Ultrastructural study of nucleolar changes in rat embryos during diapause and reactivation.

Changes in nucleolar ultrastructure were studied in preimplantation rat embryos before diapause (d 5), during diapause (d 6, 7 and 8) and after reactivation brought about either by in vitro culture for 24 h (d 7 and 8) or by estradiol-17 beta treatment of the mothers (d 11). Before diapause, fully developed nucleoli contained several low-density fibrillar centers surrounded by a dense fibrillar component and an abundant granular component. This type of nucleolus indicates a high activity of ribosomal RNA synthesis. During diapause, nucleoli revealed a disorganization of the fibrillar and granular elements typical of a diminution in transcriptional activity. During reactivation, nucleoli progressively returned to a reticulo-fibrillar configuration characteristic of the onset of intense transcriptional activity. It is concluded that the structure of the nucleolus in rat preimplantation embryos corresponds to the level of transcriptional activity and is a reliable model for studying structure-function relationships during early development.