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The follicular phase in pigs: Follicle populations, circulating hormones, follicle factors and oocytes^1,2

ARS, USDA, Biotechnology and Germplasm Laboratory, Beltsville, MD 20705

³ Correspondence: e-mail: dave{at}anri.barc.usda.gov.

The predominant pattern of follicle development in pigs is characterized by continuous activation, slow growth to the antral stage, and rapid growth to 4 to 5 mm followed by atresia. The only time that this pattern is broken is when a small portion of the follicle population is selected for ovulation. The mechanisms that regulate the selection of ovulatory follicles are not well understood. However, the ovulatory cohort shifts from FSH to LH dependence at the expense of the nonovulatory follicles as indicated by the following: 1) decreased secretion of FSH, and 2) decreased expression of the FSH receptor and increased expression of the LH receptor. The selection of ovulatory follicles may be dependent on the interaction of members of the intraovarian IGF system to maintain a high level of IGF-I bioavailability. The maintenance of a proliferating population of antral follicles is critically dependent on circulating FSH. A naturally or experimentally induced increase in circulating FSH levels results in an increase in antral follicles; conversely, decreased secretion of FSH is followed by a decrease in the number and in health status of antral follicles. Gonadotropin treatment with eCG or PG600 triggers selection of ovulatory follicles, and although these treatments do not increase litter size, they are beneficial for treatment of anestrus and, in conjunction with hCG or GnRH analogs, provide better control of the time of ovulation. The use of porcine FSH has not increased ovulation rate or improved oocyte developmental competence. To improve reproductive efficiency in the future, research should be directed toward obtaining more knowledge about genetic and physiological regulation of ovulatory follicle selection and the effect of follicle development on oocyte developmental competence.

Key Words: Follicle • Follicle Stimulating Hormone • Oocyte • Ovulation • Pigs

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