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Factors regulating apoptosis during folliculogenesis in pigs¹

Germplasm and Gamete Physiology Laboratory, Agricultural Research Service, U. S. Department of Agriculture, Beltsville, MD 20705

^2. Correspondence: fax: 301/504-5123; E-mail dave{at}lspi.barc.usda.gov.

Abstract

In pigs, female germ cell numbers decrease by 70% between fetal d 50 postcoitum and d 300 postpartum. Apoptosis, a form of programmed cell death, is recognized as the mechanism of germ cell death and follicle degeneration (atresia) through all stages of folliculogenesis. The hallmark of apoptosis is internucleosomal cleavage of genomic DNA caused by proteolytic induction deoxyribonuclease activity. We measured apoptosis in granulosa cells by DNA fluorescence flow cytometry, densitometry of fluorescently labeled internucleosomal DNA fragments, and by immunohistochemical analysis of 3' end labeling on frozen tissue sections. The incidence of atresia in antral follicles averages 55%. However, during the 3-d period of the estrous cycle when follicles are selected to ovulate in pigs, only 15% of the follicles present at the beginning of the process survive; the decrease in follicle number is associated with a 60 to 70% decrease in secretion of FSH. Expression of aromatase protein and inhibin subunit transcripts, follicular fluid estradiol, and granulosa cell proliferation are negatively correlated with the extent of granulosa cell apoptosis. In cultured granulosa cells FSH and IGF-I were shown to be anti-apoptotic. A general caspase inhibitor inhibited apoptosis, indicating that caspases were active during culture of porcine granulosa cells. The evolutionary significance for the production of so many germ cells and their subsequent loss is unknown. Up to 70% of the oocytes in 3- to 5-mm follicles have resumed meiotic maturation. In contrast, most oocytes selected to ovulate are in meiotic arrest. Therefore, apoptosis and follicle atresia may be required to eliminate oocytes that have not remained in meiotic arrest.

^1. Mention of a trade name or proprietary product does not constitute a guarantee or warranty by the USDA and does not imply approval to the exclusion of others mentioned.