The significance of the high variation in numbers of follicles produced during reproductive cycles in humans and cattle is unknown. 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 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.

We hypothesize that granulosa and theca cells from growing dominant follicles, with relatively high intrafollicular concentrations of estradiol, have a greater expression of genes involved in inhibiting apoptosis pathways and lower expression of genes involved in apoptosis pathways than growing subordinate follicles with lower estradiol concentrations. Using the well-characterized bovine dominant follicle model, we collected granulosa and theca cells from individual dominant and the largest subordinate follicle 3 days after initiation of a follicular wave in four animals. Based on ultrasound analysis, both follicle types were in the growth phase at the time of ovariectomy. However, dominant follicles were larger (9.8 ± 1.0 versus 7.6 ± 0.6 mm in diameter, P < 0.05) and had greater intrafollicular concentrations of estradiol (132.2 ±3 8.5 versus 24.1 ± 12.1 ng/ml, P < 0.05), compared with the largest subordinate follicles. We used bovine cDNA microarrays, which contained a total of 1400 genes, including a subset of 53 genes known to be involved in apoptosis pathways, to determine which apoptosis and marker genes from each of the four dominant versus subordinate follicles were potentially differentially expressed. Using a low stringency-screening criterion, 22 genes were identified. Quantitative real-time polymerase chain reaction confirmed that 16 of these genes were differentially expressed. Our novel results demonstrate that the high intrafollicular concentrations of estradiol in growing dominant follicles were positively associated with enhanced expression of mRNAs in granulosa cells for aromatase, LH receptor, estradiol receptor β, DICE-1, and MCL-1, compared with granulosa cells from subordinate follicles (all survival-associated genes). In contrast, the relatively low intrafollicular concentrations of estradiol in growing subordinate follicles were positively associated with enhanced expression of mRNAs in granulosa cells for β glycan, cyclo-oxygenase-1, tumor necrosis factor α, caspase-activated DNase, and DRAK-2, and in theca cells for β glycan, caspase 13, P58(IPK), Apaf-1, BTG-3, and TS-BCLL, compared with granulosa or theca cells from dominant follicles (genes that are all associated with cell death and/or apoptosis). We suggest that that these genes may be candidate estradiol target genes and that they may be early markers for the final stages of follicle differentiation or initiation of apoptosis and thus selection of dominant follicles during follicular waves.

Contents: Mammals such as cattle, swine, sheep and humans are born with a highly variable number of ovarian follicles and oocytes in the ovaries that dwindle during ageing and are never replenished. This variation in the ovarian reserve is reflected in the numbers of antral follicles in the ovaries at all ages after birth. As numbers of follicles in ovaries are determined during gestation, the role of maternal nutrition and health during gestation (at time of ovarian development in their foetuses) has been investigated as factors that may impact oogonia proliferation and thus follicle numbers post‐natally. These studies have found that both nutrition and health impact numbers of follicles in their offspring. The idea that numbers of follicles and oocytes in ovaries impact fertility is a long‐held belief in reproductive biology. This has recently been tested in cattle, and it has been shown that cows with a relatively high number of antral follicles in ovaries have higher pregnancy rates, shorter calving to conception intervals and fewer artificial inseminations during the breeding season compared with cows with a lower number of follicles, and similarly, heifers with many follicles had higher pregnancy rates than those with fewer follicles. Studies summarized in this review highlight the importance of the maternal environment during gestation in determining the size of the ovarian reserve in their offspring and also the contribution of the ovarian reserve to subsequent fertility in cattle.