Effect of timing on in vitro growth of bovine oocyte derived from early antral follicle

Abstract

There are thousands of follicles in bovine ovaries, however, it is solely restricted number of oocytes enable to sufficiently obtain growth phase and reach final measure to become fully-grown oocyte. In the natural physiologic environment, most of the follicles residing in the ovaries have tendency to arrest at the germinal vesicle (GV) stage, while some have been undergoing a degeneration process named Atresia. Therefore, an in vitro growth (IVG) system has been established in which growing oocytes are adequately supported to reach maturity and acquire completely meiotic developmental competence. Furthermore, an inhibitor known as adenosine (ADO), has been introduced in in vitro system to impede meiotic resumption and eventually prepares oocytes complete materials to achieve maturation stage. This study was conducted to investigate the optimal time for early antral follicles derived from 1- 2mm in diameter and the effects of ADO on growing oocytes cultured in different IVG durations. Oocyte-cumulus granulosa cell complexes (COGCs) and oocyte-cumulus cell complexes (COCs) are collected from early antral follicles were examined in four IVG durations, 1 day, 2 days and 3 days, respectively, and treated with two concentrations of ADO – 4mM (ADO4) and 8mM (ADO8), compared to control group (without IVG culture). The results indicated that growing oocyte measure reached 106.29 ± 0.98µm, 112.74 ± 0.51µm, 113.32 ± 0.47µm after 1 day, 2 days and 3 days of growth culture. Moreover, it was also clearly shown the poverty in cumulusgranulosa cell proliferation in all treated groups. Besides, there was a significant difference 1-day, 2-day, and 3-day culture groups in term of meiotic morphology. The percentage of oocytes arrested at FC, SC, GVI, GVII-IV, and MI after 1-day IVG was 8.11%, 8.11%, 18.92%, 48.65%, and 10.81%, compared with 2-day IVG (17.86%, 3.57%, 0.00%, 64.29%, and 7.17%). On the other hand, the entire of IVG oocytes after 3 days were in GVI-IV and MI stage, 60% and 16.67%, respectively. In conclusion, it was clearly seen that meiotic-resumed activity spontaneously occurs during IVG culture that leads to the requirement of higher concentration of adenosine for impeding.