Echinococcus granulosus protoscolex formation in natural infections.

Echinococcus granulosus is a parasitic platyhelminth that is responsible for cystic hydatid disease. From the inner, germinal layer of hydatid cysts protoscoleces are generated, which are are the infective forms to the dog. Systematic studies on the cell biology of E. granulosus protoscolex formation in natural infections are scarce and incomplete. In the present report we describe seven steps in the development of protoscoleces. Cellular buds formed by a clustering of cells emerge from the germinal layer of hydatid cysts. The buds elongate and the cells at their bases seem to diminish in number. Very early on a furrow appears in the elongated buds, delimiting anterior (scolex) and caudal (body) regions. Hooks are the first fully-differentiated structures formed at the apical region of the nascent scolex. In a more advanced stage, the scolex shows circular projections and depressions that develop into suckers. A cone can later be seen at the center of the hooks, the body is expanded and a structured neck is evident between the scolex and the body. During protoscolex development this parasitic form remains attached to the germinative layer through a stalk. When fully differentiated, the stalk is cut off and the infective protoscolex is now free in the hydatid fluid.