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Ovary |
Eutheria Foundation,3 Cross Plains, Wisconsin 53528
Department of Animal Health and Biomedical Sciences,4 University of Wisconsin, Madison, Wisconsin 53706
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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8.5 mm; mares, F1 20.0 mm; Hour 0). In heifers, follicular fluid was taken from F2 at Hours 3, 6, 12, or 24; each heifer was sampled only once. In mares, sequential F2 samples were taken from each mare at Hours 0, 6, and 24 or at Hours 12 and 24. Transvaginal ultrasound guidance was used for treatment and sample collection. In heifers, IGF-1 treatment of F2 stimulated the secretion of estradiol (P < 0.05) between Hours 3 and 6 and androstenedione (P < 0.05) between Hours 3 and 12. In F2 of control heifers, estradiol decreased (P < 0.05) and androstenedione did not change significantly. In mares, IGF-1 treatment of F2 did not affect the concentrations of estradiol during the 24-h posttreatment period; androstenedione decreased (P < 0.04) in the IGF-1 group and increased (P < 0.006) in the controls. Compared with control mares, the IGF-1 group had higher (P < 0.04) activin-A at Hours 12 and 24 and higher (P < 0.0006) inhibin-A at Hour 24. After ablating F1 at Hour 24 in mares, F2 became dominant and ovulated in more mares (P < 0.0002) in the IGF-1 group (12/14) than in the control group (2/14). These results are consistent with reported temporal relationships among follicular factors during deviation in both species and indicate that IGF-1 plays a key role in controlling the temporal relationships; however, no indication was found that IGF-1 stimulated estradiol production in mares during the 24 h after treatment.
estradiol, follicular development, growth factors, ovary, steroid hormones
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Follicular-fluid concentrations of estradiol begin to increase in the future dominant follicle at a greater rate than in the future subordinate follicles near the beginning of deviation in diameter in both species (for reviews, see [1, 2]). In mares, free insulin-like growth factor 1 (IGF-1) concentration increases only in the future dominant follicle approximately at the same time as the estradiol increase. In cattle, concentrations of free IGF-1 remain elevated in the future dominant follicle and begin to decrease in the future subordinate follicles concomitantly with the estradiol increase in the future dominant follicle. The higher concentrations of free IGF-1 in the largest follicle near the beginning of deviation in cattle is temporally associated with greater proteolytic degradation of IGF binding protein (IGFBP) 4 and IGFBP-5 in the largest follicle [5]. Concomitantly, the concentrations of IGFBP-4, IGFBP-5 [5], and IGFBP-2 [6] are greater in the second-largest follicle. In mares, a protease to IGFBP-5 has been identified in estrogen-dominant follicles [7]; however, only IGFBP-2 has been studied with reference to deviation, and concentrations increased in the subordinate follicles at the beginning of diameter deviation [8]. On a temporal basis, these results indicate that estradiol and the IGF system play roles in the differential growth rates of the two largest follicles.
The interrelationships between the IGF system and steroidogenesis at the follicle level has been examined in numerous studies in vitro but not in vivo during follicle deviation. Results of tissue-culture studies in various species indicate that estradiol enhances steroidogenesis (cattle), increases the sensitivity of granulosa cells to LH and FSH (rats), and increases IGF-1 synthesis by granulosa cells (pigs and sheep; for reviews, see [2, 9]); however, the in vitro effects of estradiol on equine granulosa cells seemed equivocal [10]. In vitro studies with bovine granulosa cells indicate that IGF-1 stimulates estradiol, progesterone, androstenedione, activin-A, inhibin-A, and follistatin production and enhances sensitivity to FSH (for a review, see [11]). Results of these in vitro studies and the temporal studies are consistent with a role for estradiol and IGF-1 in deviation.
