CHROMATIN ORGANIZATION, MEIOTIC STATUS AND MEIOTIC COMPETENCE ACQUISITION IN MOUSE OOCYTES FROM CULTURED OVARIAN FOLLICLES

Changes in chromatin organization, meiotic status and the development of meiotic competence in oocytes retained within mouse ovarian follicl es from day 0 to day 6 in culture were examined. The effects of exposu re for 24 h to human luteinizing hormone (hLH) during the last day in culture was also determined. Preantral follicles from 22- to 24-day-ol d (prepubertal) mice develop antra and undergo significant growth from day 0 to day 4 in culture, after which the growth rates slow. The sta tistical significance of meiotic progression was examined using exact logistical regression analysis, which is particularly useful when the data are sparse and unbalanced. The transition from rimmed to unrimmed germinal vesicle stages was found to occur between day 2 and day 4 of follicle culture and was not influenced by exposure to hLH. Treatment with hLH caused a significant increase in the proportion of intrafoll icular oocytes resuming meiosis. Assays of meiotic competence performe d in vitro in oocytes retrieved from cultured follicles demonstrated t hat the transition from an unrimmed to a rimmed state is closely coinc ident with the acquisition and expression of meiotic competence. Forty -six per cent of competent oocytes from follicle cultures at day 3 pro gressed to metaphase II. These results indicate that the follicle cult ure system used in these studies supports the transformation of enclos ed oocytes from a precompetent to a competent state and can maintain m eiotic arrest for up to 6 days in culture. However, an increasing prop ortion of oocytes exhibit abnormal meiotic progression with continued follicle culture beyond 4 days.