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.

An intraovarian mechanism that enhances the effect of an FSH surge on recovery of subordinate follicles in heifers.

The effect of the future dominant follicle (DF), corpus luteum (CL), and side (left ovary [LO] and right ovary [RO]) on FSH-induced recovery (increase in diameter) of regressing subordinate follicles was studied in heifers. The DF of wave 2 and the largest subordinate follicle remained intact (controls, n = 14 heifers) or were ablated (n = 14 heifers) on a mean of 13 d postovulation when the DF was ∼10 mm (hour 0). Concentration of FSH (P < 0.0004) and diameter of subordinate follicles (P < 0.0002) decreased between hours -48 to 0 combined for the control and ablation groups. Thereafter, follicle diameter continued to decrease in the controls. Concentration of FSH increased (P < 0.05) and diameter of subordinates began to increase at hour 12 in the ablation group. Follicle-stimulating hormone increased to hour 24 and then returned to the hour 0 concentration by hour 72, completing the induced FSH surge. Concentration of LH began to increase at hour 0 in each group and at a similar rate between groups. Follicle recovery in the ablation group was compared among 8 subgroups as defined by the 2 sides and 4 intraovarian patterns (DF-CL pattern, both structures in same ovary; DF pattern, DF alone; CL pattern, CL alone; and devoid pattern, both structures absent). Follicle diameter increased (P < 0.05) between hours 24 and 48, and diameter at hours 24, 48, 72, and 96 involved a 3-way interaction (P < 0.0001) of pattern, side, and hour. The interaction was similar when diameter of the DF that originated from a recovered subordinate was either included or excluded in the analysis. Diameter of subordinate follicles in the ablation group at hour 96 was greater (P < 0.05) in the DF-CL/RO and DF/RO subgroups than that in the devoid/LO, devoid/RO, and CL/LO subgroups. The DF-CL/LO and CL/RO subgroups were intermediate. For follicles that decreased in diameter before hour 0, a greater (P < 0.05) percentage increased after hour 0 when the ovary contained a DF and was in the RO (DF-CL/RO and DF/RO subgroups) than for the remaining subgroups even after excluding the DF that originated from a subordinate. Results supported the hypotheses that (1) an induced FSH surge can stimulate the recovery of regressing subordinate follicles and (2) recovery of regressing subordinate follicles by FSH involves an intraovarian mechanism. Our interpretation is that the intraovarian mechanism that enhances the stimulatory effect of FSH on recovery of subordinate follicles was effective only in RO and only when it contained a DF.

Temporal relationships between minor, preovulatory, or periovulatory FSH surges and the emergence and development of 2-mm follicles of wave 1 in Bos taurus heifers.

The number and day of emergence (first detection) of 2-mm follicles and the number and day when the 2-mm follicles reached 3-, 4-, 5-, and 6-mm during wave 1 were determined every 0.5 d (n = 9 heifers). Emergence of the follicles at each of the indicated diameters was normalized to the beginning and ending nadir and the peak of each of a minor FSH surge, the preovulatory surge, and the periovulatory surge. Relative to the day of ovulation (day 0), the minor FSH surge, preovulatory surge, and periovulatory surge encompassed (nadir to nadir) days -7.0 to -2.5 (peak, day -4.0), days -2.5 to -0.5 (peak, day -1.0), and days -0.5 to 4 (peak, day 0), respectively. Distinct mean nadirs occurred between the minor and preovulatory surges and between the preovulatory and periovulatory surges. A small percentage of 2-mm follicles (12%) and 3-mm follicles (2%) emerged during the minor FSH surge. The 4-mm follicles emerged during the preovulatory surge (24% of follicles) and periovulatory surge (76%). The 5-mm and 6-mm follicles emerged only during the periovulatory surge. The first increase (P < 0.05) in number of 2-, 3-, and 4-mm follicles began at 1.5, 1.0, and 0 d, respectively, before the nadir at the beginning of the preovulatory surge. The first increase (P < 0.05) in number of 5- and 6-mm follicles began at 0.5 and 0 d, respectively, before the intervening nadir between the preovulatory and periovulatory surges. Results demonstrated that each of the 3 surges including the minor surge contributed to the emergence of follicles at various diameters during wave 1. The emergence of 2-mm follicles during the descending portion of the minor surge indicated that smaller follicles (eg, 1 mm) apparently emerged during the major portion of the minor surge. The increasing diameter of the 2 largest follicles was not interrupted during the distinct intervening nadir between the preovulatory and periovulatory FSH surges.

