cytoplasmic extensions, (b) an additional complex system of muscles operating the three pairs of hooks, (c) a pair of large penetration glands, (d) a small core of germinative cells from which the next larval stage develops and (e) a primitive nervous system. 

It is not intended here to describe the ultrastructure of the oncosphere in detail as this is well covered in the above accounts but the following organ systems involved in the process of penetration require further consideration. 

The initial penetration of the oncosphere in H. diminuta appears to be largely mechanical as a result of the destruction of the columnar epithelium of the insect gut by hook movements (888), but as indicated above, lysis may also be involved. The same pattern is likely to be followed by other species in other invertebrate and vertebrate hosts. 

The cyclophyllidean oncosphere is well supplied with musculature and the general pattern of muscles appears to be similar in most species. So in both Hymenolepis citelli (which penetrates an invertebrate gut) and E. granulosus (which penetrates a vertebrate gut) there are 16 somatic muscle cells (146, 839). However, E. granulosus has 16 hook muscle cells but H. citelli has only 13. In E. granulosus, the hook muscles have been shown to insert at the collar and base of the hooks and at the basal lamina of the embryonic epithelium. Each pair of hooks has three muscle systems associated with it (a) a protractor system, for hook extensions (b) an abductor system, which draws the hooks together and (c) a retractor system which pulls the hooks into the body (839). 

Rather surprisingly, it was found (16), that the presence of glucose in the maintenance saline had no effect on survival (Figs. 8.19), which suggests that (like some trematode cercariae) the larvae are non-feeding during the hexacanth penetration phase. It is possible that the appropriate systems for active transport or diffusion do not develop until after penetration. Since so little is known regarding the metabolism of the oncosphere (see Chapter 7) it is not profitable to speculate further on this question without more experimental data. 

The immunobiology of this parasite is complicated by the fact that hosts can be infected (a) directly, via egg infections, or (b) indirectly, via cysticercoids (in beetles). In the direct life cycle, eggs hatch in the duodenum and oncospheres immediately penetrate the villi and develop into cysticercoids (Fig. 11.6). When fully developed, these break out of the villi, attach and... 

Fig. 11.7. Pattern of immune responses of an oncosphere of a taeniid cestode (e.g. Echinococcus granulosus) during initial penetration of the hatched oncosphere (initiating early immunity ) and its subsequent migration to, and establishment in, its definitive site, where it is subject to late immunity . The exact site of hatching (in man) is unknown most eggs probably hatch in the upper duodenum.

Attention has already been drawn to the fact that immunity in the H. nana/rodent system represents an unique situation in that the oncosphere develops in a villus after penetration, so that the penetration and final encystment sites are essentially the same. Passive protection against H. nana in mice can be induced by serum transfer, not only prior to the oncospheral invasion, but also after invasion has taken place. Oncospheral agglutination has also been shown to occur in H. nana-infected mouse serum but never in uninfected sera (555). 

Barker, I. K. (1970). The penetration of oncospheres of Taenia pisiformis into the intestine of the rabbit. Canadian Journal of Zoology, 48 1329-32. 

Hymenolepis nana the fine structure of the penetration gland and nerve cells within the oncosphere. Parasitology, 82 445-58. 

Penetration glands of oncospheres of Hymenolepis diminuta (Cestoda). Histochemical studies. Bulletin de I Academie Polonaise des Sciences, Class II, 25 619-22. 

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