Oocyte quality is a key limiting factor in female fertility, yet we have a poor understanding of what constitutes oocyte quality or the mechanisms governing it. The ovarian follicular microenvironment and maternal signals, mediated primarily through granulosa cells (GCs) and cumulus cells (CCs), are responsible for nurturing oocyte growth, development and the gradual acquisition of oocyte developmental competence. However, oocyte-GC/CC communication is bidirectional with the oocyte secreting potent growth factors that act locally to direct the differentiation and function of CCs. Two important oocyte-secreted factors (OSFs) are growth-differentiation factor 9 and bone morphogenetic protein 15, which activate signaling pathways in CCs to regulate key genes and cellular processes required for CC differentiation and for CCs to maintain their distinctive phenotype. Hence, oocytes appear to tightly control their neighboring somatic cells, directing them to perform functions required for appropriate development of the oocyte. This oocyte-CC regulatory loop and the capacity of oocytes to regulate their own microenvironment by OSFs may constitute important components of oocyte quality. In support of this notion, it has recently been demonstrated that supplementing oocyte in vitro maturation (IVM) media with exogenous OSFs improves oocyte developmental potential, as evidenced by enhanced pre- and post-implantation embryo development. This new perspective on oocyte-CC interactions is improving our knowledge of the processes regulating oocyte quality, which is likely to have a number of applications, including improving the efficiency of clinical IVM and thereby providing new options for the treatment of infertility.

The environment that the cumulus oocyte complex (COC) is exposed to during either in vivo or in vitro maturation (IVM) can have profound effects on the success of fertilisation and subsequent embryo development. Glucose is a pivotal metabolite for the COC and is metabolised by glycolysis, the pentose phosphate pathway (PPP), the hexosamine biosynthesis pathway (HBP) and the polyol pathway. Over the course of oocyte maturation, a large proportion of total glucose is metabolised via the glycolytic pathway to provide substrates such as pyruvate for energy production. Glucose is also the substrate for many cellular functions during oocyte maturation, including regulation of nuclear maturation and redox state via the PPP and for the synthesis of substrates of extracellular matrices (cumulus expansion) and O-linked glycosylation (cell signalling) via the HBP. However, the oocyte is susceptible to glucose concentration-dependent perturbations in nuclear and cytoplasmic maturation, leading to poor embryonic development post-fertilisation. For example, glucose concentrations either too high or too low result in precocious resumption of nuclear maturation. This review will discuss the relevant pathways of glucose metabolism by COCs during in vivo maturation and IVM, including the relative contribution of the somatic and gamete compartments of the COC to glucose metabolism. The consequences of exposing COCs to abnormal glucose concentrations will also be examined, either during IVM or by altered maternal environments, such as during hyperglycaemia induced by diabetes and obesity.Reproduction (2010) 139 685-695

Oocyte and embryo metabolism are closely linked with their subsequent developmental capacity. Lipids are a potent source of cellular energy, yet little is known about lipid metabolism during oocyte maturation and early embryo development. Generation of ATP from lipids occurs within mitochondria via beta-oxidation of fatty acids, with the rate-limiting step catalyzed by carnitine palmitoyl transferase I (CPT1B), a process also requiring carnitine. We sought to investigate the regulation and role of beta-oxidation during oocyte maturation and preimplantation development. Expression of Cpt1b mRNA, assessed by real-time RT-PCR in murine cumulus-oocyte complexes (COCs), increased following hormonal induction of oocyte maturation and ovulation in vivo with human chorionic gonadotropin (5 IU) and in embryos reaching the blastocyst stage. Beta-oxidation, measured by the production of (3)H(2)O from [(3)H]palmitic acid, was significantly increased over that in immature COCs following induction of maturation in vitro with epidermal growth factor (3 ng/ml) and follicle-stimulating hormone (50 mIU/ml). The importance of lipid metabolism for oocyte developmental competence and early embryo development was demonstrated by assessing the rate of embryo development following inhibition or upregulation of beta-oxidation with etomoxir (an inhibitor of CPT1B) or L-carnitine, respectively. Inhibition of beta-oxidation during oocyte maturation or zygote cleavage impaired subsequent blastocyst development. In contrast, L-carnitine supplementation during oocyte maturation significantly increased beta-oxidation, improved developmental competence, and in the absence of a carbohydrate energy supply, significantly increased 2-cell cleavage. Thus, carnitine is an important cofactor for developing oocytes, and fatty acids are an important energy source for oocyte and embryo development.

The capacity of fully grown oocytes to regulate their own microenvironment by paracrine factors secreted by the oocyte (oocyte-secreted factors, OSFs) may in turn contribute to oocyte developmental competence. Here, we investigated if OSFs have a direct influence on oocyte developmental competence during in vitro maturation (IVM). Bovine cumulus-oocyte complexes (COCs) were aspirated from abattoir-derived ovaries and matured in serum-free medium. COCs were either co-cultured with denuded oocytes (DOs) or treated with specific OSFs: recombinant bone morphogenetic protein 15 (BMP15) and/or growth differentiation factor 9 (GDF9). Following maturation, embryos were fertilized and cultured in vitro and blastocyst development and cell number were assessed on day 8. Co-culturing intact COCs with DOs did not affect cleavage rate, but increased (P<0.001) the proportion of cleaved embryos that reached the blastocyst stage post-insemination from 39% to 51%. OSFs also altered blastocyst cell allocation as co-culture of COCs with DOs significantly increased total and trophectoderm cell numbers, compared to control COCs. BMP15 alone, GDF9 alone or the two combined all (P<0.05) increased the proportion of oocytes that reached the blastocyst stage post-insemination from 41% (controls) to 58%, 50% and 55%, respectively. These results were further verified in neutralization experiments of the exogenous growth factors and of the native OSFs. Follistatin and the kinase inhibitor SB-431542, which antagonize BMP15 and GDF9, respectively, neutralized the stimulatory effects of the exogenous growth factors and impaired the developmental competence of control COCs. These results demonstrate that OSFs, and particularly BMP15 and GDF9, enhance oocyte developmental competence and provide evidence that OSF regulation of the COC microenvironment is an important determinant of oocyte developmental programming.