Complexities of follicle deviation during selection of a dominant follicle in Bos taurus heifers.

Follicle deviation during a follicular wave is a continuation in growth rate of the dominant follicle (F1) and decreased growth rate of the largest subordinate follicle (F2). The reliability of using an F1 of 8.5 mm to represent the beginning of expected deviation for experimental purposes during waves 1 and 2 (n = 26 per wave) was studied daily in heifers. Each wave was subgrouped as follows: standard subgroup (F1 larger than F2 for 2 days preceding deviation and F2 > 7.0 mm on the day of deviation), undersized subgroup (F2 did not attain 7.0 mm by the day of deviation), and switched subgroup (F2 larger than F1 at least once on the 2 days before or on the day of deviation). For each wave, mean differences in diameter between F1 and F2 changed abruptly at expected deviation in the standard subgroup but began 1 day before expected deviation in the undersized and switched subgroups. Concentrations of FSH in the wave-stimulating FSH surge and an increase in LH centered on expected deviation did not differ among subgroups. Results for each wave indicated that (1) expected deviation (F1, 8.5 mm) was a reliable representation of actual deviation in the standard subgroup but not in the undersized and switched subgroups; (2) concentrations of the gonadotropins normalized to expected deviation were similar among the three subgroups, indicating that the day of deviation was related to diameter of F1 and not F2; and (3) defining an expected day of deviation for experimental use should consider both diameter of F1 and the characteristics of deviation.

Temporality of two-way functional coupling between FSH and follicles in heifers.

The intervals from removal of FSH suppressants by follicle ablation to an increase in FSH and from an increase in FSH to an increase in follicle diameter were determined hourly during follicular wave 1 in a follicles-intact group (n = 7) and a follicles-ablated group (n = 20). Follicles other than the largest subordinate follicle of wave 1 (SF1) were ablated when the dominant follicle of wave 1 (DF1) was about 11 mm (hour 0) and FSH concentration was basal. In an ablated-regressed subgroup (n = 5), SF1 diameter decreased constantly during hours 0 to 8. In an ablated-maintained subgroup (n = 15), SF1 decreased for 2 or 3 hours and then increased (n = 11) or increased constantly (n = 4). Average diameter of SF1 during 8 hours was greater (P < 0.01) in the maintained subgroup (8.3 ± 0.07 mm) than in the regressed subgroup (7.3 ± 0.09 mm). Concentration of FSH increased (P < 0.02) similarly between the two ablated subgroups but did not change during the 8 hours in the follicles-intact group. In each wave in the ablated-maintained subgroup with a decrease and then an increase in SF1 diameter (n = 11), the SF1 increase was preceded by an FSH increase. The interval from hour 0 (removal of source of FSH suppressants) to the beginning of an increase in FSH was 0.8 ± 0.3 hours. The interval from the beginning of an FSH increase to the beginning of an SF1 diameter increase was 2.8 ± 0.4 hours. The immediate coupling and decoupling between follicles and FSH may be essential aspects of follicle deviation during selection of DF1 by impeding the growth rate of the future SF1 before it can attain the developmental stage needed for continued growth during low FSH.

Characteristics and functions of a minor FSH surge near the end of an interovulatory interval in Bos taurus heifers.

