Microarray Analysis of Bovine Oocyte from Distinct Follicle Sizes Reveals Gradual Transcripts Accumulation.

In vitro maturation and in vitro fertilization of bovine oocyte obtained from slaughterhouse-derived ovaries is a routine procedure in many labs to study various questions related to reproductive biology. However, even after more than 25 years of improvements of this system, from optimization of culture media to morphological selection of the best cumulus-oocyte complex, success rate remains quite low with an average blastocyst rate around 30%. One model of oocyte competence implies the follicular size where the oocyte originates. Effectively, higher blastocyst rate are obtained with oocyte from larger follicles compare to the smaller one. Furthermore, it is well known that during folliculogenesis, the oocyte needs to accumulate all the transcripts needed to ensure the development until the maternal to embryonic transition. Therefore, the aim of this project is to compare the transcriptome of oocyte originating from follicle of specific sizes in a sequential way. Bovine slaughterhouse-derived ovaries were used and oocytes were collected and divided in four groups: oocytes from follicles < 3 mm of diameter; 3 to 5 mm; 5 to 8 mm and > 8 mm. Three pools of ten oocytes per group were used to perform a transcriptomic analysis with a bovine embryo-specific 44K Agilent slide (EmbryoGene). We used a reference design (<3 mm group as a reference) to compare with the three other groups. Concerning the differentially expressed genes, we found 12, 710 and 240 probes up or down-regulated (fold-change >2; p < 0.05) in the contrast <3mm vs 3-5mm; <3mm vs 5-8mm and <3mm vs >8mm respectively. When we looked at the number of probes that are common between these three contrasts, we found 1667 probes in common (fold change ≠1, p<0.05). Interestingly, the majority of these probes appears to change either in one direction or in other across the follicular stages studied (812 probes with a positive fold change for three contrasts and 722 probes with a negative fold change in three contrasts). For example, transcriptional adaptor 1 (TADA1), which is part of a chromatin-modifying complex, showed constant increase at the transcript level with fold change >1.5 in each contrast. While within bgcn homolog (WIBG), a key regulator of the exon jonction complex known to enhance translation of spliced mRNAs, showed constant decrease of its mRNA level along the follicular growth. These results suggest the relative similarity between the first two groups (<3mm and 3-5mm) while the other two groups reveals increasing differences compared to the reference. The majority of changes in term of differentially expressed genes in the oocyte during those stages of follicular growth seem to follow a general trend (global increase or decrease), which reinforces the idea that the competence acquisition is a gradual process. So, a comprehensive transcriptomic analysis of the oocyte from different follicle size in a sequential scheme will help us to better understand the competence acquisition during the follicular growth in an in vitro system.