Hormonal mechanisms regulating follicular wave dynamics I: Comparison of follicle growth profiles under different physiological conditions in heifers

Review: Manipulation of follicle development to improve fertility of cattle in timed-artificial insemination programs

The development of an ovulatory follicle is a fundamental premise for any reproductive management program that aims to optimize fertility in cattle. Controlling follicular development comprises the synchronized emergence of a new follicular wave, selection and growth of the dominant follicle, and synchronized ovulation of a high-quality oocyte. All these follicular events, primarily driven by gonadotropin secretion, occur under a very dynamic hormonal environment. In this sense, controlling follicular development demands essentially a precise manipulation of the hormonal environment to modulate gonadotropin secretion. Furthermore, the effectiveness of hormonal manipulation strategies in the management of follicular development depends on specific particularities of each situation, which can vary widely according to genetic groups (Bos taurus vs Bos indicus), nutritional, metabolic, and reproductive status. In this regard, the constant search for the refined synchrony between the hormonal treatments and reproductive events, considering these distinctions and particularities, have provided valuable information that contributed to the development of efficient reproductive programs. This manuscript discusses the physiological bases behind the development of fine-tuned timed-artificial insemination protocols for beef and dairy cattle that resulted in great improvements in reproductive efficiency of beef and dairy herds.

Temporality of ovarian steroids and LH/FSH pulse profiles encompassing selection of the dominant follicle in heifers†

The tested hypotheses were (1) LH/FSH pulses and F2 diameter are diminished by P4 and, (2) E2 increases during the transition to deviation and alters LH/FSH pulses. On Day 5 (Day 0 = ovulation), heifers were randomized into an untreated group (HiP4, n = 11), and a prostaglandin analog treated group (NoP4, n = 10). On Day 6, a follicular wave was induced by follicle ablation. Ultrasound and blood collections were performed every 12 h from Days 7 to 11. Blood was collected every 15 min for 10 h on Day 9 (largest follicle expected to be ~7.5 mm). Estradiol was ~75% greater (0.36 ± 0.14 vs 0.63 ± 0.19 pg/mL) in heifers with F1 ≥ 7.2 mm than in heifers with F1 < 7.2 mm. The HiP4 had smaller second largest follicle (F2) diameter, lower estradiol (P = 0.06), LH pulse baseline and peak concentrations (P < 0.007), in addition to half the frequency of LH/FSH pulses (4.1 ± 0.3 vs 9.6 ± 0.7 in 10 h) than the NoP4. Within HiP4, heifers with F1 ≥ 7.2 mm had ~25% fewer (P = 0.03) LH pulses compared to heifers with F1 < 7.2 mm. In contrast, within the NoP4, heifers with F1 ≥ 7.2 mm had ~75% greater LH (P = 0.05) and FSH (P = 0.08) pulse amplitude. We propose that greater F2 diameter at deviation in low P4 is related to greater LH baseline and peak concentrations, and greater frequency of LH/FSH pulses. A greater increase in E2 after F1 reaches ~7.2 mm results in further stimulation of LH/FSH pulse amplitude. Elevated P4 not only diminished frequency of LH/FSH pulses but also converted an E2 increase into a negative feedback effect on LH/FSH pulse frequency leading to smaller F2 at deviation.

Follicle Selection in Mares as a Model for Illustrating the Many Hormonal and Biochemical Interactions That Drive a Single Physiological Mechanism

The mechanism for selection of the future dominant or ovulatory follicle in mares involves a relatively abrupt separation in growth rates between the future dominant follicle and several subordinate follicles and is termed diameter deviation. The event is used to illustrate that a coordinated complex of many follicular, hormonal, and biochemical factors interact and interbalance during a single physiological mechanism. For example, a positive effect of follicle stimulating hormone (FSH) on development of all follicles during the growing phase can later involve a positive effect of luteinizing hormone (LH) but apparently only on the future dominant follicle. In turn, the developing and future dominant follicle produces estradiol which at appropriate times and degrees reduces FSH concentrations to accommodate follicle functions at certain levels of FSH. Meanwhile, the estradiol prevents LH from increasing from a useful to an adverse concentration. These interactions enmesh with the production and roles of other factors (e.g., inhibin, insulin-like growth factor) during follicle selection. The wide array of morphological, hormonal, and biochemical activities occur in harmony even when in the same tissue and often at the same time.