The apparent function of a minor FSH surge based on temporality with follicular events was studied in 10 heifers with 2 follicular waves per interovulatory interval. Individual follicles were tracked from their emergence at 2 mm until their outcome was known, and a blood sample was collected for FSH and LH assay every 12 h from day -14 (day 0 = ovulation) to day 4. A minor FSH surge occurred in each heifer (peak, day -4.6 ± 0.2). Concentration of LH increased (P < 0.05) during the FSH increase of the minor surge but did not decrease during the FSH decrease. A minor follicular wave with 8.2 ± 2.0 follicles occurred in 6 of 10 heifers. The maximal diameter (mean, 3.4 ± 0.9 mm) of 77% of the minor-wave follicles occurred in synchrony on day -4.4 ± 0.4. Most (59%) of minor-wave follicles regressed before ovulation and 41% decreased and then increased in diameter (recovered) on day -1.9 ± 0.3 to become part of the subsequent wave 1. A mean of 3.7 ± 0.9 regressing subordinate follicles from wave 2 recovered on the day before or at the peak of the minor FSH surge. The growth rate of the preovulatory follicle decreased (P < 0.02) for 3 d before the peak of the minor FSH surge and then increased (P < 0.03). Concentration of LH increased slightly but significantly temporally with the resurgence in growth rate of the preovulatory follicle. A minor LH surge peaked (P < 0.0002) on day 3 at the expected deviation in growth rates between the future dominant and subordinate follicles. Results indicated on a temporal basis that the recovery of some regressing subordinate follicles of wave 2 was attributable to the minor FSH surge. The hypothesis was supported that some regressing follicles from the minor follicular wave recover to become part of wave 1.

Functional angiocoupling between follicles and adjacent corpus luteum in heifers.

In single ovulating cattle, ipsilateral versus contralateral interovarian relationships refer to a dominant follicle (DF) and CL in the same versus opposite ovaries. The ipsilateral relationship consists of the DF-CL and the devoid (no DF or CL) intraovarian pattern, and the contralateral relationship consists of the DF pattern and the CL pattern. The DF-CL pattern involves positive effects on both the DF and CL when adjacent (≤3-mm apart) versus separated as follows: greater diameter of DF (e.g., 10.5 ± 0.4 vs. 9.0 ± 0.4 mm), greater percentage of the DF wall with color Doppler signals of blood flow (40.2% ± 2.0% vs. 24.5% ± 1.9%), greater cross-sectional area of the CL (2.2 ± 0.1 vs. 1.8 ± 0.2 cm(2)), and greater percentage of the entire CL with blood flow signals (51.8% ± 1.2% vs. 42.5% ± 3.1%). Additional examples of positive coupling are (1) future DF on Day 0 (day of ovulation) is closer to the CL than the future largest subordinate and (2) diameter of growing follicles on Day 0 and the growth rate on Days 0 to 2 are greater for follicles that are adjacent than separated from the CL. An example of a negative intraovarian effect is decreasing diameter and loss of future DF status of a largest follicle when adjacent to a regressing CL. The impact of the continuity of ovarian angioarchitecture during the periovulatory follicular wave was exemplified in 17 of 18 waves by conversion of an ovary with only the preovulatory follicle to the postovulatory DF-CL pattern. Functional angiocoupling from commonality in angioarchitecture of the DF and adjacent CL would account for both the positive two-way coupling between DF and CL during the luteal phase and the negative effect of a regressing CL on an adjacent follicle during luteolysis.

Stimulation of regressing subordinate follicles of wave 2 with a gonadotropin product in heifers.

