Experimental schistosomiasis mansoni: characterization of connective tissue cells in hepatic periovular granulomas.

Connective tissue cells present in periovular granulomas, induced in mice livers by schistosomal infection, were studied in situ and in an in vitro culture after cell migration from explanted granulomas. They were compared to cells of the adjacent hepatic tissue. Connective tissue cells in granulomas and granuloma-derived primary cell lines were characterized as myofibroblasts. The presence of lipid droplets in cells of early granulomas, and comparison with adjacent perisinusoidal cells, indicated their origin through activation and mobilization of lipocytes from the adjacent hepatic tissue. This origin was confirmed in long-standing cultures of granuloma-derived cells, in the stationary phase of growth, in which myofibroblasts could return spontaneously into the fat-storing phenotype.

Ultrastructural localization of nucleolar organizers during oogenesis in Xenopus laevis using a silver technique.

Silver staining at the electron microscopic level of the nucleolar organizers was carried out on Xenopus laevis oocytes at various stages of oogenesis. The results indicate that a positive reaction takes place exclusively in the dense fibrillar component of the extrachromosomal nucleoli. This constituent undergoes morphological changes of distribution and architecture, which have been correlated with modifications of the transcriptional activity of the nucleoli. When nucleolar activity is reduced, during previtellogenesis, this constituent appears as dense homogeneous spherules well-segregated from the granular component. In contrast, when nucleolar activity is high, during vitellogenesis, it forms an heterogeneous area with an ill-delimited outline: it is organized into a fibrillar core with emerging skein-like strings. It thus seems that this constituent remains silver-stained throughout oogenesis. These findings suggest that the method used would allow one to follow the evolution of the nucleolar organizer region (NOR) topography during oogenesis. Moreover, they point out facts that have relevance to the problem of the correlation between Ag stainability of NORs and nucleolar transcriptional activity.