NUCLEUS ULTRASTRUCTURE AND TRANSCRIPTIONAL ACTIVITY OF BOVINE OOCYTESIN 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. Th e aim of the present study was to describe the nuclear ultrastructure and transcriptional activity of primordial to early tertiary follicula r oocytes from bovine adult ovaries. Small blocks of ovarian cortex we re incubated in medium enriched with H-3-uridine for 30 min. Subsequen tly, the tissue blocks were fixed in Karnowsky's fixative, dehydrated, epon embedded, sectioned (2 mu m), processed for autoradiography, and examined under light microscopy. Sections showing preantral follicles with presumptive oocyte nucleoli were reembedded for transmission ele ctron 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 cub oidal granulosa cells, 3) primary, with a single layer of cuboidal gra nulosa cells, 4) secondary, with a complete or incomplete bilayer of c uboidal cells, and 5) tertiary,with more than two layers of granulosa cells delineating one or more intercellular cavities. The granulosa ce lls of all follicle classes were transcriptionally active. However, th e oocytes did not display transcriptional activity, as measured by the present means, until the secondary and tertiary follicular stages. Th e oocyte nucleolus was granular in the primordial follicles. Following follicular activation, fibrillar centres invaded the nucleolus and, i n the early tertiary follicle, numerous fibrillar centres were distrib uted throughout the nucleolus. In conclusion, the oocyte nucleolar fun ction is gradually activated at follicle activation, and oocyte transc ription is initiated at approximately the time of the secondary follic le stage. 46:208-215, 1997. (C) 1997 Wiley-Liss, Inc.