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Ovary |
Department of Veterinary Biomedical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5B4
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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0.5 days after treatment: during the interwave interval of waves 1 and 2 without delaying the emergence of wave 2. The growth characteristics and serum estradiol concentrations did not differ (P > 0.05) between oFSH-induced waves and waves induced by endogenous FSH peaks. We concluded that, unlike in cattle, the largest follicle of a wave in sheep has limited direct effect on the growth of other follicles induced by exogenous oFSH. In addition, the largest follicle of a wave may possibly not influence the rhythmicity of follicular wave emergence, as it does in cattle.
estradiol, follicle-stimulating hormone, follicular development
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Ovarian antral follicles reaching ovulatory diameters of 5–7 mm in diameter grow in a wavelike pattern in cyclic [10–15] and seasonally anestrous [16–18] sheep. In the ewe, a follicular wave has been defined as one or more follicles that emerge from a pool of small (2–3 mm in size) antral follicles and grow to 
5 mm in diameter before regression or ovulation [13, 14, 16]. There are typically three to four waves during each ovine estrous cycle, with waves emerging every 3–5 days [13, 14]. Transient peaks in mean serum FSH concentrations have been shown to precede wave emergence [12–14, 16, 17].
Evidence in sheep for the existence of the powerful follicular dominance seen in cattle is equivocal [15, 19, 20]. There was no increase in the number of small (1–3 mm in diameter) antral follicles at the onset of a follicular wave in ewes and no obvious subordinate follicles [14]. Coculturing large and small ovine antral follicles did not result in atresia of the small follicles [21]. In the ewe, two or more follicles have been shown to grow in a wave, and antral follicles from the final and penultimate waves of the estrous cycle have been shown to ovulate at the same time [13, 15, 22]. However, when cycling ewes were treated with low levels of progesterone for several days, the life span of large antral follicles was prolonged, and emergence of follicular waves was suppressed [10]. Increased LH secretory pulsatility during the luteal phase of the estrous cycle also prolonged the life span of the largest antral follicles in ewes [10, 23, 24]. Ablation of the largest antral follicles in the ewe is followed by a peak in FSH secretion, but not by synchronous emergence of a new follicular wave [25]. Superovulatory treatment with FSH in ewes, when a large antral follicle was present, resulted in a reduced ovulatory response compared with treatment applied in the absence of a large follicle; however, the presence of a large follicle did not reduce the number of large follicles growing as a result of treatment [26].
The objective of the present study in ewes was to see if, as demonstrated in cattle [8], the presence of a large growing antral follicle(s) would inhibit the emergence of a new follicular wave in response to a physiological dose of oFSH. The hypothesis was that, as appears to be the case in cattle [8], at least part of the ability of a growing (dominant) follicle to suppress the growth and emergence of other follicles is by a direct effect (i.e., it cannot be overcome with exogenous oFSH administered during the growth phase of the largest follicle of a wave). In anestrous ewes, estradiol and presumably inhibin production by large antral follicles are lower compared with cyclic ewes [27], which may influence the occurrence and/or degree of follicular dominance [15]. Therefore, the present experiment was performed on both cyclic and seasonally anestrous ewes.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Experimental animals and husbandry The experimental procedures were performed according to the guidelines of the Canadian Council on Animal Care. Eleven adult, anestrous Western white face ewes (mean body weight = 75.3 ± 1.9 kg) were used in the present study during midanestrus (June–July). Ewes were kept in sheltered pens and fed a maintenance diet of hay, with water and cobalt iodized salt bars available ad libitum.
Hormone preparation The oFSH used was NIDDK-oFSH-18. Each 1 mg of oFSH has a biological potency of FSH equivalent of 65.6 x NIH-oFSH-S1 or 1640 IU and biological potency of LH equivalent of 0.1 x NIH-oLH-S1 or 106 IU. The oFSH injections were prepared in saline with 0.05% BSA (w/v; Sigma, St. Louis, MO) and 50% polyvinylppyrrolidone (w/v; Sigma).
