A morphological and histochemical study of the cystogenic cells of the cercaria ofFasciola hepaticaL.

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Abstract
The cercaria ofFasciola hepaticahas four types of cystogenic cells which can be characterized by their position and by their histochemical properties. On the basis of their histochemical reactions, each type of cell was related to a specific layer in the cyst wall of the metacercaria.The tanned protein cells occupy almost all of the ventral half of the body and contain large granules consisting of basic protein and phenols. Although the presence of a phenolase could not be demonstrated, it is considered that the granules are composed of tanned protein. Before the cercaria leaves the redia, the granules are secreted to the exterior to form a layer contained by a thin, bounding membrane. The tanned protein cells form layer I of the cyst wall.The mucopolysaccharide cells are concentrated at the dorsal and dorso–lateral surface except for a few around the ventral sucker. They contain acid and neutral mucopolysaccharides and contribute to layers II and IIIbandc.Some of the granules are secreted to the exterior before the cercaria leaves the redia, in a similar manner to the tanned protein granules.The mucoprotein cells are few in number and lie centrally between the ventral sucker and the pharynx. They give reactions for proteins and carbohydrate-protein complexes and probably help to form layers II and IIIaof the cyst wall.The keratin cells fill almost the entire dorsal half of the body and contain numerous rod-like granules. These rods are composed of a protein containing sulphydryl groups and disulphide bonds and give rise to layer IV. After secretion of the tanned protein granules to the exterior, some of the keratin cells move to the ventral part of the body formerly occupied by the tanned protein cells.The outer cyst wall exists in a preformed state in the free swimming cercaria.I am grateful to Professor J. D. Smyth and Dr J. A. Clegg of this department for advice and encouragement during this investigation and in the preparation of the manuscript. The work was done during the tenure of a Commonwealth Post-Graduate Scholarship.