Morphological characterization and meiotic competence of oocytes collected from filly ovaries

The effect of filly age on morphology of the ovaries, collected oocytes and their capacity for in vitro maturation (IVM) was examined. The ovaries of slaughtered fillies were classified into three groups, according to filly age: (I) <10 month old (<10MF); (II) approximately 1 year old (1YF); and (III) approximately 1.5 year old (1.5YF). The ovaries of mares were used as a control group. Ovarian morphology and collected oocytes were evaluated. Only oocytes with expanded (Ex) and compact (Cm) cumuli were used for IVM. In <10MF, 1YF, 1.5YF and mare groups, corpora lutea were found in the ovaries of 9.3%, 36.7%, 59.6% and 80.9% females, respectively (P < 0.001). Based on this observation, we found that about 37% of fillies reach puberty at approximately 12 months of age. No relationship was found between filly age and morphology of the oocytes obtained. In comparison to mares, fewer (P < 0.05) Cm oocytes were collected from filly ovaries. Among Cm groups, fewer filly (28.4-35.5%) than mare oocytes (50.0%) reached metaphase II stage, but the difference was only significant when compared to oocytes of the <10MF group (P < 0.05). In the Ex groups, a similar proportion of oocytes of fillies (40.8-51.1%) and mares (48.4%) attained the metaphase II stage. In conclusion, in the culture conditions applied, Cm oocytes of fillies younger than 10 months showed lower meiotic competence than mare oocytes. Oocytes of older fillies showed meiotic competence similar (P > 0.05) to mare oocytes. Both filly and mare oocytes with expanded cumuli displayed the same capacity for IVM.