Effect of elevating luteinizing hormone action using low doses of human chorionic gonadotropin on double ovulation, follicle dynamics, and circulating follicle-stimulating hormone in lactating dairy cows

Double ovulation and twin pregnancy are undesirable traits in dairy cattle. Based on previous physiological observations, we tested the hypothesis that increased LH action [low-dose human chorionic gonadotropin (hCG)] before the expected time of diameter deviation would change circulating FSH concentrations, maximum size of the second largest (F2) and third largest (F3) follicles, and frequency of multiple ovulations in lactating dairy cows with minimal progesterone (P4) concentrations. In replicate 1, multiparous, nonbred lactating Holstein dairy cows (n = 18) had ovulation synchronized. On d 5 after ovulation, all cows had their corpus luteum regressed and were submitted to follicle (≥3 mm) aspiration 24 h later to induce emergence of a new follicular wave. Cows were then randomized to NoP4 (untreated) and NoP4+hCG (100 IU of hCG every 24 h for 4 d after follicle aspiration). Ultrasound evaluations and blood sample collections were performed every 12 h for 7 d after follicle aspiration. All cows were then treated with 200 μg of GnRH to induce ovulation. In replicate 2, cows (n = 16) were resubmitted to similar procedures (i.e., corpus luteum regression, follicle aspiration, randomization, ultrasound evaluations every 12 h, GnRH 7 d after aspiration). However, cows in replicate 2 received an intravaginal P4 device that had been previously used (∼18 d). Only cows with single (n = 15) and double (n = 16) ovulations were used in the analysis. No significant differences were detected for frequency of double ovulation, follicle sizes, and FSH concentrations across replicates (NoP4 vs. LowP4 and NoP4+hCG vs. LowP4+hCG), so data were combined. Double ovulation was 40% for control cows with no hCG (CONT) and 62.5% with hCG (hCG). Double ovulation increased as the maximum size of F2 increased: <9.5 mm and 9.5-11.5 mm (7.7%) and ≥11.5 mm (94.1%). The hCG group had more cows with F2 > 11.5 (69%) than with 9.5 ≥ F2 ≤ 11.5 (25%) and F2 < 9.5 (6%). In agreement, F2 and F3 maximum size were larger in the hCG group, but FSH concentrations were lower after F1 > 8.5 mm compared with CONT. In contrast, FSH concentrations were greater before deviation (F1 closest value to 8.5 mm) in cows with double ovulations than in those with single ovulations, regardless of hCG treatment. In addition, time from aspiration to deviation was shorter in cows with double rather than single ovulation and in cows treated with hCG as a result of faster F1, F2, and F3 growth rates before diameter deviation. In conclusion, greater FSH and follicle growth before deviation seems to be a primary driver of greater frequency of double ovulation in lactating cows with low circulating P4. Moreover, the increase in follicle growth before deviation and in the maximum size of F2 during hCG treatment suggests that increased LH may also have a role in stimulating double ovulation.

Effects of eCG and FSH in timed artificial insemination treatment regimens on estrous expression and pregnancy rates in primiparous and multiparous Bos indicus cows

Effects were evaluated in Bos indicus cows of eCG and FSH on follicular growth, estrous expression, and pregnancy per artificial insemination (P/AI) as a result of fixed-time artificial insemination (TAI). In Experiment 1, extent of timing-of-ovulation synchronization among cows was evaluated after imposing an estrogen/progesterone-based treatment regimen. At progesterone device removal (D8), cows were administered: eCG, or FSH or served as untreated Controls. In Experiment 2, percentage of cows P/AI was evaluated when the Experiment 1-treatment regimen was imposed. On D10, all cows were artificially inseminated. In Experiment 3, cows were assigned to two treatment groups (Control and eCG) on D8 to evaluate percentage of cows P/AI and estrous expression. In Experiment 1, follicular dynamics were similar among treatment groups. In Experiment 2, follicular growth was greater (P =  0.0001) with the eCG treatment. There was an interaction of treatment × parity (P =  0.007) on percentage of cows P/AI. There was a greater percentage of primiparous cows P/AI in the eCG-treated than Control and FSH-treated cows. There was a greater percentage of eCG-treated multiparous cows pregnant as a result of TAI than Control cows. There was an interaction of treatment × parity (P =  0.005) on P/AI in Experiment 3, in which the eCG effect was more pronounced in primiparous cows. Treatment with FSH, therefore, was not as effective as eCG in stimulation of follicular growth or enhancing percentage of cows pregnant as a result of TAI. Physiological effects of eCG, however, were also more evident in primiparous cows.

