In vitro development of hydatid cysts from posterior bladders and ruptured brood capsules of equine Echinococcus granulosus.

A variety of morphological forms developed during monophasic culture of ruptured brood capsules of equine Echinococcus granulosus. Attached to some protoscoleces were small posterior bladders, with a surface ultrastructure of truncated microtriches, and these never developed a laminated layer. In contrast, small free vesicles of similar size and ultrastructure, but devoid of an attached protoscolex, produced a layer by 20-30 days. The surface ultrastructure of both these forms is similar to that of the protoscolex attachment stalk and the brood capsule lining, and posterior bladders and small free vesicles are considered to be derived from portions of the stalk +/- part of the brood capsule lining retained at protoscolex release. Also present were large vesicles interpreted from their size and surface anatomy as ruptured, everted brood capsules. When devoid of attached protosoleces these formed cysts each with a laminated layer, whereas similar vesicles with several attached protoscoleces did not develop a layer, or only very occasionally after 70 or more days. In contrast, a few large vesicles with a single attached protoscolex produced a laminated layer. The possibility that protoscolex attachment to stalk remnants or brood capsules inhibits the cystic development of these structures is discussed.