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 ±â€¯0.1 mm/d) than when in the left ovary (DF/LO, 1.2 ±â€¯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 ±â€¯0.2 mm) than in the conventional class (6.3 ±â€¯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.

Endogenous and exogenous effects of PGF2α during luteolysis in mares.

An inhibitor of PGF2α biosynthesis (flunixin meglumine, FM) was used to study the role of endogenous PGF2α on the luteolytic effect of exogenous PGF2α in mares. A 2-h infusion of PGF2α at a constant rate (total dose, 0.1 mg) on Day 10 (ovulation = Day 0) was used to mimic the maximal concentrations of a spontaneous pulse of a PGF2α metabolite (PGFM). Treatment with FM (1.7 mg/kg) was done 1 h before and 5 h after the start of PGF2α infusion. In hourly blood samples beginning 1 h before the start of PGF2α infusion, progesterone decreased (P < 0.05) similarly by 5 h in each of the PGF2α and PGF2α+FM groups but not in the controls (n = 5). In a study of spontaneous luteolysis, the same FM dose was given every 6 h from Day 13 until Day 17 or earlier if CL regression was indicated by an 80% decrease in luteal blood-flow signals. Blood was sampled for progesterone assay each day and 8 h of hourly blood sampling was done each day to characterize PGFM concentrations and pulses. Progesterone (P4) was lower (P < 0.05) in controls than in an FM group (n = 7) by Day 15. Luteolysis (P4 < 1 ng/mL) ended on Days 14-19 in individual controls. In contrast, luteolysis did not end until after Day 20 in 4 of 7 FM-treated mares. In the three mares with completion of luteolysis before Day 20 in the FM group, the interval from beginning to end of luteolysis was longer (P < 0.02) (4.5 ± 0.6 days) than in the controls (3.0 ± 0.4 days). During 8-h sessions of hourly blood sampling on Day 14, concentration of PGFM was significantly lower in the FM group for the minimal, mean, and maximal per session. Pulses of PGFM were identified by a CV methodology on each day in 7 of 7 and 3 of 7 mares in the controls and FM group, respectively. The four FM-treated mares without a CV-identified pulse were the four mares in which luteolysis did not occur before Day 20. In mares with detected pulses, PGFM was lower at each nadir and at the peak (86% lower) in the FM group than in controls, but the interval between nadirs or base of a pulse was not different between groups. Hypothesis 1 that endogenous PGF plays a role in the luteolytic effect of exogenous PGF2α was not supported. Hypothesis 2 that an inhibitor of PGF2α biosynthesis prevented or minimized the prominence of PGFM pulses and increased the frequency of persistent CL was supported.

Effect of number of 6-mm predeviation follicles and intraovarian patterns on right-side ovulation in heifers.

The mechanisms underlying the greater frequency of ovulation from right ovary (RO) than left ovary (LO) were studied in 145 Bos taurus heifers. Diameter deviation during a follicular wave is indicated by continued diameter increase in the future preovulatory follicle (PF) and a decrease in diameter of future subordinate follicles. The PF (ovulation) was in the RO (63/105, 60%) more frequently (P < 0.04) than in the LO (40%) for ovulatory wave 2 of two-wave interovulatory interval (IOI) but not for wave 3 of three-wave IOI (RO, 45%; LO, 55%). The difference between waves 2 and 3 was associated with a greater (P < 0.01) number of 6-mm follicles during predeviation in the RO (2.3 ± 0.2 follicles) than LO (1.8 ± 0.1) for wave 2 but not for wave 3. Combined for ovulatory waves 2 and 3, the PF developed in the ovary with more 6-mm follicles during more (P < 0.0001) ovulatory waves (72%) than when the PF developed in the ovary with fewer follicles (11%). Ovulation from RO was more frequent (P < 0.02) when both PF and CL were in RO (46/72, 64%) than in LO (36%), but not when PF was alone in RO without the CL. Results supported the hypotheses that: (1) the number of 6-mm predeviation follicles developing during the ovulatory wave is greater for RO and (2) the number of predeviation follicles is greater for the ovary in which the preovulatory follicle subsequently develops.

