ULTRASTRUCTURAL INVESTIGATIONS ON THE VITELLARIA OF THE DIGENEAN DICROCOELIUM-DENDRITICUM

The paired vitellaria of the parasitic plathelminth Dicrocoelium dendr iticum are composed of numerous follicles each of which contains vitel locytes at different stages of maturation and is enveloped by a basal lamina-like structure and a cytoplasmic sheath. The differentiation pr ocess of vitellocytes has been subdivided into three stages on the bas is of morphological and functional characteristics. Stage I vitellocyt es have a high nucleo/cytoplasmic ratio and a poorly differentiated cy toplasm mainly packed with free ribosomes. Stage II vitellocytes diffe rentiate and increase in volume. Extensive RER and small Golgi complex appear and produce vesicles with an electron-dense content which fuse and give rise to large multigranular inclusions. Stage III vitellocyt es are about to enter the vitelloduct, their cytoplasm is almost compl etely filled with the multigranular inclusions whose content reacts po sitively to the test for polyphenols. The inclusions are therefore int erpreted as egg-shell globules. Mature vitellocytes also contain a sma ll number of lipid droplets which are sometimes surrounded by a few po lysaccharide particles, but completely lack protein yolk globules. The role of vitellocytes of D. dendriticum in egg-shell formation and emb ryo nutrition is discussed.