Ultrastructural investigations on the vitellaria of the digenean Dicrocoelium dendriticum.

The paired vitellaria of the parasitic plathelminth Dicrocoelium dendriticum are composed of numerous follicles each of which contains vitellocytes at different stages of maturation and is enveloped by a basal lamina-like structure and a cytoplasmic sheath. The differentiation process of vitellocytes has been subdivided into three stages on the basis of morphological and functional characteristics. Stage I vitellocytes have a high nucleo/cytoplasmic ratio and a poorly differentiated cytoplasm mainly packed with free ribosomes. Stage II vitellocytes differentiate 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 vitellocytes are about to enter the vitelloduct, their cytoplasm is almost completely filled with the multigranular inclusions whose content reacts positively to the test for polyphenols. The inclusions are therefore interpreted as egg-shell globules. Mature vitellocytes also contain a small number of lipid droplets which are sometimes surrounded by a few polysaccharide particles, but completely lack protein yolk globules. The role of vitellocytes of D. dendriticum in egg-shell formation and embryo nutrition is discussed.