Functional morphology of the metacercarial cyst of Bucephaloides gracilescens (Trematoda: Bucephalidae)

Abstract

SUMMARYThe metacercarial cyst wall of Bucephaloides gracilescens is bilayered. It comprises an inner granular layer of pepsin-resistant carbohydrate and protein material secreted by the parasite, and an outer fibrous and cellular capsule susceptible to pepsin digestion and elaborated by the host. The capsule of cysts from the orbits and nasal regions of the whiting host are substantially thicker than those from the cranial cavity. Cells in the outer portion of the capsule resemble fibrocytes engaged in the synthesis and secretion of collagen; collagen fibres form a loose meshwork in the intercellular space. The cells incorporated [3H]proline in vitro, but not [3H]tyrosine. The cells of the inner portion of the capsule are closely packed and possess pyknotic nuclei; desmosomes and cytofilaments are numerous and probably impart strength and elasticity, respectively, to the cyst wall. Host cells are necrotic at the interface with the inner cyst wall, possibly due to compression or isolation from oxygen and nutrients. Differences in the levels of incorporation of trace-labelled substrates by encysted metacercariae were related to the site of infection and to the thickness and permeability of the cyst wall.