Changes in ovarian, follicular, and oocyte morphology immediately after the onset of puberty are not accompanied by an increase in oocyte developmental competence in the pig

Abstract

This study was conducted to determine whether ovarian morphology and developmental competence of in vitro-matured (IVM) oocytes is immediately affected by the onset of puberty in the pig. Ovaries of peri-pubertal pigs were sorted into two groups according to the presence or absence of corpora lutea presence (CL and NCL, respectively. Ovary dimensions, follicle diameter and number, and oocyte diameter (with and without zona pellucidae) were determined. The developmental competence of in vitro-matured oocytes from these two groups was evaluated following parthenogenetic activation and culture in vitro. CL ovaries were significantly ( P<0.01) larger than NCL ovaries (width: 22.3±0.9 mm versus 15.9±0.4 mm, length: 33.2±1 mm versus 24.1±0.4 mm). Although CL ovaries had fewer antral follicles in total compared with NCL ovaries (21.1±1.8 mm versus 46.8±2.2 mm), they had a similar number of follicles 3–8 mm in diameter. The mean diameter of follicles that were aspirated was greater for CL ovaries than for NCL ovaries (4.5±0.1 mm versus 3.3±0.02 mm). Oocytes from CL ovaries were greater in diameter compared with those from NCL ovaries (zona retained: 159±1.3 μm versus 146.1±1.5 μm, zona free: 124.7±1.8 μm versus 113.1±1.6 μm). No differences were found between oocytes from CL and NCL ovaries for rates of meiotic maturation (91.6±3.2% versus 92.4±3.2%), cleavage (88.4±11% versus 90.7±2.6%) and blastocyst formation (21.0±3.7% versus 23.7±5.7%). Therefore, the onset of puberty coincides with immediate changes in ovarian morphology, increased ovary size, follicle and oocyte diameter, but not with improved oocyte developmental competence. This suggests that the higher developmental competence usually observed in adult oocytes is acquired gradually and requires exposure to multiple estrus cycles.