Treatment administration Treatment involved two subcutaneous injections of either oFSH (0.5 µg/kg) or vehicle given 8 h apart. This treatment regimen was based on preliminary trials (involving different doses of oFSH and different time intervals between two injections) and was designed to result in an FSH peak of physiological amplitude [13, 28]. In preliminary trials, oFSH injections did not result in any increase in serum LH concentrations (P > 0.05). Six ewes were injected with oFSH (oFSH-treated group), and five control ewes were injected with vehicle only.
A 4-mm follicle, which grew from the pool of 2- to 3-mm follicles, was detected with ultrasonography, and it was designated as a wave 1 follicle. Such a follicle had to have emerged 3–5 days after the emergence of the previous follicular wave (i.e., at the normal time interval for waves in ewes) [28]. The first injection of oFSH or vehicle was given 24 h after the detection of the 4-mm follicle of wave 1, but only if this follicle remained at 4 mm or grew further. The second injection was given 8 h after the first. Timing of the treatment was designed to give oFSH injections during the growth phase of the largest follicle of wave 1 and the expected time of the interwave nadir in endogenous FSH secretion.
Experiment 2: Breeding Season
Experimental animals and husbandry Eleven adult, cyclic Western white face ewes (mean body weight = 81.2 ± 2.3 kg; maintained as explained in experiment 1) were used in the present study during the breeding season (October–November). Estrus detection was done twice daily (0800 and 2000 h) using crayon-harnessed vasectomized rams.
Treatment administration Treatment involved two subcutaneous injections of either oFSH (0.5 µg/kg) or vehicle given 8 h apart (see experiment 1 for details). Five ewes were injected with oFSH (oFSH-treated group), and six control ewes were injected with only vehicle.
A 4-mm follicle, which grew from the pool of 2- to 3-mm follicles, was detected with ultrasonography and was designated as wave 1 of the estrous cycle. Such a follicle had to have emerged around the day of ovulation (i.e., mean day of emergence of the first wave of the cycle) [13, 14]. The first injection of oFSH or vehicle was given 12 h after the detection of the 4-mm follicle of wave 1, but only if this follicle remained at 4 mm or grew further. The second injection was given 8 h after the first. The timing of the oFSH injection in experiment 2 (cyclic ewes) was advanced by 12 h as compared with experiment 1 (anestrous ewes) to create more precisely an FSH peak during the period corresponding to the nadir in endogenous FSH secretion and during the middle of the growth phase of the largest follicle of a wave (wave 1).
Transrectal Ovarian Ultrasonography and Blood Sampling
Ewes, in both experiments 1 and 2, were scanned twice daily (0800 and 2000 h) using a high-resolution real time B-mode ultrasound equipment (Aloka SSD-900; Aloka Co., Ltd., Tokyo, Japan) connected to a 7.5-MHz transducer until a follicular wave emerging after treatment was identified (i.e., when a new follicle that could be tracked back to a 2- to 3-mm diameter began to grow and reached a 5-mm diameter after treatment). Subsequently, ewes were scanned daily until the largest follicle of the wave emerging after treatment regressed to a 2- to 3-mm diameter (i.e., end of the follicular regression phase). During each scanning session, relative position and diameter of all follicles 1-mm diameter and corpora lutea were sketched on ovarian charts. In addition, all ovarian images were recorded on high-grade video tapes (Fuji S-VHS, ST-120 N; Fujifilm, Tokyo, Japan) for retrospective analysis of ovarian data. Blood samples (10 ml) were collected from all ewes by jugular venipuncture into Vacutainers (Becton Dickinson, Rutherford, NJ) just before each scanning session.
