Follicular Growth and Atresia during the Last Half of the Luteal Phase of the Guinea Pig Estrous Cycle: Relation to Serum Progesterone and Estradiol Levels and Utero-Ovarian Blood Flow*

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Abstract
The relationships among follicular growth, changes in serum progesterone (P) and estradiol (E) levels, and utero-ovarian blood flow (OBF) through the guinea pig ovary were examined during days 7-16 of the luteal phase (day 0 = estrus) of the estrous cycle and during the subsequent ovulatory (proestrous-estrous) period. Follicles were classified as either viable or atretic based on strict criteria and grouped according to diameter. No changes in follicular growth were observed between days 7-9 of the cycle when serum P levels were elevated and OBF was at low rates. Between days 13-15 when serum P levels were low and both OBF and serum E levels were rising, there was a dramatic increase in the number of viable follicles present in all follicle classes. As the percentage of viable follicles increased, the number of atretic follicles in each size population decreased. Peaks in the number of large follicles (300-450 .mu.m), OBF, and serum E levels were observed during the subsequent ovulatory period. As luteal activity declines during the last half of the estrous cycle, follicular recruitment and growth are stimulated. The concomitant elevations in OBF suggest a supportive role for this parameter in follicular development. In turn, the subsequent elevation in serum E levels serves as an index of follicular maturation. The elevated P levels during the luteal phase of the estrous cycle may either directly or indirectly, through the regulation of gonadotropin secretion, regulate follicular growth in the guinea pig.

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