Formation of the cyst wall and related changes in the structure of the tegument of<i>Bucephalus haimeanus</i>(Lacaze-Duthiers, 1854) during its metamorphosis from the cercarial to the metacercarial stage

Abstract

SUMMARY The formation of the cyst wall surrounding Bucephalus haimeanus and the related changes in the parasite tegument during its metamorphosis from the cercarial to the metacercarial stage have been investigated by means of experimental infections in Gobiusculus flavescens . The initial fibrous cyst wall is formed from secretions produced by both the anterior gland cells and the tegument of the parasite. These secretions gradually become compacted against the surrounding hepatic cells until by the 20–30th day post-infection the 3-zoned inner layer, characteristic of the fully developed cyst wall, is formed. Hepatic cells immediately adjacent to this inner cyst wall layer are disrupted by the arrival of the metacercaria and form the middle vacuolated layer. As the metacercaria grows, the cyst increases in size causing still further hepatic cells to become flattened and incorporated into the cyst structure forming the outer nucleated layer. The structure of the cercarial tegument is described. Development of the metacercarial tegument is accomplished by the sequential movement of secretory vesicles from tegumental cell bodies into the outer cytoplasmic tegument. Vesicles of the types V. 1–5 are released from the outer cytoplasmic tegument, resulting in an almost complete breakdown of this layer prior to its replacement by the tegument of the fully developed metacercaria. The latter is characterized by the V. 6–9 type vesicles, dorso-ventrally flattened spines which terminate in 5–7 digits and by sensory cilia.