Functional in vivo studies on the role of estradiol and IGF-1 in deviation are limited. Results of a recent in vivo study in heifers were interpreted to implicate a local functional role for estradiol in the deviation process, independent of its systemic negative effect on FSH concentrations [12]. With regard to in vivo studies with IGF-1, an IGF-1 analog was infused into the ovarian artery for 12 h beginning 14 days after estrus in ovarian autotransplanted sheep [13]. Mean plasma concentrations of estradiol were higher in the treated than in the control ewes but not until about 36 h after the end of treatment. In cattle, recombinant human (rh) IGF-1 was infused into the ovarian stroma for 7 days beginning the day after ovulation via an implanted osmotic minipump [14]. The ovaries were removed after 7 days, and the IGF-1 infusion had increased the estradiol concentrations in the follicular fluid of small follicles (2–5 mm) but apparently had no effect in large follicles or on progesterone or androstenedione concentrations in either small or large follicles. Total ovarian weight, number of follicles, and diameter of the two largest follicles were not affected significantly. The results of the in vivo studies on the effect of IGF-1 on ovarian steroidogenesis are difficult to assess because individual follicles were not targeted directly. Furthermore, none of the studies were focused on the time of follicle deviation.
In the present studies, the hypothesis that IGF-1 stimulates the production of estradiol in heifers and mares near the time of diameter deviation was tested in vivo. Stimulation of other follicular-fluid factors and the development of dominance were also evaluated. Transvaginal ultrasound was used to inject IGF-1 into the follicular fluid of the second-largest follicle at the expected beginning of deviation between the two largest follicles.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Animals and Ultrasonography
The heifers were Holsteins 24–36 mo of age weighing 360–500 kg. The mares were mixed breeds of ponies 10–17 yr of age weighing 250–320 kg. The feeding programs, protocols for inducing luteolysis, and equipment and techniques for transrectal ultrasound scanning of ovaries have been described for heifers [4] and mares [15]. In heifers, prostaglandin F2
(Lutalyse; Pharmacia and Upjohn Co., Kalamazoo, MI) was given at middiestrus, and scanning was done thereafter at 24-h intervals to detect ovulation. After ovulation, scanning was done at 12-h intervals to measure the diameter of the four largest follicles during development of the follicular wave that begins during the periovulatory period. In mares, a new follicular wave was induced by ablation of all follicles 6 mm 10 days postovulation, as previously validated and described [15]. Scanning was done every 24 h postablation, and the diameters of the three largest follicles were recorded. In both species, the largest follicle and second-largest follicle of the wave were designated F1 and F2 when F1 reached 8.5 mm in heifers and 20.0 mm in mares (expected beginning of deviation; Hour 0). Transvaginal ultrasound targeting was used for treatment of F2 at Hour 0 and for sampling follicular fluid of F2 at defined intervals as described for heifers [16, 17] and mares [18, 19]. In heifers, a separate animal was used for each sampling hour, and in mares, sequential sampling was done in each mare at different hours.
Experiment 1: Heifers
Thirty-two heifers were randomized at Hour 0 in a 2 (groups) x 4 (hours) factorial design, yielding four heifers in each group at each hour. In a control group, the F2 was punctured at Hour 0, but nothing was injected. In an IGF-1 group, the F2 was injected at Hour 0 with 20 µl of rh-IGF-1 (10 µg/µl; Genentech, San Francisco, CA). The entire follicular-fluid content was collected from F2 at Hours 3, 6, 12, or 24. End points were F2 follicular-fluid concentrations of estradiol, progesterone, and androstenedione.
Experiment 2: Heifers
This experiment was done to confirm the Hour-6 estradiol results of experiment 1. In addition, 20 µl of physiologic saline (0.9% NaCl w/v) was injected into F2 in the control group (n = 4) at Hour 0 rather than using follicle puncture only. The IGF-1 group (n = 4) was treated as described for experiment 1. The end point was follicular-fluid concentration of estradiol in F2 at Hour 6.
