Bovine oocyte diameter in relation to maturational competence and transcriptional activity

The aims of the present series of experiments were to establish a possible relationship between bovine oocyte diameter and follicle size, investigate the developmental ability of oocytes of different diameter groups, and investigate the relationship between oocyte diameter and RNA transcriptional activity of the oocyte. Follicles were recovered from slaughterhouse ovaries by mechanical dissection, measured, and assigned to one of the following size categories: > or = 4 mm, < 3-4 mm, < 2-3 mm, < 1-2 mm, and < 1 mm. Oocytes were liberated from the follicles and their diameters recorded. The correlation coefficient between oocyte diameter and follicle size was 0.32 (P < 0.0001). Oocytes were recovered by aspiration and/or slicing of slaughterhouse ovaries and divided into four groups ( < 100 microns, 100- < 110 microns, 110- < 120 microns, and > 120 microns) based on diameter. Oocytes were processed through standard procedures for in vitro maturation and stained in order to assess nuclear development. Rates of in vitro development to metaphase II were 21.2%, 42.3%, 75.9%, and 80.7%, respectively, for the four groups. On a separate occasion immature oocytes from the above diameter groups were cultured in the presence of 3H-uridine for 45 min and scored for degree of RNA synthesis as indicated by the presence of autoradiographic labeling. Oocytes < 110 microns showed a greater degree of 3H-uridine incorporation than those > or = 110 microns, suggesting that they were involved in RNA synthesis and therefore still in the growth phase.