Progesterone Regulation of Primordial Follicle Assembly in Bovine Fetal Ovaries
Nilsson, Eric E.
Skinner, Michael K.
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Fertility in mammals is dependant on females having an adequate primordial follicle pool to supply oocytes for fertilization. The formation of primordial follicles is a process known as ovarian follicular assembly. In rats and mice Sex steroids have been shown to inhibit follicle assembly both in vitro and in vivo, with assembly occurring after birth when the pups are removed from the high-steroid maternal environment. In contrast, primordial follicle assembly in other species, such as cattle and humans, occurs during fetal development before birth. The objective of the current study is to determine if progesterone levels regulate primordial follicle assembly in fetal bovine ovaries. Ovaries and blood were collected from bovine fetuses and correlated to the developmental age of the fetus. The mid-gestation stages between 62 and 170 days were used. Primordial follicle assembly directly correlated with this stage of fetal development ranging from 5% (62d) to 85% (170d) assembled follicles. Interestingly, ovarian progesterone concentration was found to decrease with increasing fetal age and correlated to increased primordial follicle assembly. Ovarian estrogen levels also decreased and correlated with fetal age. Fetal bovine ovary cultures were used to investigate the direct actions of progesterone. Treatment with progesterone significantly decreased primordial follicle assembly. Observations indicate that sex steroids regulate ovarian primordial follicle assembly in cattle, as they do in rodents. The speculation is made that the progesterone inhibition of primordial follicle assembly is critical to prevent premature development of follicles during gestation and this is conserved from rodents to humans