Experiment 3: Mares
Mares were randomized at Hour 0 (F1 20.0 mm) into control and IGF-1 groups (n = 7/group). At Hour 0, F2 was injected with either 50 µl of physiologic saline (control group) or 50 µl of rh-IGF-1 (10 µg/µl; IGF-1 group). A sample of follicular fluid (40 µl) was taken from F2 before treatment at Hour 0 and sequentially thereafter from each mare at Hours 6 and 24. A 20-ga/25-ga needle combination as described for heifers [16] was used for injection and sampling. Diameter of F2 was recorded just before treating or sampling the follicle. End points for F2 were diameter and follicular-fluid concentrations of estradiol, androstenedione, and progesterone.
Experiment 4: Mares
Mares were randomized at Hour 0 into two groups (n = 14/group) with the same treatment regimens as for experiment 3; however, a larger sample (90 µl) of follicular fluid was taken from F2 at Hour 12 and again at Hour 24. After sampling F2 at Hour 24, F1 was ablated by aspirating follicular contents using transvaginal ultrasonography, as previously described [20]. Aspiration was performed to determine whether follicle growth and the rate of ovulation for F2 or experimental assumption of dominance by F2 was different between the control and IGF-1 groups. End points were diameters of F1 and F2, F2 ovulation rate, and F2 concentrations of estradiol, androstenedione, progesterone, activin-A, and inhibin-A.
Hormone Assays
Follicular-fluid samples were centrifuged (500 x g for 10 min), decanted, and stored (-20°C) until assay. Follicular-fluid samples were assayed for estradiol, progesterone, androstenedione, activin-A, and inhibin-A using commercially available kits that have been modified and validated for use with bovine [21] and equine [8] follicular fluid in our laboratory. For experiment 1, intra- and interassay coefficients of variation (CVs) for progesterone were 13.4% and 4.5%, respectively. All other CVs for all hormones and experiments were <10%. The sensitivities for estradiol, androstenedione, progesterone, activin-A, and inhibin-A were <0.7 pg/ml, <2.0 pg/ml, <0.2 ng/ml, <0.2 ng/ml, and <1.6 ng/ml, respectively.
Statistical Analyses
The follicular and hormonal data were challenged for extreme values with the Dixon outlier test [22] and for normality with the Kolmogorov-Smirnov test. When the normality test result was significant (P < 0.05), data were transformed by either natural logarithm or square root. In experiment 2, follicular-fluid concentration of estradiol was examined between groups within Hour 6 using an unpaired t-test. In the other experiments, end points were examined among groups within hour using a general linear model (GLM) and within group among hours using a mixed linear model with a repeated statement to account for the autocorrelation between sequential measurements (SAS Institute, Cary, NC). Follicular-fluid hormone concentrations were examined for the effect of group, hour, and an interaction using either GLM or an unpaired t-test. When a significant (P < 0.05) effect of group or a group x hour interaction was detected, unpaired t-tests were used to locate the mean differences between groups within an hour and paired t-tests were used between hours within a group. The results are given as the mean ± SEM. A probability of 
0.05 indicated that a difference was significant and probabilities between >0.05 and 0.1 indicated that a difference approached significance.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Follicular-fluid concentrations of estradiol, androstenedione, and progesterone in F2 at Hours 3, 6, 12, and 24 after follicle puncture alone (control) or intrafollicular injection of rh-IGF-1 at Hour 0 are shown with the results of the statistical analyses (Fig. 1). The main effect of group for estradiol resulted from a higher (P < 0.05) concentration averaged over all hours in the IGF-1 group (300.0 ± 84.0 ng/ml) than in the control group (81.8 ± 39.4 ng/ml). Concentrations in the IGF-1 group were significantly higher than those in the control group (P < 0.05) at Hour 12 and approached significance (P < 0.08) at Hours 6 and 24. Within groups, concentrations increased (P < 0.05) between Hours 3 and 6 in the IGF-1 group and decreased (P < 0.05) between Hours 6 and 12 in the control group. The group x hour interaction for androstenedione was attributable primarily to a progressive increase (P < 0.05) in concentrations between Hours 3 and 12 in the IGF-1 group, so that concentrations were higher (P < 0.05) at Hour 12 than those in the control group. A lower mean concentration in the IGF-1 group than in the control group at Hour 3 approached significance (P < 0.06). There were no significant main effects or interactions for progesterone concentrations.
