Development competence and relative transcript abundance of oocytes derived from small and medium follicles of prepubertal gilts

The objective of this study was to examine the competence of mature oocytes aspirated from small follicles (SF, <2 mm in diameter) and medium follicles (MF, 3–6 mm) of abattoir-derived prepubertal gilt ovaries. Oocytes were selected by the presence of the first polar body (1pb) after IVM in a chemically defined medium, for sperm penetration, pronuclear formation, cleavage rate, and development to the blastocyst stage. Relative transcript abundance of genes associated with regulation of oocyte maturation (AURKA, AURKB, and MOS), fertilization (ZP3 and ZP4), maternal effect (NALP9 and HSF1), and anti-apoptosis (BCL2) were also examined in oocytes at germinal vesicle (GV) and metaphase-II (MII) stages. In SF, compared with MF, the maturation rate post-IVM was lower (P < 0.05), but there were no differences in sperm penetration rate (78.2% and 68.5% at 6 hours after insemination and 90.8% and 91.9% at 9 hours after insemination, P = 0.51 and P = 0.67, respectively), the percentage of oocytes that formed both female and male pronuclei (27.9% and 25.8% at 6 hours after insemination and 79.4% and 76.1% at 9 hours after insemination), or cleavage rate at 48 hours after insemination (85.9% and 89.7%, respectively, P = 0.46), whereas blastocyst formation rate was lower (P < 0.05) in oocytes from SF versus MF (14.7% and 31.0%). Transcript abundances decreased (P < 0.05) in all genes examined between the GV and MII stages, although only transcript abundance for MOS was lower (P < 0.05) in GV oocytes from SF versus MF. In conclusion, mature oocytes from SF and MF of prepubertal gilts with a visible 1pb had similar fertilizability in vitro and relative transcript abundance of nine genes. However, follicle size affected meiotic competence, early embryonic development to the blastocyst stage, and transcript abundance of the MOS gene.