The recovery of regressing wave-2 subordinate follicles was studied by treating heifers with a gonadotropin product that had about 84% and 16% of follicle-stimulating hormone and luteinizing hormone activity, respectively. A treated group (n = 8) received a single dose of 50 mg (2.5 mL) of the gonadotropin product, and a control group (n = 8) received 2.5 mL of saline vehicle. The group assignment of heifers was not known to the ultrasonographer who tracked the follicles and measured follicle diameters. Follicle measurements began on the day of expected follicle deviation in wave 2 (largest follicle closest to 8.5 mm), and treatment (hour 0) was given on Day 13.4 ± 0.2 (Day 0 = ovulation) when the dominant follicles of waves 1 and 2 were 14.1 ± 0.3 mm and 10.7 ± 0.1 mm, respectively. Subordinate follicles of wave 2 that had regressed to a 3-mm category (3.0-3.9 mm) or 4-mm category by hour 0 decreased in diameter for at least 48 h before hour 0, whereas follicles that were in the 5-mm or 6-mm categories at hour 0 did not change significantly in diameter during the previous 48 h. About 55% of the follicles that had regressed to the 3-mm and 4-mm categories at hour 0% and 78% of the follicles in the 5-mm and 6-mm categories increased in diameter after gonadotropin treatment, whereas follicles in the control group continued to decrease (regress) in diameter. The follicles for each of the 4 diameter categories were greater (P < 0.05) in diameter 9 h after treatment in the treated group than in the control group. The dominant follicle of wave 1 and the largest subordinate follicle of wave 2 in the treated group also increased in diameter so that diameter was greater (P < 0.05) than in the controls at hour 9. The results demonstrated that subordinate follicles of wave 2 that had decreased in diameter (regressed) for at least 48 h retained the capability to recover as indicated by a diameter increase when exposed to a gonadotropin product.

Systemic effect of follicle-stimulating hormone and intraovarian effect of the corpus luteum on complete regression vs recovery of regressing wave-2 follicles in heifers.

Each subordinate of the second follicular wave (wave 2) was monitored, and the outcome was classified as fully regressed (decreased in diameter to 2 mm) or recovered (decreased initially and then increased to become a growing follicle of the subsequent wave 1). The changing diameter of each follicle after emergence at 2 mm and plasma concentration of follicle-stimulating hormone were determined every 12 h from the day of ovulation (Day 0) to 4 d after the subsequent ovulation in heifers with 2 follicular waves per interovulatory interval (n = 10). The number and percentage of wave-2 subordinates that initially regressed and then recovered (7.2 ± 1.0 follicles; 33.2 ± 5.1%) were less (P < 0.0008) than the number and percentage that completely regressed (15.0 ± 1.7; 66.8 ± 5.1%). Follicles that later recovered initially reached maximal diameter on a later day (P < 0.0001) after emergence at 2 mm (4.3 ± 0.2 d) and at a larger (P < 0.0001) diameter (5.8 ± 0.2 mm) than follicles that completely regressed (3.2 ± 0.1 d; 4.7 ± 0.1 mm). The follicle-stimulating hormone surge that stimulated wave 2 began earlier and was more sustained in a subgroup with a high percentage of recovered follicles (61%) than in a subgroup with a low percentage (24%). Recovery began on Day -1.0 ± 0.1 when the follicles had regressed to 3.7 ± 0.1 mm. Diameter of subordinate follicles on Day -6 or before the expected days of luteolysis was greater (P < 0.05) when in the corpus luteum (CL) ovary than when in the non-CL ovary. During expected luteolysis, more follicles (P < 0.008) per ovary continued to regress when ipsilateral to the CL (9.2 ± 1.1 follicles) than when contralateral (5.8 ± 1.1), and more follicles (P < 0.02) recovered from regression when contralateral to the CL (5.0 ± 0.8) than when ipsilateral (2.2 ± 0.6). The hypothesis that the CL has a local effect on the development, regression, and recovery of the subordinate follicles of wave 2 was supported.

Mechanism for greater frequency of contralateral than ipsilateral relationships between corpus luteum and ovulatory follicle for wave 3 in heifers.

