Transcriptional activity and ultrastructure in bovine oocytes

The oocyte growth phase includes development of organelles and inclusions a s well as a period of oocyte transcription. Oocyte transcription including nucleolar function (rRNA-synthesis) is activated in the secondary follicle and is maintained up to an oocyte diameter of about 110 mm in the 3 mm tert iary follicle. At a diameter of 100 to 110 mm, the oocyte gradually achieve s the competence to undergo meiotic maturation and sustain embryonic develo pment. Ln the dominant follicle the oocyte undergoes further ultrastructura l modifications through a process that we refer to as "capacitation". The f inal oocyte maturation up to metaphase Pi after LH stimulation of the ovula tory follicle is the culmination of the previous processes and equips the o ocyte with the cell biological apparatus specialized for fertilization and initial embryonic development.