Follicle blood flow and FSH concentration associated with variations in characteristics of follicle selection in heifers.

Selection of the dominant follicle during a follicular wave is manifested by diameter deviation. At deviation (day 0), growth rate continues for the future dominant follicle (F1) and begins to decrease for the largest subordinate follicle (F2). The percentage of color-Doppler blood-flow signals in the wall of F1 and F2 and the temporality between FSH concentration and F1 and F2 diameter were determined daily in waves 1 and 2 in 24 Holstein heifers. Diameter and blood flow were compared among classes of deviation: (1) conventional (F2 ≥ 7.0 mm on day 0), (2) F2-undersized (F2 < 7.0 mm on day 0), and (3) F1,F2-switched (F2 larger than F1 on days -1 or 0). A class-by-day interaction for diameter of F2 (P < 0.004) and for blood-flow percentage of F2 (P < 0.02) represented greatest values on days -1 or 0 in the switched class and greater values in the conventional than undersized class. Changes were similar between diameter and blood flow in F1 and F2 before deviation. Blood flow in F2 decreased sooner than diameter after deviation indicating that a decrease in vascular perfusion preceded a decrease in diameter. Relationships between F1 and FSH in conventional deviation were similar between waves 1 and 2 for (1) growth rate of F1 on days -1 to 0, (2) interval from emergence of F1 at 4 mm to deviation, and (3) decrease in FSH on days -2 to 0. Relationships between F2 diameter and FSH were dissimilar between classes and between waves 1 and 2 indicating other hormones or factors are also involved in the complex control of F2. For example, the growth rate of F2 was greater (P < 0.05) for conventional than undersized class during wave 1 but similar between classes during wave 2. The FSH surge 2 was similar in profile and prominence between classes but the interval from the FSH peak of surge 2 to deviation was shorter (P < 0.05) in the undersized class (1.5 ±â€¯0.3 d) than in the conventional class (2.3 ±â€¯0.3 d). This was a novel finding and accounted for some of the dissimilarities in growth rate of follicles between classes in wave 2. Results did not support the hypothesis that the extent of blood flow in the wall of future dominant and largest subordinate follicles before deviation is an earlier indicator of follicle destiny than diameter. Results supported the hypothesis that follicle dynamics and FSH concentrations before deviation are temporally associated within conventional and undersized deviation classes but the temporality differs between classes.

Factors affecting side of ovulation in heifers and mares-A comparative study.

Effect of side (left ovary, LO; right ovary, RO) and intraovarian presence of the regressing corpus luteum (CL) on the preovulatory follicle (PF) and ovulation were studied using ultrasonic- records of complete interovulatory intervals (IOI) for 500 IOI in heifers and 485 IOI in mares. Only IOI with a recording of side of a single PF and a single CL at each end of the IOI were used. In heifers, ovulation at the end of the IOI occurred more frequently (P < 0.0001) from RO (n = 285, 57%) than LO (n = 215, 43%). In mares, frequency of ovulation did not differ between RO (n = 250. 52%) and LO (n = 235, 48%). In heifers but not in mares, the number of predeviation follicles has been reported to be greater in RO than LO. In heifers, RO ovulation was more frequent when CL was in PF ovary (66%) than when CL was in opposite ovary (50%). There was no difference in side of ovulation (RO, 50%, LO, 50%). In mares, there were no significant main effects or interactions for frequency of ovulation involving side and intraovarian presence of the CL. On a species comparative basis, the present and reported results supported hypothesis 1 that a greater number of predeviation follicles in RO than in LO is associated with the predilection for RO ovulation. Results also supported hypothesis 2 that ovulation from RO in heifers is more frequent when RO contains the regressed CL than when the CL is in LO.

Gonadotropin concentrations associated with variations in diameter deviation during follicle selection in Holstein heifers.

