The role of the proestrous surge of FSH and LH in the initiation of follicular maturation in the cyclic hamster was studied. Blockade of gonadotropin action was confined to the period of the surge by injecting specific antisera to FSH or LH at the beginning of the surge, followed by injection of the respective hormone at the end of the surge to neutralize any persisting excess antibody. The criteria used to assess the effect of such an acute neutralization of FSH or LH surge on follicular development were: 1) the occurrence of ovulation on the expected day of estrus at the end of the next cycle, considering that ovulation is the physiological endpoint of normal follicular development, and 2) the rate of [3H]thymidine incorporation into DNA in vitro by the non-luteal tissue of ovaries on the day after the surge (i.e., estrus). The latter was based on our observation that during the normal estrous cycle, the highest rate of incorporation of [3H]thymidine into ovarian DNA occurred at 1200 h of estrus, about 19 h after the proestrous peak of gonadotropins. Both of these criteria were found to be affected by neutralization of either the surge of FSH or LH on proestrus. Further, the need for LH could not be substituted by exogenous estrogen. The results obtained demonstrate the need for endogenous surges of both FSH and LH for initiation of the normal cyclical process of follicular maturation in adult hamsters.
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