Identification of the Ovulatory Follicle in the Ewe: Associated Changes in Follicular Size, Thecal and Granulosa Cell Luteinizing Hormone Receptors, Antral Fluid Steroids, and Circulating Hormones during the Preovulatory Period*

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Abstract
The presence of LH receptors in both thecal and granulosa cells appears to be required in preovulatory follicles. Since Finn and Suffolk sheep have significantly different ovulation rates, the number of Graafian follicles with LH receptors in both cell types was investigated to determine the relationship between the number of corpora lutea (CL) per ewe and the number of follicles with LH receptors in both cell types. Follicles with this characteristic LH binding were classified as ovulatory. All other follicles were considered to be nonovulatory. Follicular size, LH/hCG receptor concentrations in thecal and granulose cells, and antral fluid (AF) steroid concentrations were studied in ovulatory and nonovulatory follicles at two periods of the follicular phase of the estrous cycle in Finn and Suffolk sheep. Sheep were studied either before preovulatory gonadotropin surge or during the ascending limb of the preovulatory gonadotropin surge. The mean ovulation rate (2.7 CL per Finn ewe and 1.3 CL per Suffolk ewe) was equal to the mean number of ovulatory follicles on a pair of ovaries in each of the two breeds of sheep. Significantly more estradiol was secreted by the ovary containing the ovulatory follicle than from the other ovary of the pair. The rate of ovarian estradiol secretion and LH/hCG receptor levels in the thecal and granulosa cells of the ovulatory follicle increased as the preovulatory period progressed, and both were accompanied by increasing peripheral LH levels. The ovulatory follicle was significantly larger than nonovulatory follicles, and in ovulatory follicles there was a significant correlation between AF estradiol concentrations and the number of LH/hCG receptors in thecal and granulosa cells. There was no increase in granulosa cell LH/hCG receptor levels or rate of estradiol secretion in nonovulatory follicles during the preovulatory rise in circulating LH. In contrast, AF testosterone increased significantly in nonovulatory follicles. In summary, the presence of LH/hCG receptors in thecal and granulosa cells was found to be a useful marker for the identification of the ovulatory follicle in the ewe. It is suggested that the increase in concentrations of thecal and granulosa cell LH receptors during the preovulatory period in the ewe is part of the final maturational process that ensures that ovulation will occur in response to the LH surge.

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