M.A.R. Siddiqui

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.

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.

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.