Strobilisation, sexual differentiation and maturation

With the exception of a few species with progenetic plerocercoids (e.g. Schistocephalus) it is only within the definitive host that cestode tissue develops in a strobilar direction, and it is self-evident that strobilar differentiation must be induced by factors present in the intestinal environment. Clearly, any number of known parameters of the intestine (Fig. 3.1, p. 37) 

Contrasting results have also been obtained with different species of Mesocestoides. In M. corti, the chief factors inducing strobilar differentiation in vitro appear to be anaerobic conditions (Fig. 10.12, p. 280) and a pH greater than 7.4 (606,607,806). Yet in M. lineatus, the presence of trypsin in the culture media is required for sexual differentiation (382). These phenomena are discussed in further detail in Chapter 10. Whatever the nature of the stimulus which initiates sexual differentiation, there is now strong experimental evidence that it operates via the neurosecretory system. It was speculated, many years ago, that in E. granulosus, the contact stimulus operated via a neurosecretion which in turn induced the release of a strobilisation organiser (796). 

An elegant demonstration that the larval/adult transformation, resulting in sexual differentiation, is, in fact, associated with a neurosecretory mechanism has been made by Gustafsson and co-workers (207, 278, 280, 281,283). They showed that a clear activation of the peptidergic neurones took place when plerocercoid larvae of Diphyllobothrium dendriticum were transferred from the poikilothermic intermediate fish host to the final homeothermic bird host. The effect could also be reproduced by cultivating 

Stage 0 Undevelopedprotoscolex. This stage is represented by the freshly evaginated, but undeveloped, protoscolex. 

Stage 2 Second proglottis. This stage too is clearly defined by the appearance of an interproglottid partition (but see p. 5). 

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