Oocyte growth, capacitation and final maturation in cattle

Abstract The oocyte growth phase includes a series of modulations of organelles and inclusions, as well as a period of oocyte transcription, which are necessary for the oocyte to achieve meiotic and developmental competence. Oocyte transcription including nucleolar function (rRNA-synthesis) is activated in the secondary follicle and is maintained up to an oocyte diameter of about 110 μm in the 3 mm tertiary follicle. At a diameter of 100 to 110 μm, the oocyte gradually achieves the competence to undergo meiotic maturation and sustain embryonic development. In the dominant follicle the oocyte undergoes further ultrastructural modifications and attains full developmental competence through a process that may be termed “capacitation”. The final oocyte maturation up to metaphase II after LH stimulation of the ovulatory follicle is the culmination of the previous processes and equips the oocyte with a haploid chromosomal compartment and the cell biological apparatus specialized for fertilization and initial embryonic development.