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.

Ultrastructural study of the spermatogenesis and mature spermatozoa of Dicrocoelium dendriticum (Plathelminthes, Digenea).

The spermatogenesis and spermatozoon of Dicrocoelium dendriticum were studied by transmission electron microscopy. Peripheric accessory cells project between germ cells. Each spermatogonium gives rise to 32 spermatozoa. The stages in spermiogenesis include development of the zone of differentiation, appearance of the intercentriolar body flanked by two centrioles from each of which a free axoneme and a striated rootlet grow, outgrowth of the differentiation zone to form the median cytoplasmic process and migration of the nucleus and mitochondria into it, and rotation of the flagella and subsequent proximodistal fusion of the three projections to form a monopartite spermatozoon. The spermatozoon possess two incorporated axonemes with the "9 + 1" pattern typical of those in trepaxonematid plathelminths. beta-Glycogen particles accumulate in the spermatozoa after they have separated from the cytophore as revealed by Thiery's method. This study confirms in a further family, Dicrocoeliidae, the constant pattern of spermiogenesis and spermatozoon structure in Digenea.

Intracytoplasmic ciliary elements in epidermal cells of Syndesmis echinorum and Paravortex cardii (Platyhelminthes, Dalyellioida).

Epidermal cells of Syndesmis echinorum and Paravortex cardii contain many intracytoplasmic ciliary components: clusters of centrioles disorganized and incomplete short axonemes composed of loosely organized microtubules of irregular lengths, fully formed axonemes though some with fewer than nine doublets, and ciliary rootlets. Furthermore, conspicuous dense granules are found in solitary groups in the cytoplasm. Clusters of dense granules are also closely associated with Golgi complexes and developing axonemal microtubules. Since the dense granules decrease in number as the axonemes increase, it is likely that the granules are involved in the formation of axonemal microtubules. Ciliary elements are especially abundant in epidermal cells of Paravortex cardii embryos, some of them resembling those previously described by several authors in differentiating ciliated cells engaged in centriologenesis and ciliogenesis. Attention has been focused on the relative proportion and position of these elements, as well as the different morphology and several assembling states that they exhibit in epidermal cells of adult S. echinorum and adults and embryos of P. cardii. A functional interpretation of some of the findings is given, which allows us to suggest a sequence of ciliogenetic events that occur in epidermal cells of both species.