Diameter deviation or selection of the future dominant follicle (F1) from the future largest subordinate follicle (F2) during a follicular wave occurs when F1 is 8.5 mm (expected deviation, day 0). Deviation has been classified as conventional (F2 ≥ 7.0 mm), F2-undersized (F2 < 7.0 mm), and F1,F2-switched (F2 larger than F1 on day -1 or 0). Concentrations of gonadotropins were compared within and among deviation classifications in waves 1 and 2 in 48 heifers. A three-way (wave 1 compared with 2, classification, day) analysis indicated no effect of wave 1 compared with 2 on F2 or FSH. An interaction of classification by day for F2 diameter (P < 0.001) and FSH concentration (P < 0.005) was primarily from differences on day -1. Rankings on day -1 from greatest to least for F2 diameter were switched, conventional, and undersized and for FSH concentration were undersized, conventional, and switched. Lower FSH concentration in conventional compared with undersized deviations during the decline in the FSH surge was presumed to represent greater output of FSH inhibitors by larger follicles. The incline in FSH surge 2 began significantly later for undersized than for conventional deviation. Switched deviation was associated significantly with emergence of F2 before F1, lower FSH concentration during the decline in the FSH surge, and earlier occurrence of the post-surge FSH nadir. Results supported the hypothesis that diameter differences among deviation classifications are temporally associated with differences in FSH concentration within each classification. These novel findings may complicate studies on the mechanisms of follicle selection.

Concentrations of follicle stimulating hormone associated with follicle selection, number of follicles, and ipsilateral vs contralateral relationships in mares.

Diameter deviation or follicle selection during a follicular wave begins with continuous growth rate of F1 (dominant or selected follicle; DF) and decreasing growth rate of F2 (largest subordinate). Intraovarian patterns based on presence or absence of the DF and CL are DF-CL, DF, CL, and devoid (neither DF nor CL). The DF and CL relationships in a pair of ovaries are ipsilateral (DF-CL pattern and devoid pattern) and contralateral (DF pattern and CL pattern). The effects of patterns and relationships on F1, F2, and FSH during deviation were determined in 21 mares. Diameters of F1 and F2 at expected beginning of deviation (F1, 22.5 mm; day 0) defined deviation classifications of conventional (F2 ≥ 18.5 mm on day 0), F2-undersized (F2 < 18.5 mm), and F1,F2-switched (F2 larger than F1 on day -1 or 0). During the decline in the FSH surge, an interaction (P < 0.001) of deviation classification and day indicated that FSH concentration was greater (P < 0.05) in undersized than in conventional deviations on days -3 and -1 and intermediate in switched deviation. Low FSH during days -3 to -1 in switched deviation began to increase on day -1 during switching in diameter rank between F1 and F2. The number of subordinate follicles per wave that attained a maximum of 11 to <16 mm was greatest in the DF-CL and devoid patterns and in ipsilateral (4.2 ±â€¯0.5 follicles) than in contralateral (2.5 ±â€¯0.7) relationships (P < 0.03). Concentration of FSH was greater (P < 0.04) on days -4 to 5 in ipsilateral (5.3 ±â€¯0.2 ng/mL) than in contralateral (4.2 ±â€¯0.4 ng/mL) relationships consistent with more follicles 11 to < 16 mm. In a survey (N = 63), an ipsilateral vs contralateral relationship was associated with 17 vs 17 conventional deviations and 7 vs 22 nonconventional deviations (P < 0.04), and conventional deviation (54%) was more frequent (P < 0.0001) than undersized (21%) or switched (25%) deviations. Results supported the hypotheses that (1) diameter deviation can be classified in mares into distinct categories based on diameter and rank of the future largest subordinate at expected deviation, (2) the number of subordinate follicles is influenced by ipsilateral vs contralateral relationships of DF and CL, and (3) conventional deviation is more common when the relationship between DF and CL is ipsilateral. Knowledge on follicle diameters for the different deviation classifications should be considered in studying follicle selection especially when terminal follicle sampling is done before expected deviation.

Follicles and gonadotropins during waves 2 and 3 in three-wave interovulatory intervals in Bos taurus heifers.