Timing of early resynchronization protocols affects subsequent pregnancy outcome in dairy cows

The aim of this study was to evaluate the outcomes of an early resynchronization protocol (Resynch) initiated at different timepoints after timed artificial insemination (TAI) and with unknown pregnancy status. Holstein cows (n = 164) were submitted to the following TAI protocol: D0, insertion of an intravaginal progesterone (P4) device and 2 mg im estradiol benzoate (EB); D8, removal of P4 device and treatment with 0.5 mg im sodium cloprostenol (PGF); D9, 0.1 mg im Lecirelin (LEC); and D10, TAI1. Cows were then randomly assigned to Resynch protocols starting either on day 20 (Resynch20D, n = 82) or 25 after TAI1 (Resynch25D, n = 82) with the insertion of a new P4 device and EB treatment. In both groups, P4 device was removed on day 8 after the beginning of Resynch, the same day of pregnancy diagnosis by ultrasonography. In pregnant cows there was no further action. Non-pregnant cows were treated with 0.5 mg im PGF, had a blood sample collected for serum P4 analysis and we measured and recorded the size of the largest follicle and the presence or absence of a corpus luteum (CL). One day later, cows were treated with 0.1 mg im LEC and TAI2 occurred 12-14 h later. The diameter of the largest follicle and serum P4 were compared between groups by ANOVA for the main effects of treatment, presence of a CL, and their interaction, whereas pregnancy per artificial insemination (P/AI) and the percentage of cows with a CL on the day of ultrasonography were analyzed using the Chi-square test. Follicle diameter on day 8 of Resynch was greater for cows in the Resynch20D group compared with Resynch25D (15.9 ± 3.9 vs 12.2 ± 2.5 mm, respectively; P = 0.046). The Resynch25D group had a greater percentage of cows with a CL (51.9 vs 18.9%, respectively; P = 0.0008) and higher serum P4 (2.8 ± 1.1 vs 1.7 ± 0.8 ng/mL; P = 0.041) at the end of the protocol compared with Resynch20D. P/AI at TAI1 was 35.4 and 36.6% (P > 0.10) for cows enrolled in Resynch20D and Resynch25D groups, respectively. P/AI to TAI2, after Resynch protocols, was greater in Resynch25D than Resynch20D (44.2 vs 22.6%, respectively; P < 0.05). In conclusion, starting an early resynchronization protocol 25 days after TAI increases P/AI compared with starting 20 days after TAI, and this was associated with a presumed greater proportion of cows with a functional CL at the moment of P4 device removal.

Selection of fewer dominant follicles in Trio carriers given GnRH antagonist and luteinizing hormone action replaced by nonpulsatile human chorionic gonadotropin†

Studying selection of multiple dominant follicles (DFs) in monovulatory species can advance our understanding of mechanisms regulating selection of single or multiple DFs. Carriers of the bovine high fecundity Trio allele select multiple DFs, whereas half-sib noncarriers select a single DF. This study compared follicle selection during endogenous gonadotropin pulses versus during ablation of pulses with Acyline (GnRH antagonist) and luteinizing hormone (LH) action replaced with nonpulsatile human chorionic gonadotropin (hCG) treatment in Trio carriers (n = 28) versus noncarriers (n = 32). On Day 1.5 (Day 0 = ovulation), heifers were randomized: (1) Control, untreated; (2) Acyline, two i.m. doses (Days 1.5 and D3) of 3 μg/kg; (3) hCG, single i.m. dose of 50 IU hCG on Day 1.5 followed by daily doses of 100 IU; and (4) Acyline + hCG. Treatments with nonpulsatile hCG were designed to replace LH action in heifers treated with Acyline. Acyline treatment resulted in cessation of follicle growth on Day 3 with smaller (P < 0.0001) maximum follicle diameter in Trio carriers (6.6 ± 0.2 mm) than noncarriers (8.7 ± 0.4 mm). Replacement of LH action (hCG) reestablished follicle diameter deviation and maximum diameter of DFs in both genotypes (8.9 ± 0.3 mm and 13.1 ± 0.5 mm; P < 0.0001). Circulating follicle stimulating hormone (FSH) was greater in Acyline-treated than in controls. Finally, Acyline + hCG decreased (P < 0.0001) the number of DFs from 2.7 ± 0.2 to 1.3 ± 0.2 in Trio carriers, with most heifers having only one DF. This demonstrates the necessity for LH in acquisition of dominance in Trio carriers (~6.5 mm) and noncarriers (~8.5 mm) and provides evidence for a role of GnRH-induced FSH/LH pulses in selection of multiple DFs in Trio carriers and possibly other physiologic situations with increased ovulation rate.