Hormone Assays
Blood samples were allowed to clot for 18–24 h at room temperature, and serum was harvested and stored at -20°C until assayed. Serum samples were analyzed by validated radioimmunoassay for circulating concentrations of FSH [29] and estradiol [30]. The ranges of the standard curves were 0.12–16 ng/ml for FSH and 1.0–50 pg/ml for estradiol. The sensitivities of assays (defined as the lowest concentration of hormone capable of significantly displacing labeled hormone from the antibody; unpaired t-test, P < 0.05) were 0.1 ng/ml and 1 pg/ml for FSH and estradiol, respectively. The intraassay coefficients of variation (all samples were analyzed in a single assay) were 11.6% or 2.9%, respectively, for reference sera, with a mean FSH concentration of 0.39 ng/ml or 1.50 ng/ml. The intra- and interassay of coefficients of variation were 16.5% and 7.8% or 14.5% and 8.9%, respectively, for sera with mean estradiol concentration of 3.5 or 12.0 pg/ml. Peaks in daily serum concentrations of FSH and estradiol were determined using the cycle-detection program [31]; blood samples taken every 12 or 24 h were analyzed separately to avoid false-positive peaks because of infrequent sampling.
Data Analysis
A follicular wave consists of a follicle or a group of follicles that emerge and grow from 2 or 3 mm in diameter to 5 mm (growth phase) and remain at their maximum diameter (static phase) before regression to 2 or 3 mm in diameter (regression phase) [13]. Emergence was restricted to a 24-h period [14]. The follicular wave during which growth phase the treatment was administered was designated as wave 1. Any follicular wave induced by treatment was designated as wave A. The follicular wave emerging after the induced wave (oFSH treated ewes) or the follicular wave emerging after 4–5 days after the emergence of wave 1 (control ewes) was designated as wave 2. The time of follicular wave emergence was determined in relation to time of first injection of treatments.
The inter-wave interval was defined as the interval between the time of wave emergence (i.e., time at which the largest follicle[s] of a wave was 2 or 3 mm in diameter) of two consecutive follicular waves. The length of the growth, static, and regression phases were considered for waves 1 and A, but only the growth phase was considered for wave 2; not all ewes had a regressing follicle in wave 2 on the last day of the experiments. The mean daily numbers of small follicles (1 mm, but 
3 mm in diameter) were centralized to the time of first injection of the treatment and analyzed for the period from 2 days before to 2 days after the time of injection.
Mean serum concentrations of FSH were aligned to the time of first injection of oFSH or vehicle and analyzed for the period from 2.5 days before to 7 days after the treatment. The peak serum concentrations of estradiol following emergence of each follicular wave and the interval between the time of follicular wave emergence and the time of the peak in serum estradiol concentrations were calculated for all ewes.
Statistical Analyses
Serum FSH concentrations and daily numbers of small follicles were analyzed for effects of time, group, and time x group, by two-way repeated measures analysis of variance (RM ANOVA; SigmaStat Statistical Software, version 2.0 for Windows 95, NT, and 3.1, Chicago, IL). The characteristics of estradiol peaks and various follicular parameters for waves 1 and 2 were compared between oFSH-treated and control ewes by two-way RM ANOVA. Additionally, comparisons of waves 1, A, and 2 detected in oFSH-treated ewes were made by one-way RM ANOVA. Multiple comparisons were made by the method of Fisher least significant difference. Results are reported as least square means ± SEM. Statistical significance was defined as P < 0.05.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Administration of exogenous oFSH Injections of oFSH or vehicle were given to oFSH-treated and control ewes at 1.7 ± 0.2 and 2.1 ± 0.1 days after emergence, respectively (P > 0.05), and at 0.9 ± 0.4 and 0.4 ± 0.2 days before the end of the growth phase of the largest follicle of wave 1, respectively (P > 0.05). At the time of treatment, oFSH-treated and control ewes had basal serum FSH concentrations of 1.6 ± 0.2 and 1.4 ± 0.2 ng/ml, respectively (P > 0.05; Fig. 1A). Analysis of serum FSH concentrations from 2.5 days before to 7 days after the first injection of oFSH or vehicle (time 0) showed no main effect of group (P > 0.05), but a significant effect of time and a group x time interaction (P < 0.05). Serum concentrations of FSH were higher (P < 0.05) in oFSH-treated compared with control ewes at 12 and 24 h after the first injection (Fig. 1A). The mean concentration of induced FSH peak (2.8 ± 0.2 ng/ml) in oFSH-treated ewes did not differ (P > 0.05) from that of the endogenous FSH peak (2.4 ± 0.2 ng/ml), which immediately preceded the emergence of wave 1. The mean duration of the induced FSH peak (nadir-to-nadir) was 1.6 ± 0.1 days. The cycle detection analyses revealed two peaks in serum FSH concentrations in both oFSH-treated and control ewes; these peaks occurred at the same time in the ewes of both groups (Fig. 1A) and preceded the emergence of waves 1 and 2, respectively (Table 1). An additional peak (preceding wave A) was seen at 0.5 ± 0.0 days after the first injection of oFSH only in oFSH-treated ewes (Table 1 and Fig. 1A).