Observations were made on follicle dynamics and gonadotropin concentrations in anovulatory wave 2 and ovulatory wave 3 in three-wave interovulatory intervals (n = 15). Hypotheses were not used owing to inadequate availability of rationale. The future dominant follicles for waves 2 and 3 were designated DF2 and DF3 and the largest future subordinate follicles as SF2 and SF3, respectively. The day of expected diameter deviation (day 0) was defined as the day that DF2 or DF3 was closest to 8.5 mm. The first day that DF2 became smaller (P < 0.05) than DF3 was day 2 (10.7 ± 0.2 mm vs 11.8 ± 0.3 mm). The FSH surges 2 and 3 that stimulated waves 2 and 3 were similar at peak concentration, but the postsurge nadir of surge 2 occurred 1 day earlier than for surge 3. An LH increase was not temporally associated with deviation in wave 2, but an increase (P < 0.05) in LH in wave 3 began on day -1. Diameter of SF2 (6.5 ± 0.2 mm) on day 0 was less (P < 0.005) than for SF3 (7.2 ± 0.2 mm). Mean diameter of subordinate follicles in wave 2 did not differ among days. Diameter of subordinate follicles that attained ≥6 mm in wave 3 was greater (interaction, P < 0.02) by day 3 when in the right ovary (RO, 7.4 ± 0.2 mm) than when in the left ovary (LO, 5.6 ± 0.2 mm). The frequency of a conventional classification of deviation (future SF greater than 7.0 mm on day 0) was less (P < 0.001) for wave 2 (1 of 15 waves) than for wave 3 (8 of 15 waves). Novel observations involving DF2 and DF3 were (1) before deviation, diameter of DF2 vs DF3 and an incline in FSH surge 2 vs surge 3 were similar and (2) after deviation, smaller diameter of DF2 vs DF3 by day 2 was associated with an earlier cessation (nadir) in FSH surge 2 vs surge 3 and an absence of an LH increase during deviation. Novel observations involving subordinate follicles ≥6 mm were (1) before deviation, diameters were similar between waves 2 and 3 in association with the similar incline in FSH surges 2 vs 3 and (2) after deviation, a greater diameter increase of subordinates occurred in RO than in LO for wave 3, but an increase did not occur for either ovary in wave 2. The characteristics of diameter deviation were profoundly different between waves 2 and 3 owing to a smaller SF2 than SF3 at deviation but similar diameter of DF2 and DF3.

Blood flow and echotextural differences between the future dominant and subordinate follicles before the beginning of diameter deviation in heifers.

Diameter deviation is the beginning of a decrease in growth rate of the largest subordinate follicle (SF) and a continuing growth rate of the dominant follicle (DF). In wave 1 in cattle, deviation begins 2 or 3 days after ovulation when the future DF is about 8.5 mm. Gray scale and power-flow Doppler ultrasound examinations were done in experiment 1 (daily examinations, n = 13) and experiment 2 (examinations every 8 h, n = 15). Blood flow and an anechoic layer in the follicle wall were normalized to the beginning of diameter deviation (day 0 or hour 0). Only waves with conventional diameter deviation (68% of waves) were used as identified by: (1) future SF greater than 7.0 mm when DF was 8.5 mm and (2) future DF and SF did not switch in diameter rank. In experiment 1, deviations in the extent of blood-flow signals and in the extent of anechoic layer began on the same day as deviation in diameter. In experiment 2, deviations in diameter, surface area (πd2), and anechoic layer began in synchrony, and deviation in blood-flow signals began 16 h earlier. Blood-flow deviation before diameter deviation was shown by (1) a first difference (P < 0.02) between follicles at hour -16 and (2) development during the hours -24 to 0 of a greater (P < 0.05) percentage difference between follicles in blood-flow signals (11.1 ± 2.3%) than in surface area (7.4 ± 0.7%) or diameter (4.5 ± 0.4%). Results supported the hypothesis that the extent of blood flow in the future dominant and subordinate follicles deviates before diameter deviates. A similar hypothesis for anechoic layer was not supported; diameter and anechoic layer deviated in synchrony.