Four readily identifiable changes in appearance of corpora lutea (Stages I to IV) occur during a bovine estrous cycle. Accuracy of estimating the stage of an estrous cycle by appearance of corpora lutea was determined in a double-blind study. One investigator observed estrus in a group of heifers while another with no prior knowledge of reproductive histories of the heifers estimated stage of the estrous cycle by visual inspection of their corpora lutea. Stage of the estrous cycle (Stage I, days 1 to 4; Stage II, days 5 to 10; Stage III, days 11 to 17; Stage IV, days 18 to 20) was estimated correctly in 41 of 48 heifers. The correlation between estimated and actual days of the estrous cycle was .81. In addition, concentrations of progestins and weights of corpora lutea during estimated stages of the estrous cycle were similar to many other investigations. Stages of the estrous cycle in heifers can be estimated from appearance of corpora lutea.

BACKGROUND: The significance of the high variation in numbers of follicles produced during reproductive cycles in humans and cattle is unknown. METHODS: We selected beef heifers with high (25) or low (15) numbers of ovarian follicles and determined the association with alterations in FSH and estradiol concentrations, as well as responsiveness to superstimulation and embryo quality. The variation in follicle numbers was also compared with oocyte quality in natural cycles using IVF and abattoir sourced bovine ovaries. RESULTS: Results show that: (i) FSH was lower (P < 0.03) in animals with high compared with low follicle numbers per follicle wave; (ii) after superovulation, in the high versus low follicle number group, the number of oocytes/embryos recovered after insemination (10.6 + + + + + 2.7 versus 4.7 + + + + + 0.7) and the number of transferable embryos (5.4 + + + + + 1.3 versus 3.8 + + + + + 0.8) per animal were greater (P < 0.05), whereas the proportion of transferable embryos (50.7% versus 79.8%) was lower (P < 0.05); (iii) in unstimulated animals, the numbers of high-quality oocytes harvested and in-vitro fertilized oocytes developing into blastocysts were up to 4-fold greater (P < 0.05) for ovaries with high versus low numbers of follicles, but the proportions of oocytes developing into blastocysts were similar in the two groups. CONCLUSION: Phenotypic classification based on numbers of follicles may be useful to improve superovulation procedures. The lower proportion of transferable embryos following superovulation of ovaries with high numbers of follicles is probably not the result of differences in the quality of oocytes before superovulation.

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.

The mechanism whereby the inherently high variation in ovary size and the total number of high-quality oocytes in ovaries (ovarian reserve) impact on ovarian function and fertility, diagnostics to measure the size of the ovarian reserve and the factors that cause variation in the ovarian reserve are unknown. Our results show that cattle can be phenotyped reliably based on the number of antral follicles growing during follicular waves (antral follicle count, AFC). Young adult cattle with a consistently low v. a high AFC have smaller gonads, a markedly diminished ovarian reserve and many other phenotypic characteristics usually associated with ovarian aging and infertility. A powerful new approach based on a single measurement of serum concentration of anti-Müllerian hormone (AMH) is described to test the longstanding hypothesis that the size of the ovarian reserve is positively associated with fertility. Also, new evidence shows that maternal environment has a critical role in regulation of the high variation in the ovarian reserve and perhaps fertility in offspring. These results support the conclusion that the inherently high variation in the ovarian reserve, potentially caused by alterations in the maternal environment, has a negative impact on ovarian function that may result in suboptimal fertility in young adult cattle, and a single AMH measurement can be used reliably in future studies to determine if fertility is suboptimal in young adult cattle with low circulating AMH concentrations and a correspondingly diminished ovarian reserve.

The effects of estradiol, FSH and LH on ovarian follicular development and granulosa cell differentiation were examined in the immature rat hypophysectomized on day 24 of age. Administration of estradiol to hypophysectomized rats for 4 days stimulated the growth of large preantral follicles with a concomitant 1.5-fold increase in FSH receptor content and a 4-fold decrease in LH receptor content in the granulosa cells. When highly purified hFSH was administered alone, receptor content for FSH increased progressively for 4 days while receptor for LH remained essentially unchanged. However, when rats were pretreated with estradiol, the response of follicles to FSH was markedly enhanced as indicated by the appearance of large, antral follicles and elevated receptor content for both FSH and LH. Receptor content for FSH increased markedly in response to hFSH following only one day of estradiol pretreatment, while receptor content for LH increased most rapidly in response to hFSH after 3 days of estradiol pretreatment. LH administered to rats possessing large preovulatory follicles caused luteinization of granulosa cells and a marked decline in receptor content for both gonadotropins within 24 h. Receptor content remained low even 48 h after LH administration when granulosa cells were fully luteinized. These results indicated that follicular development and granulosa cell differentiation are dependent on steroid-protein hormone regulation of hormone specific receptors.