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Follicular wave emergence The mean time of wave emergence, in relation to the time of treatment (Day 0) for waves 1 and 2 did not differ between oFSH-treated and control ewes (P > 0.05; Table 1 and Fig. 2A). An additional wave (wave A) emerged at 0.6 ± 0.2 days after the first injection of oFSH in oFSH-treated ewes (Table 1 and Fig. 2A). The mean day of emergence for wave A was 2.2 ± 0.2 days after emergence of wave 1 and 0.5 ± 0.2 days before the end of the growth phase of the largest follicles in this wave in oFSH-treated ewes.
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Estradiol concentrations There was no significant effect (P > 0.05) of wave, group, or a wave x group interaction for the mean interval between the day of follicular wave emergence and the day on which peak serum estradiol concentrations occurred, or the mean peak estradiol concentrations, following the emergence of follicular waves 1 and 2 in both oFSH-treated and control ewes (Table 1). There were no significant differences (P > 0.05) for the parameters above between follicular waves 1, A, and 2 in oFSH-treated ewes (Table 1).
Characteristics of the largest follicle of the wave There was no significant effect (P > 0.05) of wave, group, or a wave x group interaction for the length of the growth, static, and regression phases, or for mean growth rates and maximum diameter of the largest follicles of waves 1 and 2 in both oFSH-treated and control ewes (Table 2). There was no significant effect (P > 0.05) of wave for the parameters above for waves 1, A, and 2 in oFSH-treated ewes (Table 2).
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Numbers of small antral follicles
There was no significant effect (P > 0.05) of time, group, or a time x group interaction for mean numbers of small follicles (1 mm but 3 mm in diameter) analyzed for the period from 2 days before to 2 days after the first injection of oFSH or vehicle (Day 0; Fig. 2A). The numbers of small follicles at any time point ranged from 11 to 13 in oFSH-treated ewes and from 10 to 13 in control ewes.
Experiment 2
Administration of exogenous oFSH Injections of oFSH or vehicle were given to oFSH-treated and control ewes at 1.2 ± 0.1 days and 1.3 ± 0.1 days after emergence, respectively (P > 0.05) and 1.5 ± 0.2 days and 1.8 ± 0.3 days before the end of growth phase of the largest follicle of wave 1 of the cycle, respectively (P > 0.05). At the time of treatment, oFSH-treated and control ewes had basal serum FSH concentrations of 2.4 ± 0.3 ng/ml and 2.1 ± 0.4 ng/ml, respectively (P > 0.05; Fig. 1B). Analysis of serum FSH concentrations from 2.5 days before to 7 days after the first injection of oFSH or vehicle (time 0) showed a significant main effect of group and time, and a group x time interaction (P < 0.05). Serum concentrations of FSH were higher (P < 0.05) in oFSH-treated ewes compared with control ewes at 12, 24, and 36 h after the first injection (Fig. 1B). The mean concentrations of induced FSH peaks (3.6 ± 0.3 ng/ml) in oFSH-treated ewes did not differ (P > 0.05) from the endogenous FSH peak (3.9 ± 0.3 ng/ml), which immediately preceded the emergence of wave 1 of the cycle. The mean duration of the induced FSH peak (nadir-to-nadir) was 1.7 ± 0.1 days. The cycle detection analyses revealed two peaks in serum FSH concentrations in both oFSH-treated and control ewes; these peaks occurred at the same time in the ewes of both groups (Fig. 1B) and preceded the emergence of waves 1 and 2, respectively (Table 3). An additional peak (preceding wave A) was seen at 0.5 ± 0.0 days after the first injection of oFSH only in oFSH-treated ewes (Table 3 and Fig. 1B).
