超数排卵兔的Graaf氏卵泡酸性磷酸酶的研究

本文对超数排卵免排卵前后卵泡壁中的酸性磷酸酶(ACPase)进行了研究,发情盛期卵泡壁中ACPase含量很少,只是在固缩行将脱落颗粒细胞中有较强的酶反应;排卵初期溶酶体和细胞内ACPase增加,并参与排卵过程;排卵开始后15小时,卵泡中白细胞及细胞溶酶体增多,均含有大量的ACPase,此时ACPase参与细胞的清除和修复过程。     

同源GnRH对无血清培养鸡卵泡颗粒细胞孕酮分泌的作用

选用产蛋规律的鸡，在一个产蛋序列中，排卵前１～３ｈ剖腹收集各级卵泡，分离颗粒层建立卵泡颗粒细胞无血清单层贴壁培养模型。在此基础上，用不同剂量鸡促性腺激素释放激素（ＧｎＲＨ－Ⅱ）单独处理或与羊ＬＨ（ｏＬＨ）协同处理，并使用ＧｎＲＨ－Ａｎｔａｇｏｎｉｓｔ（ＧＡ），以观察ＧｎＲＨ－Ⅱ对颗粒细胞孕酮分泌的影响。研究获得了以下结果：（１）ＧｎＲＨ－Ⅱ对鸡不同卵抱（Ｆ１、Ｆ３、Ｆ５）颗粒细胞孕酮分泌均有促进作用，并呈现剂量－反应关系；（２）ＧｎＲＨ－对ｏＬＨ促鸡卵泡颗粒细胞孕酮的分泌有明显的协同作用；（３）ＧｎＲＨ拮抗物使ＧｎＲＨ－Ⅱ的促卵泡颗粒细胞孕酮分泌作用受到阻断。     

TACE激酶在猪排卵过程中的作用机制

促黄体激素(LH)诱导猪卵泡排卵过程包括卵泡壁的破裂、颗粒细胞黄体化、卵丘细胞扩展和卵母细胞减数分裂成熟。LH受体主要在排卵前卵泡的颗粒细胞中表达,排卵刺激后表达水平明显降低。颗粒细胞和卵丘细胞表达的EGF样因子可以激活EGFR-MAPK3/1在这2种细胞中的通路。EGF样因子是由信号序列、跨膜区域和EGF区域组成,并且由特异性酶刺激释放EGF区域与EGFR互作诱导排卵过程。TACE/ADAM17是一种EGF样因子的蛋白水解酶,在FSH/LH刺激的颗粒细胞和卵丘细胞中表达并激活EGFR-MAPK3/1通路。应用特异性抑制剂或siRNA抑制技术而降低TACE/ADAM17的表达活性,则会抑制颗粒细胞黄体化、卵丘细胞扩展以及卵母细胞的成熟过程。因此,TACE/ADAM17是诱导猪排卵过程的主效基因。     

ERα免疫反应产物在山羊卵巢中的分布

为了研究不同生殖周期山羊卵巢中雌激素受体α（estrogen receptor α，ERα）的表达，本试验应用高灵敏度的免疫组织化学SP法检测了ERα免疫反应产物在不同生殖周期山羊卵巢中的分布。结果显示，ERα免疫反应产物主要存在于成熟卵泡的颗粒细胞、发情期和妊娠期生长卵泡的部分颗粒细胞、原始卵泡卵母细胞胞质、粒黄体细胞胞质及膜黄体细胞胞核、卵泡膜细胞和类固醇细胞胞质；间情期生长卵泡的颗粒细胞中的ERα免疫反应产物较少，主要存在于细胞膜上；卵母细胞中ERα免疫反应产物在初级卵泡阶段含量较少，从次级卵泡开始其含量很丰富。以上结果提示，ERα以激活不同信号途径的方式参与了对卵泡生长、排卵及妊娠维持的调控，这为研究雌激素及雌激素受体α在卵巢中的作用及其生殖调控机理提供了形态学资料。     

母鸡抑制素的产生

母鸡抑制素主要是由排卵前卵泡颗粒细胞产生的,肾上腺是又一来源,ＬＨ在刺激体外颗料细胞产生抑制素方面的比ＦＳＨ有效。去除排卵前卵泡后,血浆免疫活性抑制素显著降低,而血浆ＦＳＨ急剧升高。卵泡颗粒层中,抑制素α亚基比β（Ａ）亚基表达充分。α亚基是由近似１．７ｋｂ的ｍＲＮＡ编码,主要的８．４ｋｂβ（Ａ）－ｍＲＮＡ带在排卵前卵泡的颗粒层中表达。     