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Experiment 2: Heifers
Follicular-fluid estradiol concentrations of F2 were higher (P < 0.03) at Hour 6 after injection of IGF-1 (IGF-1 group; 209.6 ± 52.3 ng/ml) than after injection of saline (control group; 60.0 ± 22.1 ng/ml).
Experiment 3: Mares
Mean diameters and hormone concentrations of F2 and results of statistical analyses are shown in Figure 2. The main effect of hour for F2 diameter was significant and, averaged over the two groups, was characterized by a decrease (P < 0.01) between Hours 0 (20.4 ± 0.4 mm) and 6 (17.5 ± 0.9 mm) followed by an increase (P < 0.01) between Hours 6 and 24 (19.2 ± 1.1 mm). There were no other significant effects for diameter and no effects for estradiol. A significant interaction for androstenedione concentrations was attributable to a gradual increase (P < 0.006) in the controls and a decrease (P < 0.04) in the IGF-1 group. As a result, concentrations were different (P < 0.003) between groups at Hour 24. There were no significant main effects for progesterone, but the interaction of group x hour approached significance (P < 0.1). Within the IGF-1 group, progesterone concentrations increased (P < 0.04) between Hours 0 and 6 and decreased (P < 0.04) between Hours 6 and 12, and the difference between groups at Hour 6 approached significance (P < 0.07).
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Experiment 4: Mares
Diameters of F1 until ablation at Hour 24 and diameters of F2 until the first mare ovulated at Hour 96 are shown in Figure 3. There were no differences between groups in F1 diameters. Both main effects and the interaction were significant for F2. A postinjection decrease (P < 0.006) in F2 diameter occurred in both groups between Hour 0 and Hour 12, but the diameter at Hour 12 was greater (P < 0.007) in the IGF-1 group than in the controls. Thereafter, mean F2 diameter progressively increased in the IGF-1 group but did not change in the control group. Ovulation from F2 occurred in more mares (P < 0.0002) in the IGF-1 group (12/14) than in the control groups (2/14).
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Estradiol concentrations did not differ between groups. Averaged over Hours 12 and 24, follicular-fluid concentrations were greater in the IGF-1 group than in the controls for progesterone (P < 0.03), activin-A (P < 0.009), and inhibin-A (P < 0.001) and lower for androstenedione (P < 0.02). Averaged over groups, concentrations of estradiol (P < 0.04) and progesterone (P < 0.004) were lower at Hour 24 than at Hour 12. There were no other main effects. The interaction was significant only for inhibin-A (P < 0.002) and approached significance for progesterone (P < 0.08). The results of comparisons between groups and hours for each of the five follicular-fluid factors are shown in Table 1. Concentrations in the IGF-1 group compared with the controls were lower (P < 0.001) for androstenedione at Hours 12 and 24, higher (P < 0.0002) for progesterone at Hour 12, higher (P < 0.04) for activin-A at Hours 12 and 24, and higher (P < 0.0001) for inhibin-A at Hour 24.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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In mares, 15- to 25-mm follicles have been sampled once with no effect on the time of the beginning of deviation [18]; postsampling growth rate of the sampled follicle was reduced during the first day but was not altered thereafter. In the present mare experiments, F2 was sampled and then treated at Hour 0 and thereafter sampled twice (experiment 3) or treated only at Hour 0 and thereafter sampled twice (experiment 4), resulting in four or three punctures of F2, respectively, during a 24-h span. The treated follicles were reduced in diameter at Hours 6 or 12 but thereafter recovered, as indicated by increased diameter with a growth rate in experiment 4 similar to the presampling growth rate. This transient diameter effect of follicle puncture and subsequent recovery is similar to reported results for the previous development and detailed study of the effects of antral sampling in mares [18]. The transient decrease in diameter seems attributable primarily to leakage of follicular fluid as a result of puncture; the sample volume represented only about 2% of follicular-fluid volume of the punctured follicle in the mares.
