Nucleus ultrastructure and transcriptional activity of bovine oocytes in preantral and early antral follicles.

An understanding of the recruitment and growth of follicles within the bovine ovary is crucial to their successful exploitation in vitro. The aim of the present study was to describe the nuclear ultrastructure and transcriptional activity of primordial to early tertiary follicular oocytes from bovine adult ovaries. Small blocks of ovarian cortex were incubated in medium enriched with 3H-uridine for 30 min. Subsequently, the tissue blocks were fixed in Karnowsky's fixative, dehydrated, epon embedded, sectioned (2 microns), processed for autoradiography, and examined under light microscopy. Sections showing preantral follicles with presumptive oocyte nucleoli were reembedded for transmission electron microscopy. The follicles were divided into five categories: 1) resting primordial, with a single layer of flattened granulosa cells, 2) activated primordial, with a single layer of flattened and some cuboidal granulosa cells, 3) primary, with a single layer of cuboidal granulosa cells, 4) secondary, with a complete or incomplete bilayer of cuboidal cells, and 5) tertiary, with more than two layers of granulosa cells delineating one or more intercellular cavities. The granulosa cells of all follicle classes were transcriptionally active. However, the oocytes did not display transcriptional activity, as measured by the present means, until the secondary and tertiary follicular stages. The oocyte nucleolus was granular in the primordial follicles. Following follicular activation, fibrillar centres invaded the nucleolus and, in the early tertiary follicle, numerous fibrillar centres were distributed throughout the nucleolus. In conclusion, the oocyte nucleolar function is gradually activated at follicle activation, and oocyte transcription is initiated at approximately the time of the secondary follicle stage.