J.M. Baldrighi

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.

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.

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.

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.

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.

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 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.

Defective secretion of Prostaglandin F2α during development of idiopathic persistent corpus luteum in mares

Five mares that developed idiopathic persistent corpus luteum (PCL) were compared with 5 mares with apparently normal interovulatory intervals (IOIs). Progesterone (P4) and a metabolite of prostaglandin F2α (PGFM) were assayed daily beginning on the day of ovulation (Day 0). Transition between the end of an initial progressive P4 increase and the beginning of a gradual decrease in P4 occurred on mean Day 6. The gradual decrease in P4 between Days 6 and 12 was less (approached significance, P < 0.06) in the PCL group than in the IOI group. The P4 concentration on Day 12 (before luteolysis in IOI group) was greater (P < 0.05) in the PCL group than in the IOI group. In a post hoc comparison, an interaction (P < 0.04) of group by day for Days 4 to 7 indicated that the end of the progressive increase in P4 was temporally associated with a transient increase in concentration of PGFM in IOI mares but not in PCL mares. Complete luteolysis (P4 < 1 ng/mL) occurred in the IOI mares on Days 13 to 15. Partial luteolysis (mean P4 decrease, 62%) occurred in 3 of the 5 PCL mares. Normalization to the day at the end of the most pronounced P4 decrease in the IOI mares and in the 3 PCL mares with partial luteolysis resulted in a day-by-group interaction (P < 0.05) for PGFM concentration. The interaction was partly from lower PGFM concentration on the day at the end of the pronounced P4 decrease in the 3 PCL mares than in the IOI mares. The peak of a transient PGFM increase and the day at the end of the most pronounced decrease in P4 were synchronized in each IOI mare but not in any of the 3 PCL mares. In the other 2 PCL mares, partial luteolysis did not occur, and a transient increase in PGFM was not apparent. Results tentatively indicated that the relationship between P4 and PGFM may be altered as early as Day 6 in PCL mares and supported the hypothesis that prostaglandin F2α secretion is defective in mares with idiopathic PCL.

Relationships among nitric oxide metabolites and pulses of a PGF2α metabolite during and after luteolysis in mares

Hourly circulating concentrations of a PGF2α metabolite (PGFM), progesterone (P4), and LH were obtained from a reported project, and concentrations of nitric oxide (NO) metabolites (NOMs; nitrates and nitrites) were determined in eight mares. Unlike the reported project, hormone concentrations were normalized to the peak of the first PGFM pulse of luteolysis (early luteolysis), second PGFM pulse (late luteolysis), and a pulse after luteolysis. The duration of luteolysis was 23.1 ± 1.0 hours, and the peak of the first and second PGFM pulses occurred 6.5 ± 0.9 and 14.8 ± 0.8 hours after the beginning of luteolysis. Concentration of P4 decreased progressively within and between the PGFM pulses Changes were not detected in LH concentration in association with the PGFM pulses. Concentration of NOMs was greater (P < 0.05) at the peak of the PGFM pulse during early luteolysis (88.8 ± 15.0 μg/mL) than during late luteolysis (58.8 ± 9.0 μg/mL). Concentration of NOMs began to decrease (P < 0.05) 4 hours before the peak of the PGFM pulse of early luteolysis. Concentration began to increase (P < 0.05) an hour after the peak of the PGFM pulse of late luteolysis. An NOM decrease and increase was not detected during the PGFM pulse after luteolysis. On a temporal basis, results indicated that NO either is not required for luteolysis in mares or has a role in or responds only during late luteolysis. A caveat is that the relative contribution of the CL versus other body tissues to circulating concentrations of NOMs in mares has not been determined.