In the controls of both species, F2 was punctured alone or injected with physiologic saline, and therefore the effects of treatment on concentrations of follicular-fluid factors can be attributed to the IGF-1 treatment and apparently not to the injection and sampling technology. Some reservation is warranted, however, because controls without follicle puncture were not included in any of the experiments. The dose for the single injection of IGF-1 into a follicle was high when compared with the reported total IGF-1 content in follicles that were equivalent in diameter to the follicles in this study. The doses of 200 (heifers) and 500 (mares) µg of rh-IGF-1 are equivalent to about 2000 times the reported IGF-1 content of an 8.5- and 22.5-mm follicle in cattle (100 ng) [23] and mares (250 ng) [7, 24], respectively. High doses were expected to increase the likelihood that adequate free IGF-1 would be available to the cells for an adequate time to test the hypothesis of positive stimulatory effect on estradiol production. The exchange of the components of the IGF system between the follicles and the circulatory system is dynamic [25], but information on the immediate fate of IGF-1 injected into a follicle is not available. We expected that there would be some local neutralization of the biologic activity of the IGF-1 by binding proteins in the follicular fluid of F2 [25], and a large but unknown quantity would pass during an unknown time span into the circulatory/lymphatic system. Considerable study would be needed to elucidate the dynamics of the IGF-1 system under the present experimental conditions.
The intrafollicular injection of rh-IGF-1 into F2 at the expected beginning of deviation increased the follicular-fluid concentrations of estradiol in heifers, supporting the hypothesis that IGF-1 stimulates follicle production of estradiol in vivo. The estradiol response in heifers is consistent with greater differential production of both free IGF-1 and estradiol in F1 than in F2 near the beginning of natural deviation [6, 21, 26] and in F2 during the experimental assumption of dominance [21, 27] and with in vitro results on the effect of IGF-1 on estradiol production in bovine granulosa-cell cultures [11]. In controls receiving follicle puncture only or an injection of saline, estradiol concentrations decreased as expected for a follicle that on average was destined to become subordinate [2]. The IGF-1 stimulation of the production of estradiol occurred between 3 and 6 h after treatment, indicating a rapid response. The rapid response is consistent with the synchrony between the increases in IGF-1 and in estradiol that has been demonstrated in heifers near the beginning of deviation within the limitation of collecting follicular fluid an equivalent of every 2 h [26].
The mean increase in androstenedione in experiment 1 (heifers) did not reach maximum until Hour 12 or 6 h after estradiol was at maximum. Maximal mean concentration in androstenedione after the maximal concentration in estradiol is consistent with reports that androstenedione was high in bovine dominant follicles [28, 29] and that the increase occurred after deviation began or after the estradiol increase in studies of both natural [21] and experimental [21, 27] deviation. In addition, IGF-1 had a stimulatory effect on androstenedione production by bovine thecal cells in vitro [30]. The intrafollicular injection of IGF-1 did not affect progesterone concentrations in the follicular fluid at Hours 3, 6, 12, or 24. In this regard, results of studies on the temporal position of progesterone during deviation in heifers have been inconsistent [6, 21].
In mares, the IGF-1 treatment of F2 was followed by a transient increase in follicular-fluid progesterone at Hour 6 or Hour 12, with a return to control concentrations by Hour 24. The transient increase in progesterone was the first detected change in concentrations of follicular-fluid factors. The progesterone increase is consistent with a reported increase in F1 at the expected beginning of deviation in a study of temporality in mares, but the temporal increase occurred after an estradiol increase [8]. In this regard, the in vivo stimulation of progesterone production by IGF-1 in mares is consistent with the report that IGF-1 is a potent stimulator of progesterone secretion from granulosa cultures in several species, including cattle [31]; however, studies using equine granulosa cells have not been reported. The IGF-1 stimulation of progesterone in mares but not in heifers can be attributed to the species difference in the steroidogenic pathway, wherein progesterone is an intermediary in the production of androstenedione and estradiol in the 
4 pathway in mares [32] but not in the 5 pathway in heifers [33].