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Follicular wave emergence The mean time of wave emergence, in relation to the time of treatment (Day 0), for waves 1 and 2 did not differ between oFSH-treated and control ewes (P > 0.05; Table 3 and Fig. 2B). There was an additional wave (wave A) that emerged at 0.5 ± 0.0 days after the first injection of oFSH only in oFSH-treated ewes (P < 0.05; Table 3 and Fig. 2B). The mean day of emergence of wave A was 1.8 ± 0.1 days after the emergence of wave 1 and 1.0 ± 0.2 days before the end of the growth phase of the largest follicles of this wave in oFSH-treated ewes.
Estradiol concentrations There was no significant effect (P > 0.05) of wave, group, or a wave x group interaction for the interval between the day of follicular wave emergence and the day of peak serum estradiol concentrations, or for peak estradiol concentrations following the emergence of waves 1 and 2 in both oFSH-treated and control ewes (Table 3). There was no significant effect (P > 0.05) of wave on the parameters above for follicular waves 1, A, and 2 in oFSH-treated ewes (Table 3).
Characteristics of the largest follicle of the wave There was no significant effect (P > 0.05) of wave, group, or a wave x group interaction for the length of the growth, static, and regression phases, or for mean growth rate and maximum diameter of the largest follicles of waves 1 and 2 in both oFSH-treated and control ewes (Table 4). There was no significant effect (P > 0.05) of wave for the parameters above for waves 1, A, and 2 in oFSH-treated ewes (Table 4).
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Numbers of small follicles
There was no significant effect (P > 0.05) of time, group, or a time x group interaction for mean numbers of small follicles (1 mm, but 3 mm in diameter) analyzed for the period from 2 days before to 2 days after the first injection of oFSH or vehicle (Day 0; Fig. 2B). The numbers of small follicles at any time point ranged from 9 to 13 in oFSH-treated ewes and from 11 to 15 in control ewes.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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In both present experiments, treatment with oFSH induced the emergence of a new follicular wave (Fig. 2) in the presence of a large growing follicle of a previous wave (i.e., the wave in which treatment was applied; wave 1). In cattle, injection of a superovulatory dose of recombinant bovine FSH (rbFSH) on Day 5 of the estrous cycle (i.e., after the selection of the dominant follicle of the first wave of the cycle) did not rescue subordinate follicles from atresia nor did it stimulate the emergence of a new follicular wave [8]. Although there is much evidence that ovarian follicular dominance in cattle involves suppression of FSH secretion [4], the findings of Adams et al. [8] described above do not allow us to eliminate a direct effect of the dominant follicle on smaller, or subordinate, follicles. It has been suggested that ovarian follicular dominance could involve local inhibitory or regulatory factors [2, 4]. Based on the present results, however, the growing follicle of a wave in the ewe did not appear to directly inhibit the emergence and growth of smaller follicles stimulated by exogenous oFSH.
Several authors have postulated the existence of follicular dominance in cyclic ewes [12, 15, 20, 33–35]; however, other, especially more recent, evidence would bring this postulate into question. Follicular waves have been frequently found to emerge in the presence of growing ovulatory-sized follicles from a previous wave in cyclic [13, 24, 36, 37] and anestrous ewes [17]. Several reports have indicated a lack of any temporal relationship between follicular wave emergence and estradiol secretion during the anestrous period in sheep [16, 17], although follicular wave emergence was maintained and associated with rhythmic FSH peaks across anestrus [16, 17]. A large (presumably dominant) follicle did not inhibit eCG-induced growth or function of other follicles in ewes [21]. Coculturing small follicles with large follicles in sheep in a closed system did not decrease thymidine incorporation by granulosa cells of the small follicles as compared with those of small follicles cultured alone [21]. Recently, it has been demonstrated that follicles from the final and penultimate waves of the estrous cycle in sheep can ovulate together and form healthy corpora lutea [13, 15, 22]. Therefore, the largest follicles of waves in the ewe may not exert the same functional dominance as is seen in cattle.