During the last wave of the interovulatory interval (IOI), the permutations of the relationship between the ovulatory follicle and the CL (ipsilateral vs. contralateral) and the number of follicular waves (two vs. three) per IOI differ in frequency of occurrence as follows: ipsilateral relationship and two waves (34%), contralateral relationship and two waves (34%), ipsilateral relationship and three waves (8%), and contralateral relationship and three waves (24%). Deviation or the continuation in growth rate of the future ovulatory follicle and a decrease in growth rate of the future subordinate follicles begin well before luteolysis in two-wave IOIs and during luteolysis in three-wave IOIs. The largest follicle decreases in diameter and loses its dominant status before completion of deviation when it is ipsilateral and adjacent to the regressing CL during wave 3. Dominant status switches from the largest follicle in the ipsilateral ovary to the next-largest follicle which may be in either ovary. Switching accounts for the greater frequency of a contralateral follicle-CL relationship than for ipsilateral follicle-CL relationship during the ovulatory wave in three-wave IOIs. It is proposed that the phenomenon results from commonality in angioarchitecture so that the decrease in blood flow to the regressing CL is associated with a decrease in blood flow to adjacent follicles.

Stimulation of LH, FSH, and luteal blood flow by GnRH during the luteal phase in mares.

A study was performed on the effect of a single dose per mare of 0 (n = 9), 100 (n = 8), or 300 (n = 9) of GnRH on Day 10 (Day 0 = ovulation) on concentrations of LH, FSH, and progesterone (P4) and blood flow to the CL ovary. Hormone concentration and blood flow measurements were performed at hours 0 (hour of treatment), 0.25, 0.5, 1, 2, 3, 4, and 6. Blood flow was assessed by spectral Doppler ultrasonography for resistance to blood flow in an ovarian artery before entry into the CL ovary. The percentage of the CL with color Doppler signals of blood flow was estimated from videotapes of real-time color Doppler imaging by an operator who was unaware of mare identity, hour, or treatment dose. Concentrations of LH and FSH increased (P < 0.05) at hour 0.25 and decreased (P < 0.05) over hours 1 to 6; P4 concentration was not altered by treatment. Blood flow resistance decreased between hours 0 and 1, but the decrease was greater (P < 0.05) for the 100-µg dose than for the 300-µg dose. The percentage of CL with blood flow signals increased (P < 0.05) between hours 0 and 1 with no significant difference between the 100- and 300-µg doses. The results supported the hypothesis that GnRH increases LH concentration, vascular perfusion of the CL ovary, and CL blood flow during the luteal phase; however, P4 concentration was not affected.

Persistence and recovery of regressing 3-mm ovarian follicles in heifers.

The persistence and outcome of 3-mm follicles before the emergence of follicular wave 1 were studied every 6 hours in 15 heifers beginning on Day 14 (Day 0 = ovulation). A mean of 9.1 ± 1.3 persistent 3-mm follicles (P3Fs) per heifer was detected with persistence for 3.5 ± 0.1 days. The P3Fs either regressed continuously and remained in the 3-mm range (3.0-3.9 mm) or regressed but with a transient increase in diameter during regression. Some (43%) P3Fs were rescued to become growing follicles in wave 1. The number of follicles that became part of wave 1 was less (P < 0.0001) for follicles that originated from a P3F (4.2 ± 1.0 P3Fs) than for follicles that did not originate from a P3F (11.9 ± 1.6 follicles). The day of rescue of wave 1 follicles from a P3F (Day -1.1 ± 0.6) was earlier (P < 0.001) than for emergence of follicles at 3 mm that did not originate from a P3F (Day -0.5 ± 0.5). A cluster of 5.1 ± 0.6 P3Fs was identified in 10 of 15 heifers by the synchronized peaks of transient diameter increases at the 6-hour interval corresponding to Day -4.0 ± 0.3. Concentrations of FSH oscillated at 12-hour intervals with a peak (P < 0.05) 6 hours before and 6 hours after the beginning of a transient diameter increase during a P3F. Concentration of FSH was greater (P < 0.02) in heifers with a high number (11-18) of P3Fs per heifer (0.27 ± 0.02 ng/mL) than with a low number (2-9) per heifer (0.17 ± 0.008 ng/mL). Results supported the novel hypothesis that 3-mm follicles may persist for two or more days and may be rescued to become growing follicles of wave 1.