A change in follicular-fluid estradiol concentrations was not detected in either experiment in mares during the 24-h posttreatment period. Thus, IGF-1 may account for a transient increase in progesterone in mares but not for the increase in estradiol that occurs before deviation begins [18]. The hypothesis that IGF-1 stimulates an estradiol increase in the developing dominant follicle was supported in heifers but not in mares, indicating a profound species difference. The species difference found in the present IGF-1 injection experiments is reminiscent of the results of ablating F1 at the expected beginning of natural deviation, thereby causing F2 to convert to a dominant follicle in heifers [27] and mares [34]; in heifers, estradiol and IGF-1 increased in F2 simultaneously, but in mares estradiol did not begin to increase in F2 until 35 h after the beginning of the IGF-1 increase. In the present mare experiments therefore, the last sample, at Hour 24, may have been obtained too early to detect an IGF-1-induced estradiol increase.
In both mare experiments, androstenedione increased in F2 of controls in agreement with an increase in subordinate follicles but not in the dominant follicle in a study of temporality [8]. In contrast, androstenedione decreased in F2 following the local injection of IGF-1. The effects of IGF-1 treatment on androstenedione were reciprocal to those in heifers, as indicated by a decrease in the treated follicle in mares and an increase in heifers. The species difference of an increase in androstenedione in subordinate follicles of mares versus the dominant follicle of heifers has been found in studies of temporality (mares [8]; heifers [21]). The present studies can be interpreted to indicate that IGF-1 plays a direct or indirect positive role in stimulating production of androgens in the dominant follicle of heifers, as reflected by higher concentrations of androstenedione in the treated F2 and higher concentrations of estradiol, which is downstream in the steroidogenic pathway. In mares, the aromatase enzyme activity apparently is shut off in the presence of the low concentrations of IGF-1 in nontreated subordinate follicles, resulting in an accumulation of androgens. More study is needed to reconcile this species difference as it relates to steroidogenesis.
In experiment 4 (mares), activin-A concentrations increased between Hours 12 and 24 in the IGF-1-treated mares and were higher than those in the controls at both hours. Inhibin-A increased in the IGF-1 group and decreased in the controls between Hours 12 and 24, resulting in higher concentrations in the IGF-1 group by Hour 24. These results are compatible with the study of temporality in mares that demonstrated a differential increase in these two factors in the future dominant versus the future largest subordinate follicle at approximately the time of the differential increases in estradiol and free IGF-1 [8]. In a study of the experimental assumption of dominance by F2 after ablation of F1, an increase in IGF-1 occurred before an increase in activin-A or inhibin-A [34]. The present results indicate that the differential increase in these two factors in the future dominant follicle versus the future largest subordinate follicle in mares can be attributed to the stimulatory effects of IGF-1. The positive effect of IGF-1 injection on activin-A and inhibin-A in mares is consistent with results of in vitro studies on the effect of IGF-1 on secretion of activin-A and inhibin-A in bovine granulosa-cell cultures [11, 35]. However, similar studies with equine granulosa cells have not been done.
In experiment 4 (mares), F1 was ablated at the last F2 sampling time (Hour 24) in both the control and IGF-1 groups to obtain information on the effect of the injected IGF-1 on the assumption of dominance by F2. After the initial diameter decrease in both groups following injection of F2 at Hour 0, mean diameter of F2 increased in the IGF-1 group, characteristic of a dominant follicle; F2 diameter did not increase in the controls. A high rate of ovulation occurred from F2 in the IGF-1 group (86%) compared with the ovulation rate from F2 in the control group (14%). These results demonstrated that the changes in concentrations of follicular-fluid factors induced by a single intrafollicular injection of IGF-1 favored an increased incidence of the assumption of dominance by F2 after ablation of F1.
