Ultrastructural and cytochemical studies on extranucleolar bodies in rat oocytes at the preovulatory follicle stage.

At the antral follicle stage, the nucleolus is entirely composed of a homogeneous proteinic compact mass. This nucleolar compaction during oogenesis seems to be a general feature in mammalian oocytes. However, when oocyte maturation is induced by gonadotropin hormone (LH), oocytes enter into preovulatory stage. All the nucleoli are vacuolated and extranucleolar bodies appear in the germinal vesicle near the nucleolar mass. Based on the results obtained by ultrastructural cytochemical stainings, we postulate that these extranucleolar bodies originate from the nucleolar mass itself. The presence of the extranucleolar bodies could reflect the extrusion of nucleolar material, essentially ribonucleoproteins, into the ooplasm. This material could persist after fertilization in the pronuclei until the resumption of transcription at the early stage of embryogenesis.