Oocyte Development in Cattle: Factors Affecting Competence.

Abstract

The development of an oocyte which is competent to mature, be fertilized and sustain development until the embryonic genome takes control is a protracted process spanning foetal, pubertal and adult life which can be affected by factors such as nutrition, hormonal regulation, and environmental influence. By reviewing the biochemical and morphological profiles of the oocyte and its surrounding follicle as it develops from the primordial up to peri-ovulatory stage, we have concluded that the key transition stages of bovine oocyte and follicle development are the secondary follicle stage, mid antral stage and the peri-ovulatory period. In cattle, the secondary follicle stage is characterized by the activation of the oocyte transcriptome, deposition of the zona pellucida and the establishment of bidirectional communication between the granulosa cells and the oocyte, the tertiary or antral follicle stage is associated with the proliferation of oocyte organelles, intensive mRNA and rRNA transcription and the establishment of the maternal imprints and the periovulatory period is characterised by the extensive degradation or polyadenylation of mRNA transcripts, resumption of meiosis, spindle formation, chromosome alignment and segregation.We have also studied the global transcriptomic profiles of mature oocytes from several mammalian species and identified a number of common pathways that appear to be associated with the acquisition of oocyte development competence across species, these include WNT, Notch, and β-catenin signalling, polyadenylation and ubiquitinization, apoptosis, cytoskeleton rearrangement and cell cycle regulation. As a number of these pathways are regulated by progesterone (P4) responsive genes in other tissues and given the critical role of P4 in triggering oocyte maturation in frog and fish oocytes, we have investigated the role of P4 signaling during bovine oocyte maturation. Our studies indicate that P4 synthesis by the cumulus cells surrounding the oocyte during maturation is associated with oocyte competence. Initial investigations suggest that P4 protects the oocyte from the onset of apoptosis and that this is mediated via the classical progesterone receptor. However, it is likely that the affects of P4 are not confined to one pathway but are due to the regulation of some of the pathways and processes mentioned above, particularly during oocyte maturation. By understanding the essential changes that occur in the molecular and morphological profiles of the oocyte and its surrounding follicle and by identifying the key regulatory factors, we can identify critical checkpoints that may flag insult or injury to the oocyte or can be targeted to improve oocyte quality and developmental competence.