EFFECT OF MOUSE AGE AND GAP JUNCTION BREAKING OF CUMULUS-OOCYTE COMPLEXES ON OOCYTE MATURATION

The present experiments were performed to evaluate the function of cumulus cells and oocyte itself on mouse oocyte maturation in vitro. The cumulus enclosed oocyte (CEO) were isolated from immature mice stimulated with Humegon 48 hours previously and cultured in hypoxanthine (HX) medium. Three experiments were designed: (1) CEO and denuded oocyte (DO) from different age group were compared by the percentage of their germinal vesicle breakdown (GVBD) after 24 hours culture period. (2) The effect of FSH on cumu- lus cell expansion and on oocyte maturation. (3) CEO separation and rejoining or oocytes stripped from their cumulus cells were cultured to compare the function of cumulus cells and oocyte itself on the resumption of meiosis. Experiment 1 showed that with the increase of mouse age, the percentage of GVBD of both CEO and DO decreased after 24 hours culture in HX-medium. When the mouse age was over 25 days, 4mmol HX could maximally keep oocytes in germinal vesicle stage, no difference in GVBD percentage was observed between CEO and DO in each age group. Experiment 2 revealed that FSH induced both cumulus cell expansion and oocyte maturation. 68% of CEO entered into GVBD after FSH stimulation compared with 15% of control. In the last experiment, when the CEO were separated and rejoined the percentage of oocyte maturation significantly increased after the culture period. The GVBD percentage was 45 % in treatment group compared to 18 % in CEO intact group. Whereas no significant GVBD increase was observed in the cumulus stripped oocytes when they were cultured alone. These results suggested that keeping CEO intact is essential for oocyte arresting in meiotic prophase stage. The expansion or separation of cumulus cells from oocytes is linked to the maturation of oocytes and the function of cumulus cell secre- tion.