The intrafollicular injection of rh-IGF-1 into the second-largest follicle at the expected beginning of deviation (Hour 0) resulted in changes in concentrations of several follicular-fluid factors during the 24-h postinjection period. The hypothesis that IGF-1 stimulates the production of estradiol within 24 h was supported in heifers but not in mares. In heifers, estradiol increased between Hours 3 and 6 and androstenedione increased by Hour 12. In mares, estradiol did not change by Hour 24, and androstenedione did not increase as it did in the control F2. Activin-A and inhibin-A also were evaluated in mares, and both factors increased between Hours 12 and 24. In addition, the effect of the IGF-1 treatment on converting an expected subordinate follicle into a dominant follicle was examined in mares by ablating F1 at Hour 24. The treatment increased the frequency of dominance and ovulation of F2.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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2 Correspondence: O.J. Ginther, Department of Animal Health and Biomedical Sciences, 1656 Linden Dr., University of Wisconsin, Madison, WI 53706. FAX: 608 262 7420; ginther{at}svm.vetmed.wisc.edu
Received: 5 August 2003.
First decision: 25 August 2003.
Accepted: 2 September 2003.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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. Biol Reprod 2003 68:758-765This article has been cited by other articles:
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  R. Ferreira, J. F. Oliveira, R. Fernandes, J. F. Moraes, and P. B. Goncalves The role of angiotensin II in the early stages of bovine ovulation Reproduction, November 1, 2007; 134(5): 713 - 719. [Abstract] [Full Text] [PDF]
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P M. Lokman, K. A N George, S. L Divers, M. Algie, and G. Young 11-Ketotestosterone and IGF-I increase the size of previtellogenic oocytes from shortfinned eel, Anguilla australis, in vitro Reproduction, May 1, 2007; 133(5): 955 - 967. [Abstract] [Full Text] [PDF]
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M A Beg and O J Ginther Follicle selection in cattle and horses: role of intrafollicular factors. Reproduction, September 1, 2006; 132(3): 365 - 377. [Abstract] [Full Text] [PDF]
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O J Ginther, E L Gastal, M O Gastal, and M A Beg In vivo effects of pregnancy-associated plasma protein-A, activin-A and vascular endothelial growth factor on other follicular-fluid factors during follicle deviation in mares Reproduction, April 1, 2005; 129(4): 489 - 496. [Abstract] [Full Text] [PDF]
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E L Gastal, M O Gastal, M A Beg, and O J Ginther Interrelationships among follicles during the common-growth phase of a follicular wave and capacity of individual follicles for dominance in mares Reproduction, October 1, 2004; 128(4): 417 - 422. [Abstract] [Full Text] [PDF]
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 T.J. Acosta, E.L. Gastal, M.O. Gastal, M.A. Beg, and O.J. Ginther Differential Blood Flow Changes Between the Future Dominant and Subordinate Follicles Precede Diameter Changes During Follicle Selection in Mares Biol Reprod, August 1, 2004; 71(2): 502 - 507. [Abstract] [Full Text] [PDF]
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T.J. Acosta, M.A. Beg, and O.J. Ginther Aberrant Blood Flow Area and Plasma Gonadotropin Concentrations During the Development of Dominant-Sized Transitional Anovulatory Follicles in Mares Biol Reprod, August 1, 2004; 71(2): 637 - 642. [Abstract] [Full Text] [PDF]
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A. Oropeza, C. Wrenzycki, D. Herrmann, K.-G. Hadeler, and H. Niemann Improvement of the Developmental Capacity of Oocytes from Prepubertal Cattleby Intraovarian Insulin-Like Growth Factor-I Application Biol Reprod, June 1, 2004; 70(6): 1634 - 1643. [Abstract] [Full Text] [PDF]
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O.J. Ginther, E.L. Gastal, M.O. Gastal, and M.A. Beg Critical Role of Insulin-Like Growth Factor System in Follicle Selection and Dominance in Mares Biol Reprod, May 1, 2004; 70(5): 1374 - 1379. [Abstract] [Full Text] [PDF]
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