Differences between follicular waves 1 and 2 in patterns of emergence of 2-mm follicles, associated FSH surges, and ovarian vascular perfusion in heifers.

The emergence (first detection) of 2-mm follicles, FSH surges, and ovarian vascular perfusion for follicular wave 1 and surge 1 (n = 26) and wave 2 and surge 2 (n = 25) were studied daily in heifers. The day the future dominant follicle was closest to 5.5 mm was designated Day 0 for each wave. In wave 1, many 2-mm follicles (41%) emerged on Days -5 to -3, whereas FSH surge 1 did not begin until Day -3. Concentration of FSH increased abruptly in 1 day to a peak on the day of maximal number of emerging 2-mm follicles, although the day of maximal number relative to Day 0 differed among individuals. The first emergence of 2-mm follicles in wave 2 occurred concurrently with the first increase in the FSH of surge 2. In wave 1, ovarian resistance to vascular perfusion was negatively correlated (r = -0.48, P < 0.05) with a number of 2-mm follicles on Days -4 to -1 for ovaries that did not contain the preovulatory follicle; vascular perfusion increased with an increase in the number of small follicles. The following hypotheses were supported for wave 1 but not for wave 2: (1) an increase in the number of emerging 2-mm follicles of a follicular wave occurs before the beginning of an increase in FSH, (2) the day of maximal number of emerging 2-mm follicles occurs concurrently with an abrupt FSH increase on different days among individuals, and (3) the association between the number of emerging 2-mm follicles and the extent of ovarian vascular perfusion is positive.

Effects of conversion of follicular activity from wave 1 to wave 2 and proximity of wave 2 follicles to CL in heifers.

Effects of the dominant follicle (DF) of follicular wave 1 on follicles and ovarian vascular perfusion during wave 2 and the effects of intraovarian distance between a follicle and CL on follicles of wave 2 were studied daily (N = 28 heifers). Intraovarian patterns were DF1-CL and DF2-CL (DF and CL in the same ovary for waves 1 and 2, respectively), DF1 and DF2 (DF alone), CL (CL alone), and devoid (ovary with neither DF nor CL). On the basis of blood flow resistance and the number of follicles per ovary, the wave 1 patterns of DF1 versus devoid resulted in greater (P < 0.05) vascular perfusion and more (P < 0.05) follicles in wave 2 for the following patterns: (1) conversion of DF1 to DF2 than in conversion of devoid to DF2 and (2) conversion of DF1 to devoid than in conversion of devoid to devoid. On the day of emergence of wave 2 (future DF2 closest to 5.5 mm) in two-wave interovulatory intervals, the mean diameter of all follicles that were adjacent (distance, ≤1 mm) to the CL (4.4 ± 0.3 mm) was greater (P < 0.05) than that for follicles that were separated (3.4 ± 0.2 mm). The hypotheses were supported that (1) the extent of vascular perfusion for the intraovarian patterns of wave 1 affects the perfusion and the number of follicles for the patterns of wave 2 and (2) close proximity of a follicle to the CL in wave 2 has a positive effect on the follicle.

Spontaneous and experimental conversion of a regressing subordinate follicle of wave 1 to the dominant follicle of wave 2 in heifers.

