Oogonial proliferation, oogenesis, folliculogenesis and vitellogenesis in the ovary of zebrafish (danio rerio): a histological and histochemical analysis

Histological and histo chemical analysis of zebrafish ovary with reference to oogonial proliferation, oogenesis, folliculogenesis and vitellogenesis were studied. Ovaries from adult females were processed for paraffin embedding and sections (3µm) thick were cut and stained with hematoxylin-eosin and for histochemical detection of proteins, carbohydrates, mucopolysaccharides, lipids, neutral lipids and acidic lipids. Sections of ovaries exhibited oogonia and oocytes (previtellogenic and vitellogenic). Oogonia were spherical cells found in nests on the ovarian epithelium and occasionally dividing; they ranged from 9-11 µm in size. Newly transformed oocytes ranged from 20-40 µm and primary oocytes measured 40-140 µm in their size. In larger primary oocytes (>170 μm) yolk vesicles appeared beneath plasma membrane marking beginning of the vitellogenesis. Ooplasm of previtellogenic oocytes stained to mercuric bromophenol blue and alcian blue. Nuclear extrusions (nuage) stained positive to Sudan black B and Oil Red O. Extraneous yolk granules stained positive to nile blue, alcian blue, bromophenol blue and PAS. The findings reveal that oogonial proliferation and their differentiation into oocytes (oogenesis) are prevalent in adult ovary; yolk contains nuclear extrusions (RNA?) and yolk granules deposited from the periphery of the oocyte.