Oogenesis in pig ovaries during the prenatal period: ultrastructure and morphometry

Oogenesis in fetal pig ovaries comprises the successive changes from the primordial germ cells to the dictyotene oocytes in primordial ovarian follicles. In this study the observations were carried out with an electron microscope and stereological analysis was performed. At the ultrastructural level there are no differences between the primordial germ cells and oogonia, but oogonia are connected with the intercellular bridges. The onset of the dictyotene phase was accompanied by the changes in the cytoplasm of oocytes. Near the nucleus, the yolk nucleus is formed containing numerous Golgi bodies, endoplasmic reticulum (ER), mitochondria and granules. ER proliferates in contact with the external leaflet of the nuclear envelope forming the narrow ER cisterns. Between the nuclear envelope and ER cisterns, the vesicles with grey content are visible. The proliferating ER forms numerous concentric cisterns around the nucleus. Next, the most external cisterns fragment, detach, and then form the cup-like structures. These structures separate the distinct areas of cytoplasm-compartments, which contain mitochondria, ribosomes and lipid droplets. The cells of cortical sex cords of the ovary, which encloses the oocyte, form the follicles. The volume of oocytes in forming follicle increases due to the increase in the number of the cell inclusions: lipid droplets, vacuoles and yolk globules. In the oocytes of primordial ovarian follicles, the compartments are transformed into the yolk globules, which are encountered by a sheath of ER cisterns and the grey vesicles; they contain the mitochondria, lipid droplets and light vacuoles. The role of the compartments and yolk globules as metabolic units is discussed in comparison with similar structures of the mature eggs of pigs and other mammal species.