哺乳动物MAPK信号级联及其功能

丝裂素激活的蛋白激酶 (Ｍｉｔｏｇｅｎ -ａｃｔｉｖａｔｅｄｐｒｏｔｅｉｎｋｉｎａｓｅｓ,ＭＡＰＫｓ)是真核细胞中一种重要的信号转导酶 ,涉及细胞进程的许多方面。近来 ,有关哺乳动物ＭＡＰＫ信号级联在控制基因表达、细胞增殖和凋亡等生理功能的研究引起了人们的重视 ,并获得了一些新的发现 ,本文对此作一综述。1　ＭＡＰＫｓ与ＭＡＰＫ信号级联ＭＡＰＫｓ是连接细胞表面受体和细胞内靶蛋白的蛋白激酶 ,还能对化学和生理应激做出反应 ,因而调控细胞的生存和适应性。ＭＡＰＫ活性受由ＭＡＰＫ、ＭＡＰＫＫｉｎａｓｅ(ＭＡＰＫＫ ,ＭＫＫ…     

兔卵泡胆固醇定位变化的研究

用促性腺激素处理兔，取其卵泡，采用羊地黄皂苷方法固定，在电镜下观察胆固醇的这位变化，发情盛期固醇主要定位于内分泌细胞的滑面内质网、细胞膜，线粒体和脂滴上，颗粒细胞内较少。排卵初期，胆固醇在内分泌细胞中减少，在颗细胞中有增多趋势，排卵开始后１５小时，胆固醇增加，定位于内分泌细胞和颗粒细胞的滑面内质网，细胞膜，线粒体和脂滴上，排卵开始后３９小时，大部分卵泡退化，一部分卵泡高度黄体化，核工谢旺盛，滑面内     

小白鼠卵泡壁发育的形态学研究

选择性成熟的纯种小白鼠，用ＦＳＨ和ｈｃＧ处理，取其卵巢，制成光学切片，显微镜下观察卵泡壁发育的形态学变化过程。胚胎阶段发育而来的原始卵泡的扁平细胞钭发育为颗粒层细胞，颗粒细胞最初在单层上增殖细胞的数量；卵泡膜在初级卵泡在初期就开始形成，是由卵巢基质中的扁平样细胞发育而来；卵泡膜中的血管是由卵巢基质中血管伸入形成，在初级卵泡刚开始形成时就开始生长发育。     

牛卵泡颗粒细胞中生长相关基因的表达研究

随着卵巢卵泡的生长,卵泡内雌激素分泌量逐渐上升,雌激素通过其受体(Estrogen Receptors,Esrs)对卵泡的调节作用也在发生改变。该研究通过手术法获取牛卵巢上不同直径的卵泡颗粒细胞,对颗粒细胞中雌激素受体及相关基因表达进行研究。结果表明:Esr1和Esr2都能在牛卵泡颗粒细胞中表达,且随着卵泡的生长而显著下降(P0.05);Fshr的表达随着卵泡的生长而显著上升(P0.05),但在中、大型卵泡颗粒细胞中差异不显著(P0.05);Lhr在大型卵泡颗粒细胞中的表达量极显著高于小、中型卵泡(P0.01);随着卵泡的生长,Cox2在颗粒细胞中的表达显著上升(P0.05),而Hsd17b1的表达则显著下降(P0.05)。综上所述,随着卵泡生长,卵泡颗粒细胞对Esr1、Esr2作用的依赖性逐渐减弱;大卵泡在排卵前Lhr表达量急剧上升,同时诱导Cox2表达量显著上升而Hsd17b1表达量显著下降,表明大卵泡已启动排卵反应。     

兔卵泡排卵机理的研究

利用ＦＳＨ和ＨＣＧ对兔进行处理，在发情盛期、排卵初期和排卵开始后１５ｈ，解剖取出不同直径的卵泡制成光镜和电镜样品，供组织学观察研究。实验结果表明，在发情盛期，卵泡细胞代谢旺盛，内膜血管网丰富，微循环正常；在排卵初期，大部分卵泡发生微循环障碍，有的血管淤血，有的缺血，卵光细胞代谢活动下降。排卵开始后１５ｈ，卵泡微循环恢复正常，大部分卵泡退化，一部分大卵泡黄体化。     