Examination of daily ultrasound records from a previous study indicated that spontaneous conversion of a regressing largest subordinate follicle (SF) of wave 1 (SF1) to the dominant follicle (DF) of wave 2 (DF2) occurred on Day 6 or 7 (Day 0 = ovulation) in two of 28 heifers (7%). A conversion was considered definitive on the basis of no other SFs in the same ovary as SF1, thereby avoiding error in maintaining follicle identity. Spontaneous conversion appeared to involve an FSH fluctuation. In a separate study, experimental conversion of SF1 to DF2 was studied by ultrasonic imaging every 6 hours after ablating follicles other than SF1 when DF of wave 1 was close to 11.0 mm (hour 0). Diameter of SF1 decreased (P < 0.01) between hours -6 (7.8 ± 0.3 mm) and 0 (7.6 ± 0.3 mm). A decrease of 0.1 to 0.8 mm occurred in each heifer, indicating that SF1 was in early regression at hour 0. Conversion occurred in four of 12 (33%) heifers. A diameter increase (P < 0.05) in DF2 after conversion from SF1 occurred between hours 6 and 12. An increase (P < 0.05) in FSH occurred by hour 12 with and without conversion of SF1. Concentration of FSH at each of hours 30 to 48 was greater (P < 0.05) for nonconversion than that for conversion of SF1 to DF2 and greater (P < 0.05) for conversion than that for the basal concentration in controls (n = 7). The hypothesis that a regressing SF1 can be converted to DF2 by ablating other follicles was supported.

Conversion of intraovarian patterns from preovulation to postovulation based on location of dominant follicle and corpus luteum in heifers.

The conversion of preovulatory intraovarian patterns based on location of the preovulatory follicle (PF) and the associated corpus luteum (cl) to postovulatory patterns based on location of the future and established dominant follicle (DF) and corpus luteum (CL) was studied daily in 26 heifers from Days -5 to 6 (Day 0 = ovulation). The two ipsilateral preovulatory patterns were PF-cl and devoid (neither PF nor cl), and the two contralateral patterns were PF and cl. The postovulatory patterns were DF-CL, devoid, DF, and CL. For the contralateral preovulatory relationships, a conversion from PF to DF-CL and the accompanying conversion from cl to devoid occurred most frequently (17 of 18 conversions, 94%). For the ipsilateral preovulatory relationships, a conversion from PF-cl to CL and from devoid to DF occurred most frequently (6 of 8, 75%). Number of 2-mm follicles during preovulation was greatest (P < 0.05) for the devoid and PF patterns, and number of 6-mm follicles during postovulation was greatest (P < 0.05) for the DF-CL and DF patterns. Blood flow resistance at a color Doppler signal in the ovarian pedicle indicated increasing ovarian perfusion over days in the PF to DF-CL and devoid to DF conversions and decreasing perfusion in the PF-cl to CL and cl to devoid conversions. In addition to formation of the CL from the PF, it was interpreted that the conversion of patterns involved number of newly emerging 2-mm follicles per ovary before ovulation and a continuation of the preovulatory angioarchitecture into postovulation. Results supported the novel hypothesis that the four preovulatory intraovarian patterns determine the frequency of the four postovulatory patterns.

Intraovarian factors associated with switching of a future dominant follicle to a subordinate follicle during induced luteolysis in heifers.

The factors involved in the switching of a future dominant follicle (DF) to subordinate status were studied (n = 42) by induction of luteolysis with PGF2α (hour 0) when the largest follicle (F1) in follicular wave 2 was 7.0 or 8.5 mm. Combined for 7.0- and 8.5-mm groups, the frequency of switching was greater (P < 0.01) when F1 and CL were ipsilateral (10 of 28, 36%) than when contralateral (0 of 14). The frequency of switching in the ipsilateral relationship was greater (P < 0.002) when F1 and CL were adjacent (<3 mm apart; 10 of 17) than when separated (0 of 11). The difference in diameter between F1 and F2 was less (P < 0.005) when switching occurred (0.4 ± 0.1 mm) than when switching did not occur (ipsilateral, 1.3 ± 0.2 mm; contralateral, 1.1 ± 0.2 mm). Treatment at hour 0 when F1 was 7.0 mm and ipsilateral to the CL resulted in smaller diameter (P < 0.001) of F1 at hour 12 (7.6 ± 0.2 mm) than when treatment was not given (8.3 ± 0.1 mm). The hypotheses were supported that (1) switching from a future DF to a future subordinate is functionally related to luteolysis, and (2) factors involved in switching include an ipsilateral relationship between the largest follicle and CL and close intraovarian proximity of the follicle to the regressing CL. It is proposed that switching of the future DF to subordinate status during spontaneous luteolysis accounts for the reported greater frequency of the contralateral than ipsilateral relationships between the preovulatory follicle and CL in three-wave interovulatory intervals.

