SEM cryofracture study of ovarian follicles of immature rats

Abstract

Follicles were examined with the scanning electron microscope (SEM) following preparation of cryofractured specimens of rat ovaries. After perfusion fixation with 2.5% glutaraldehyde buffered at pH 7.4 with 0.1M cacodylate, ovaries were trimmed of fat and decapsulated, fixed for an additional 12 hours, washed in distilled H2O for 20 hours and postfixed in 1% OsO4 in distilled H2O for 30 minutes. Following a brief wash, ovaries were dehydrated in a linear gradient of dist. H2O/ethanol, frozen in Freon 22, cryofractured under liquid nitrogen, brought to 1 degree C in ethanol, and critical point dried from CO2. Follicles cleaved through their oocytes were examined with SEM to determine the ultrastructural characteristics of developing and atretic follicles revealed by cryofracture. Cytoplasmic arrays of parallel sheets, nucleoli and microvilli were prominent structures of cryofractured oocytes. Numerous cytoplasmic projections of granulosa cells and oocyte microvilli penetrated the zonae pellucida of preantral follicles. The numbers of oocyte microvilli were greatly diminished in large antral follicles. Other structures made prominent by cryofracture included cell nuclei, a microvillous border at the granulosa-theca boundary, and numerous holes in the cytoplasm of thecal cells which correspond in size and distribution to the liquid droplets in these cells. The soluble proteins and glycoproteins of the follicular fluid were visualized as a homogeneous granular precipitate. The disappearance of oocyte microvilli and granulosa cell projections from the zonae pellucida and the blebbing of granulosa cells at the surface of the antral cavity appeared characteristic of an early stage of follicular atresia. Increased numbers of holes in thecal cells, oocyte fragmentation, and extensive dissociation and fragmentation of granulosa cells were typical of more advanced stages of atresia.