Evidence for a local role of inhibin or inhibin alpha subunits in compensatory ovarian hypertrophy

Ovarian follicular development in the unilateral ovariectomized rat

Background and Aims:  Following unilateral ovariectomy, the contralateral ovary underwent hypertrophy and the number of ovulations remained unchanged. The aim of the present study was to elucidate the mechanism, which occurred by this compensation, through examination of ovarian follicular development in operated rats. Methods:  Rats were examined 1-8 weeks post unilateral operation. The ovaries were serially sectioned. All sections were examined and the size of each follicle was measured. Results:  Large follicles soon recovered after surgery, but it took 5 weeks for small and medium-sized follicles to recover. The rate of atresia at the 'critical point' was 31.1% in unilateral ovariectomized animals and 73.9% in the sham-operated rats. Conclusions:  These findings showed that in operated rats, the recovery of the number of follicles varied depending on the size of the follicle. Compared with the control animals, approximately twice as many large healthy follicles were saved from atresia at the 'critical point', producing a compensatory increase in the ovulation number (P < 0.01). (Reprod Med Biol 2005; 4: 89-93).

Ovarian follicular development in the unilateral ovariectomized rat

Background and Aims:  Following unilateral ovariectomy, the contralateral ovary underwent hypertrophy and the number of ovulations remained unchanged. The aim of the present study was to elucidate the mechanism, which occurred by this compensation, through examination of ovarian follicular development in operated rats. Methods:  Rats were examined 1-8 weeks post unilateral operation. The ovaries were serially sectioned. All sections were examined and the size of each follicle was measured. Results:  Large follicles soon recovered after surgery, but it took 5 weeks for small and medium-sized follicles to recover. The rate of atresia at the 'critical point' was 31.1% in unilateral ovariectomized animals and 73.9% in the sham-operated rats. Conclusions:  These findings showed that in operated rats, the recovery of the number of follicles varied depending on the size of the follicle. Compared with the control animals, approximately twice as many large healthy follicles were saved from atresia at the 'critical point', producing a compensatory increase in the ovulation number (P < 0.01). (Reprod Med Biol 2005; 4: 89-93).

Numbers of antral follicles during follicular waves in cattle: evidence for high variation among animals, very high repeatability in individuals, and an inverse association with serum follicle-stimulating hormone concentrations

The extent, causes, and physiological significance of the variation in number of follicles growing during ovarian follicular waves in human beings and cattle are unknown. Therefore, the present study examined the variability and repeatability in numbers of follicles 3 mm or greater in diameter during the follicular waves in bovine estrous cycles, and we determined if the variation in number of follicles during waves was associated with alterations in secretion of FSH, estradiol, inhibin, and insulin-like growth factor I (IGF-I). Dairy cattle were subjected to twice-daily ultrasound analysis to count total number of antral follicles 3 mm or greater in diameter throughout 138 different follicular waves. In another study, blood samples were taken at frequent intervals from cows that consistently had low or very high numbers of follicles during waves and were subjected to immunoassays. Results indicate the following: First, despite an approximately sevenfold variation in number of follicles during waves among animals and marked differences in age, stage of lactation, and season of the year, a very highly repeatable (0.95) number of follicles 3 mm or greater in diameter is maintained during the ovulatory and nonovulatory follicular waves of individuals. Second, variation in number of follicles 3 mm or greater in diameter during waves and the inverse association of number of follicles during waves with FSH are not directly explained by alterations in the patterns of secretion of estradiol, inhibin, or IGF-I. Third, ovarian ultrasound analysis can be used reliably by investigators to identify cattle that consistently have low or high numbers of follicles during waves, thus providing a novel experimental model to determine the causes and physiological significance of the high variation in antral follicle number during follicular waves among single-ovulating species, such as cattle or humans.

Regulatory role of inhibin in follicle-stimulating hormone secretion and folliculogenesis in the guinea pig

The effects of unilateral and bilateral ovariectomy and passive immunization against inhibin on follicle-stimulating hormone (FSH) secretions and follicular development in the guinea pig were investigated. Bilateral ovariectomy decreased plasma immunoreactive (ir-) inhibin rapidly and increased plasma FSH significantly. Unilateral ovariectomy decreased plasma ir-inhibin and increased plasma FSH temporarily, and doubled the number of ova released from the remaining ovary at the subsequent ovulation in guinea pigs. Injection of 1.0 ml inhibin antiserum significantly increased concentrations of plasma FSH at 6 hr onwards and the number of small follicles (100-200 microm in diameter) at 48 hr after the injection in guinea pigs bearing progesterone-containing implants. In vitro bioassay showed that inhibin antiserum could neutralize the suppression of ovarian homogenate on FSH secretion from cultured rat anterior pituitary cells. These results confirm the evidence that the ovary is the main source of inhibin secretion and both in vitro bioassay and passive immunization against inhibin show that the inhibin is a major regulator in the follicular development through FSH secretion in guinea pigs.

Secretion of ovarian inhibin and its physiologic roles in the regulation of follicle-stimulating hormone secretion during the estrous cycle of the female guinea pig

To characterize inhibin secretion during the estrous cycle in guinea pigs, the concentrations of plasma inhibin, estradiol, progesterone, and FSH were determined. A significant positive correlation was observed between inhibin and estradiol throughout the estrous cycle. Plasma inhibin and estradiol started to increase a few days before ovulation (Day 0 = day of estimated ovulation), and decreased after ovulation. These two hormones remained low during the luteal phase. The immunoreactivity of inhibin alpha, betaA, and betaB subunits was colocalized in the granulosa cells of one or two healthy large follicles in the ovary before ovulation. There was no positive reaction of inhibin alpha and beta subunits in the corpora lutea or other follicles. Ovariectomy resulted in an abrupt decrease in plasma inhibin and a significant increase in plasma FSH. Injection of anti-inhibin serum into adult female guinea pigs induced an elevation in plasma FSH in a dose-dependent manner. This report presents the first description of sequential changes in plasma inhibin and estradiol during the estrous cycle of guinea pigs. Results suggest that inhibin is secreted mainly by granulosa cells of a few healthy large follicles in the ovary and that it plays an important role in the regulation of FSH secretion during the estrous cycle in guinea pigs.