Effect of intraovarian proximity between dominant follicle and corpus luteum on dimensions and blood flow of each structure in heifers.

An intraovarian positive physiologic coupling between the extant CL and the ipsilateral preovulatory follicle (PF) or the future or established postovulatory dominant follicle (DF) was studied in 26 heifers. Ovaries were scanned by ultrasonic imaging from Day 16 (Day 0 = ovulation) of the preovulatory period until Day 6 of the postovulatory period. Hemodynamics of the follicles and CL were assessed by color-Doppler ultrasonography. When the PF and CL were ipsilateral compared with contralateral, blood-flow resistance in wall of the PF was lower (P < 0.04) on Days -2 and -1, and percentage blood-flow signals in the CL approached being greater (P < 0.08) on Days -4 to -1. During the postovulatory period, percentage of DF wall with blood-flow signals (44.1 ± 1.2% vs. 31.4 ± 2.8%) and percentage of CL with blood-flow signals (51.8 ± 1.2% vs. 42.5 ± 3.1%) were each greater (P < 0.05) when the two ipsilateral structures were adjacent (distance between antrum and CL wall, ≤ 3 mm) than when separated. On Day 0, the distance between follicle and ipsilateral CL was less (P < 0.02) for the future DF than for the future largest subordinate. Growth rate between Days 0 and 2 averaged over all growing follicles was greater (P < 0.01) when the follicles were ≤3 mm from the CL (1.1 ± 0.1 mm/day) than when farther from the CL (0.9 ± 0.1 mm/day). Results supported the hypotheses that (1) a positive intraovarian coupling occurs between the PF or postovulatory DF and the extant CL and (2) the coupling is enhanced when the ipsilateral DF and CL are in close proximity.

Switching of largest follicle from dominant to subordinate status when follicle and CL are in same ovary in heifers.

It has been reported that early development of wave 3 in three-wave interovulatory intervals occurs during the luteolytic period, and the frequency of the ipsilateral relationship between the preovulatory follicle and CL is lower (e.g., 33%) than for the contralateral relationship (67%). In this study, luteolysis was induced with PGF2α when the largest follicle of wave 2 reached 8.5 mm or more (diameter at expected deviation). A two-way interaction (P < 0.004) of follicle (first follicle to reach 8.5 mm and the next-largest follicle) by group (ipsilateral and contralateral relationship between the 8.5-mm follicle and CL) represented smaller posttreatment diameter difference between the two follicles within the ipsilateral group than within the contralateral group; the 8.5-mm follicle was smaller and the next-largest follicle was larger in the ipsilateral than in the contralateral group. Switching in the destiny of the 8.5-mm follicle from dominant to subordinate occurred in 3 of 8 (41%) and 0 of 5 (0%) heifers in the ipsilateral and contralateral groups, respectively. These novel findings supported the hypothesis that follicle deviation during luteolysis may result in decreased diameter of the largest follicle and increased frequency of switching of the largest follicle from future dominant to subordinate status when the follicle and CL are ipsilateral. Support for the switching hypothesis indicated that the reported lower frequency of an ipsilateral than contralateral relationship between the preovulatory follicle and CL for wave 3 of three-wave interovulatory intervals can be attributed to development of the wave during luteolysis.