Injection of oFSH in the present study did not alter the number of small antral follicles. The presence of experimentally induced large follicles in sheep has been shown to reduce the number of small follicles and block the emergence of follicular waves [10]. However, in that experimental approach [10], the characteristics of the follicles with prolonged life span were essentially nonphysiological in that they grew to unusually large diameters and did not produce estradiol throughout the period of their apparent dominance. In addition, in a recent ultrasonographic study, there was no increase in the numbers of small follicles during periods encompassing follicular wave emergence in cyclic ewes [14]. Follicular wave emergence in cattle is clearly associated with a transient increase in numbers of small, 3- to 4-mm follicles, from which the dominant follicle is selected [38, 39]; this would not appear to be the case in sheep.
In addition, injection of oFSH in the present study did not affect the development of wave 1 follicles, as neither the growth of the largest follicle nor estradiol secretion during the growth of wave 1 was affected by the treatment (Fig. 2, A and B; Tables 1– 4). The lack of effect of FSH on large growing follicles has been previously demonstrated in sheep [19, 26] and in cattle [8].
In cattle, the FSH peak that stimulates the emergence of a follicular wave occurred late in the static phase of the dominant follicle of the previous wave [3, 40] when it was undergoing atresia [41]. In sheep, a follicular wave emerged at the end of the static phase or early regression phase of the largest follicle from the previous wave [12]. If an apparent dominant follicle in sheep strictly regulated the inter-wave interval by direct (follicle to follicle) or indirect (by regulating FSH secretion) means, then follicular waves should not be able to emerge at different times in the life span of a previous follicular wave. However, in the present study the follicular wave induced by exogenous oFSH (wave A) emerged during the growth phase of the previous wave (wave 1) and did not disrupt the rhythmic pattern of occurrence of the next endogenous FSH peaks (Tables 1 and 3) and the emergence of a follicular wave (wave 2; Tables 1 and 3; Fig. 2, A and B). In addition, it is interesting that the second follicular waves (waves 2) stimulated by endogenous FSH peak emerged at the end of the growth phase of the oFSH-induced waves (wave A; Fig. 2, A and B), the period at which its estradiol secretory ability is expected to be maximal [12, 18, 42–45]. This again brings into question the importance of follicular dominance as a regulator of follicular dynamics in the ewe.
In summary, in cyclic and anestrous ewes the largest growing follicle of a wave failed to inhibit the stimulation of emergence of a new follicular wave by a physiological dose of oFSH given at the expected time of the endogenous inter-wave FSH nadir. The largest follicle of the oFSH-induced wave did not disrupt the rhythmic pattern of endogenous FSH secretion or emergence of the next follicular wave. The largest follicle of the oFSH-induced wave did not differ from those of follicular waves stimulated by endogenous FSH peak in its morphological attributes and estradiol secretory ability. The injection of a physiological dose of oFSH did not alter the number of small antral follicles in ewes. Thus, we conclude that unlike in cattle, the large growing follicle of follicular waves in ewes has limited direct effect on the emergence of other antral follicles induced by physiological concentrations of oFSH. In addition, the large growing follicle of a wave may possibly not influence the rhythmicity of endogenous FSH secretion and follicular wave emergence, as it does in cattle.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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2 Correspondence. FAX: 306 966 7376; norman.rawlings{at}usask.ca
3 Current address: Department of Biomedical Sciences, Ontario Veterinary College, University of Guelph, Guelph, ON, Canada N1G 2W1
Received: 27 September 2003.
First decision: 27 October 2003.
Accepted: 17 November 2003.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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) ewes. FSH concentrations were centralized to the time of the first injection (Day 0) of the two injections oFSH or vehicle given 8 h apart and were considered from 2.5 days before to 7 days after the first injection. First injections were given 24 h (A; nonbreeding season, experiment 1) or 12 h (B; breeding season, experiment 2) after the largest follicle of a wave (nonbreeding season) or the first wave of the estrous cycle (breeding season) reached 
, oFSH-induced wave) in oFSH-treated ewes and waves 1 (