The aim of this study was to determine the relationships between early follicular development, the time at which follicles appear in early stages of follicular development, and changes in the serum concentrations of FSH in the female bovine fetus. Thirty-five female bovine fetuses aged between 59 and 285 days, as estimated from the crown-rump length, were obtained from dams at an abattoir. Serum samples were separated from fetal blood obtained from the heart. Fetal ovaries were removed and weighed. The content of oestradiol in one of the fetal ovaries and the serum concentrations of FSH and oestradiol were determined using radioimmunoassay. Sections of the other ovary were examined histologically for the appearance and number of follicles. The follicles were divided into four stages: primordial, primary, secondary and early antral. The appearance of primordial, primary, secondary and early antral follicles was observed at day 74, day 91, day 120 and day 150, respectively. Serum concentrations of FSH in female bovine fetuses increased between day 120 and day 150 of gestation. Fetal serum concentrations of oestradiol increased from day 120. The number of early antral follicles increased from day 180 together with an increase in the fetal ovarian content of oestradiol. These findings indicate that, in the female bovine fetus as well as in adult cows, the number of follicles and stages of follicular development are associated with changes in the concentration of FSH.
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