Hormonal mechanisms regulating follicular wave dynamics II: Progesterone decreases diameter at follicle selection regardless of whether circulating FSH or LH are decreased or elevated

Selection of a single dominant follicle is morphologically manifested by diameter deviation between the future dominant follicle (F1) and the future largest subordinate follicle (F2). Conventional deviation is defined as F2≥7 mm when F1 reaches ∼8.5 mm whereas, undersized deviation is if F2<7 mm when F1 reaches ∼8.5 mm. Greater frequency of undersized deviation has been temporally associated with greater circulating progesterone (P4) and greater FSH but reduced LH in observational studies. Experiment 1 was conducted to directly test if elevating P4 increased the likelihood of undersized deviation and altered circulating concentrations of LH and FSH. Experiment 2 was conducted to test if increasing LH action by treatment with exogenous porcine LH or human chorionic gonadotropin (hCG) in the presence of elevated P4, would stimulate growth of F2 and increase the likelihood of conventional deviation. Ovaries were evaluated by ultrasound and blood samples collected every 12 h after development of a new wave following follicle ablation on D6 (D0 = ovulation). Data were normalized to F1≥7.5 mm and compared using SAS software. In experiment 1 (n = 20), the CL was regressed by prostaglandin F2α treatment and heifers were randomized on D6 into control (no P4 treatment) or P4 treatment (75 mg every 12 h for 5.5 d) beginning when F1 reached ∼3 mm (P4-3 mm group) or ∼6 mm (P4-6 mm group). The P4 treatment significantly increased the frequency of undersized deviation from 0% (controls) to 54%, decreased LH by 44%, and increased FSH by 32%. In experiment 2 (n = 27) heifers were randomized on D6 into control (saline) or treatment with the LH analogs - pLH (1.25 mg porcine LH/12 h) or hCG (160 IU initially and subsequently 96 IU/24 h). Treatment with LH analogs significantly increased P4 (control, 4.6 ± 0.3 ng/mL; pLH, 6.6 ± 0.4 ng/mL; and hCG, 8.9 ± 0.4 ng/mL) and decreased FSH (control, 0.46 ± 0.03 ng/mL; combined-pLH/hCG, 0.34 ± 0.02 ng/mL). However, F1 and F2 diameter and frequency of conventional (37%) and undersized (48%) deviations were similar between the control and combined-pLH/hCG groups. In conclusion, elevated P4 was directly linked to undersized deviation but the P4 effect on decreasing F2 diameter occurred independently of the P4 effects on FSH and LH concentrations.

Role of intraovarian mechanisms and side of ovary on characteristics of follicle selection in Bos taurus heifers

Intraovarian effects on diameter of future dominant follicle (DF or F1) and future largest subordinate follicle (F2) during the few days before selection of the future DF and subordinate follicles were studied in 147 bovine interovulatory intervals. Follicle selection involves diameter deviation or the beginning of separation of growth rates between F1 and F2. Diameter deviation is classified as conventional (F2 ≥ 7.0 mm when F1 is 8.5 mm or at expected deviation) and as undersized (F2 < 7.0 mm when F1 is 8.5 mm). Diameter separation of F1 and F2 in conventional and undersized deviations is characteristically abrupt and gradual, respectively. The predeviation diameter of F2 when located in an ovary that later becomes the F1 intraovarian patterns of DF-CL, devoid (ovary without a DF or CL), DF alone, or CL alone and in left or right ovaries (LO, RO) was compared between conventional and undersized deviations. In conventional deviation, ovaries with the future DF (combined DF-CL and DF patterns) were associated with greater (P < 0.02) predeviation growth rate of F2 when the DF was in the right ovary (DF/RO, 1.6 ± 0.1 mm/d) than when in the left ovary (DF/LO, 1.2 ± 0.1 mm/d). The F2 was in DF/RO more frequently (75%, P < 0.002) than in non-DF/RO. When F2 was in the future devoid F1 pattern and F1 was 6 mm, F2 was smaller (P < 0.002) in the undersized class (5.3 ± 0.2 mm) than in the conventional class (6.3 ± 0.1) but not when F2 was in one of the other future F1 patterns. Only the devoid pattern was greater in frequency (P < 0.03) in the undersized class than in the conventional class. The novel hypothesis was supported that location of F2 in ovaries with different future F1 intraovarian patterns and on different sides affects the predeviation diameter and growth rate of F2 and thereby the frequencies of conventional and undersized deviations during follicle selection.