Ovarian Follicular Development
Nine prepubertal female cattle (Angus, n = 5; Saler, n = 2; Braler, n = 2; 9 mo of age at the initiation of the experiment; 258 ± 25 kg) were used in the study. Age at puberty for each female was determined retrospectively on the basis of the time when luteal function was initially detected (concentrations of progesterone > 1 ng/ml of blood plasma). buy birth control online
Development of the largest ovarian follicle of postpu-bertal cattle can be classified into selection, growth, and plateau phases, and all follicles that do not ovulate subsequently go through an atretic phase. Circulating concentrations of estradiol are elevated during the growth phase of dominant follicles when compared with circulating concentrations during plateau and atretic phases. Associated with this increase in circulating concentrations of estradiol is a greater frequency of release of LH pulses.
Ovariectomy of prepubertal cattle results in an increase in frequency of release of LH pulses. This post-ovariectomy increase in frequency of LH pulses can be inhibited by administration of estradiol. The number of receptors for estradiol in the medial basal hypothalamus decreases prepuberty, and this decrease may result in the decreased negative feedback of estradiol on release of LH pulses. Thus there is thought to be a change in sensitivity to estradiol due to decreased receptors at the hypothalamus in prepubertal cattle, with estradiol produced by ovarian follicles regulating secretion of LH by inhibiting the putative hypothalamic pulse generator for LHRH. Consequently, frequency of release of LH pulses increases between 50 days prepuberty and onset of puberty.