FACTORS REGULATING APOPTOSIS DURING FOLLICULOGENESIS IN PIGS

Authors: Guthrie, Howard; GARRETT, W - 1265-10

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/24/2000
Publication Date: N/A
Citation: N/A

Interpretive Summary: Female germ cell numbers decrease during the life of the pig with a decrease from 1.1 to .5 million between d 50 of gestation and the time of birth. Apoptosis, a form of programmed cell death, is recognized as the death mechanism for germ cells and follicles through all stages of folliculogenesis. In antral follicles, germ cell loss is through follicular ratresia, or degeneration, initiated by apoptosis among follicular granulos cells. The incidence of atresia is normally about 50% in antral follicles. During the three day period of the estrous cycle when follicles are selected to ovulate in the pig, only 20% of the follicles present on the ovaries at that time survive and are selected to ovulate. The rational for the production of so many germ cells and their subsequent loss is unknown. One explanation may be that functional life span of the oocyte becomes much shorter after it becomes fully grown. Up to 70% of oocytes have resumed meiotic maturation before follicle numbers have been reduced prior to ovulation. In contrast, most oocytes selected to ovulate are in meiotic arrest until the preovulatory LH surge. Therefore, apoptosis and follicle atresia may be required to eliminate oocytes that no longer developmentally competent even if they are fertilized. Additional research to define the functional lifespan of the oocyte and its relationship to follicle growth and atresia may reveal methods to generate more developmentally competent oocytes and increase reproductive efficiency in swine.

Technical Abstract: Female germ cell numbers decrease by 70% between day 50 of gestation in the fetus and d 300 post-partum. Apoptosis, a form of programmed cell death, is recognized as the death mechanism of germ cells and for follicle degeneration (atresia) through all stages of folliculogenesis. The hallmark of apoptosis is DNA cleavage into internucleosomal fragments by activation of a specific cell death pathway. We measured apoptosis in follicles by DN fluorescence flow cytometry, densitometry of DNA fragments following electrophoresis on agarose gels, and on frozen tissue sections immunohistochemically. Apoptosis in oogonia and preantral follicles is 3- 5%. During the development of antral follicles, germ cell loss is through follicular atresia initiated by apoptosis in granulosa cells. The incidence of atresia is normally 50%, but during the three day period of the estrous cycle when follicles are selected to ovulate, only 20% of the follicles present on the ovaries at that time survive and are selected to ovulate. The decrease in follicle number is driven by a 60% decrease in secretion of FSH. Estradiol, aromatase, and inhibin were inversely related to the extent of granulosa cell apoptosis. In cultured granulosa cells FSH and IGF-I were shown to be anti-apoptosis. A caspase inhibitor, benzyloxycarbonyl-valinyl-alaninyl-aspartyl fluoro methylketone, attenuated apoptosis by 70% indicating that caspases were active during apoptosis in porcine granulosa cells. Apoptosis in granulosa cells is negatively induced by withdrawal of survival factors and positively induced by ligand binding to plasma membrane receptors.