We have examined changes in chromatin organization, meiotic status and the acquisition of meiotic competence in oocytes retained within mouse ovarian follicles for 0 - 6 days in culture. Preantral follicles from 22-24 day old (prepubertal) or 31 day old (peripubertal) mice develop antra and undergo significant growth from 0 - 4 days in culture after which the growth rates slow down. Oocyte meiotic status was evaluated by chromatin organization at day 0 and day 6 of follicle culture and the effects of a 24 hour exposure to LH during the last day of culture were determined. In prepubertal mice, meiotic arrest was maintained after 6 days and the majority of germinal vesicles (GVs) exhibited a transition from unrimmed to rimmed nucleoli. LH caused meiotic progression of intrafollicular oocytes. In peripubertal mice, a partial transition from unrimmed to rimmed stages occurred in GV stage oocytes and meiotic arrest was not maintained as well as in follicles from prepubertal mice. The transition from rimmmed to unrimmed GV stages was found to occur between days 3 and 4 of follicle culture and was not influenced by exposure to LH. LH treatment, however, did cause an increase in the proportion of intrafollicular oocytes resuming meiosis. In vitro assays of meiotic competence in oocytes retrieved from cultured follicles demonstrated that the transition from an unrimmed to a rimmed state at day 3 to day 4 of culture is strongly correlated with the acquisition and expression of meiotic competence. These results indicate that the ovarian culture system employed in these studies supports the transformation of enclosed oocytes from a pre-competent to a competent state and can maintain meiotic arrest for up to 6 days in culture. Treatment with LH overrides the mechanism of meiotic arrest and may interfere with the ability of enclosed oocytes to complete meiotic arrest.
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