Combinations of genetic, environmental, and management factors are suspected to explain the loss in fertility observed for over 20 years in dairy cows. In some cases, IVF is used. When compared with in vivo embryo production, IVF resulted in low success rates until the FSH coasting process (FSH starvation after superstimulation) was introduced in 2002. Increased competence associated with FSH withdrawal of aspirated oocyte for in vitro maturation and IVF has not been optimized nor explained yet. The goal here was to determine and characterize the optimal oocyte competence acquisition window during the coasting period by determining blastocyst rates and follicular cohort development. Commercial milking cycling cows (nZ6) were stimulated with 3 days of FSH (6!40 mg NIH Folltropin-V given at 12 h intervals) followed by a coasting period of 20, 44, 68, or 92 h. Each animal was exposed to the four conditions and served as its own control. At the scheduled time, transvaginal aspirations of immature oocytes were performed followed by IVF of half the oocytes. The outcomes were as follows: i) FSH coasting was optimal at a defined period: between 44 and 68 h of coasting; ii) The best estimated coasting duration was w54G7 h; iii) Under these conditions, the best statistical blastocyst rate estimation was w70%; iv) Between 44 and 68 h of coasting, follicle size group proportions were similar; v) Follicle diameter was not linearly associated with competence. In conclusion, coasting duration is critical to harvest the oocytes at the right moment of follicular differentiation.

Various morphological and cytological traits of oocytes and their surrounding cumulus cells may be used to select oocytes for assisted reproduction. However, even with careful selection, successful IVF and subsequent embryo development remain uncertain. The factors that ensure oocyte competence are unclear and other approaches to assessing developmental potential must be explored. With the constant development of the molecular toolbox, genomic/transcriptomic analysis is becoming a more and more interesting approach to understand oocyte quality on the basis of RNA composition. Using bovine and mouse models as well as human oocytes of known developmental potential, various efforts are underway to characterize the mRNA profile of the competent oocyte using microarray technology. The proliferation of gene expression data sets raises new opportunities to identify the mechanisms involved in this complex phenotype, which should lead to improved techniques of assisted reproduction. Although several molecular markers of oocyte quality are known, translating these into cellular functions remains challenging, largely due to the poor correlation between mRNA level and protein synthesis. Unlike most somatic cells, the oocyte can store mRNA for days, with transcriptional activity remaining at a halt during the 4-5 days beginning before ovulation and ending with embryonic genome activation. This review provides an overview of the transcriptomic data obtained from oocytes of different quality as well as interesting avenues to explore in order to improve our understanding of oocyte competence.

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Follicle size is recognized as a predictor of the potential for the enclosed oocyte to yield an embryo following in vitro maturation and in vitro fertilization. Oocytes from larger follicles are more likely to reach the blastocyst stage than those from smaller follicles. A growing oocyte accumulates all the transcripts needed to ensure development until the maternal embryonic transition, and this accumulation must be completed before the period of transcriptional arrest. Accordingly, the transcriptomes of bovine germinal-vesicle-stage oocytes collected from follicles of increasing sizes (<3, 3-5, >5-8, and >8 mm) were evaluated, using the EmbryoGENE bovine transcriptomic platform (custom Agilent 4 × 44 K), to better understand transcriptional modulation in the oocyte as the follicle becomes larger. Microarray analyses revealed very few differences between oocytes from small follicles (<3 vs. 3-5 mm), whereas an important number of differences were detected at the mRNA level between oocytes from larger follicles. Weighted gene correlation network analysis allowed for the identification of several hub genes involved in crucial functions such as transcriptional regulation (TAF2), chromatin remodeling (PPP1CB), energy production (SLC25A31), as well as transport of key molecules within the cell (NAGPA, CYHR1, and SLC3A12). The results presented here thus reinforce the hypothesis that developmental competence acquisition cannot be seen as a simple one-step process, especially in regards to the modulation of mRNA. Mol. Reprod. Dev. 83: 558-569, 2016. © 2016 Wiley Periodicals, Inc.