Ultrastructural characteristics of the vitellarium of Brandesia turgida (Brandes, 1888) (Digenea: Pleurogenidae) and an examination of the potential usefulness of such vitelline traits in digenean systematics

Transmission electron microscopical observations were made on the vitelline structure of the digenean Brandesia turgida (Brandes, 1888) collected from crypts within the intestinal wall of the frog Pelophylax ridibundus (Pallas, 1771). Ultrastructural details of the vitelline follicles of B. turgida include: (a) the presence within the vitellarium of a single type of cell, i.e. vitellocytes at different stages of their development; (b) a narrow region between the vitellocytes filled with the processes of surrounding parenchymal cells; and (c) the occurrence of the junctional complexes between the vitellocytes and the surrounding parenchymal cells at the periphery of the vitelline follicles. It is shown that the vitelline globules and a few lipid droplets are the main inclusions resulting from vitellocyte synthetic activity. The limited amount of nutritive reserves in the vitellocytes can be explained by the nature of the parasite’s life-cycle, which is characterized by fully-embryonated intrauterine eggs containing a fully-formed miracidium. Despite the small number of digenean species studied in relation to their vitelline cytoarchitecture, two structural patterns of the vitellarium can be elucidated; these are examined in terms of digenean systematic relationships.