Submitted to: Journal of Reproduction and Fertility
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 3, 2001
Publication Date: N/A
Interpretive Summary: Female germ cell numbers in the pig decrease by 70% between fetal day 50 postcoitum and day 300 postpartum. 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 atresia, or degeneration, initiated by apoptosis among follicular granulosa 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 ebeen 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 in the pig decrease by 70% between fetal day 50 postcoitum and day 300 postpartum. Approximately 55% of antral follicles are degenerating (atretic) except during the 3-day period prior to oestrus when only 15% survive to ovulate. Apoptosis, a form of programmed cell death, is recognized as the mechanism of germ cell death and follicle atresia through all stages of folliculogenesis. The internucleosomal cleavage of genomic DNA caused by caspase induction of deoxyribonuclease activity was measured in porcine granulosa cells by DNA fluorescence flow cytometry, densitometry of fluorescently labeled internucleosomal DNA fragments, and immunohistochemical analysis of 3prime end labeling of deoxyribonuclease nicked DNA on frozen tissue sections. Follicular atresia during the 3-day period prior to oestrus is associated with a 60 to 70% decreased secretion of FSH. In granulosa cells apoptosis is associated with hdecreased cell proliferation, and reduced production of oestradiol and inhibin. In cultured porcine granulosa cells FSH and IGF-I are anti- apoptotic and a caspase inhibitor blocked apoptosis providing evidence of caspase activity. Oocytes in most follicles have resumed meiotic maturation, therefore one role for apoptosis and follicle atresia may be to act as barrier to ovulation of oocytes that have not remained in meiotic arrest.