大约克母猪排卵规律的研究

通过对大约克母猪在发情不同时间分批屠宰测定表明，大约克母猪排卵时间是在发情开始后４８小时左右至７２小时之前这一范围。并采用杜洛克和大约克公猪分别在母猪发情的不同阶段先后配种，从所产仔猪的遗传特征鉴别结果判断，排卵大多集中在发情３６小时后至６０小时左右范围内，与屠宰测定基本一致。据上试验结果，确定掌握适配期为母猪发情开始后３６－４８小时期间第一次配种，间隔１２－２４小时内复配一次。经两年配种实践取得良好效果，情期受胎率分别达到９１．２７％（２０９／２２９）和９３．３６％（２６７／８６）。     

Influence of vitamin A injection before mating on oocyte development, follicular hormones, and ovulation in gilts fed high-energy diets

Previous research revealed that treatment with vitamin A approximately 5 d before ovulation may increase litter size in weaned sows and improve embryonal survival in gilts fed high-energy diets that reduced embryonal survival. For the current study, the hypothesis was that administration of vitamin A before ovulation would alter development of follicles and oocytes in a way favorable to enhanced embryonal survival. (Landrace x Large White) x (Duroc x Hampshire) gilts (n = 44) were fed 11.0 Mcal ME x gilt(-1) x d(-1) beginning 7 d after second estrus and given (i.m.) corn oil or 1 x 10(6) IU of vitamin A (retinyl palmitate) on d 15 after second estrus. Gilts were checked for estrus every 4 h, mated naturally at third estrus, and assigned randomly to undergo midventral laparotomy beginning at 24 to 28, 28 to 32, 32 to 36, or 36 to 40 h after onset of third estrus. At laparotomy, ovulated oocytes and early-stage embryos were recovered from oviducts, and ovaries were removed for aspiration of oocytes and granulosa cells from unovulated follicles. Oocytes and embryos were stained for assessment of stage of development. Granulosa cells were cultured to assess their ability to secrete progesterone. Follicular fluid was assayed for progesterone, estradiol-17beta, IGF-I, and PGF2alpha. Treatment with vitamin A altered development of oocytes and embryos by decreasing the percentage at the germinal vesicle stage and increasing the percentage at advanced stages. Mean stage of development was increased by vitamin A, but variation in stage was decreased. Among follicles matched by meiotic stage of oocyte, follicular fluid concentrations of progesterone, IGF-I, and PGF2alpha were greater in vitamin A-treated gilts than in controls, but treatment with vitamin A in vivo did not affect LH-stimulated or unstimulated secretion of progesterone by granulosa cells in vitro. These data provide evidence that vitamin A may influence embryonic development by advancing resumption of meiosis and altering follicular hormonal environment during follicle maturation.     

Timed Artificial Insemination by Combining Estrous Behavior Observation With Deslorelin Treatment in Jennies

The purpose of this study was to determine the optimal time for ovulation induction and artificial insemination (AI) based on the relationship between estrous behavior and ovulation in jennies. Thirty-two jennies were teased by one jackass for 1 hour per day during 46 days and estrous behaviors were recorded, while the follicular development and ovulation was examined by ultrasound. Furthermore, another 31 jennies were teased by one jackass as the teasing group (group T), which were injected with Deslorelin at 2 and 4 days after the onset of estrus, and AI was performed at 8 hours after each injection. Moreover, Ultrasound was performed on the follicle development of 23 jennies as the ultrasonography group (group U). Injection with Deslorelin when the follicle diameter ≥ 30 mm, and AI was performed at 8 hours later. The results showed that mouth clapping was the specific estrous behavior of jennies and indicated the beginning of estrus. The mean time for jennies to develop dominant follicles (≥30 mm) after the onset of estrus was 3.5 ± 1.3 days, and the mean time between the onset of estrus and ovulation was 5.1 ± 1.5 days. Estrous behaviors ended 0.5 ± 1.2 days after ovulation. After AI, there were no significant differences in ovulation (96.8% vs. 91.3%) and conception rates (40.0% vs. 38.1%) between group T and U. The optimal breeding time of jennies can be determined by jackass teasing and hastening ovulation by Deslorelin injection.     

同期排卵处理母牛体温和活动量变化规律及效果分析

为探究同期排卵处理母牛体温和活动量变化规律及不同同期排卵技术处理效果，指导同期排卵技术优化。本研究自动监测了18头20月龄左右同期排卵（GnRH-PG-GnRH）处理荷斯坦母牛和17头产后40~60 d预同期排卵（PG-PG-GnRH-PG-GnRH）处理荷斯坦母牛的体温和活动量，应用自动检测系统进行母牛发情监测。结果发现，同期排卵处理母牛发情时阴道温度平均升高（0.43±0.20）℃，持续（12.37±2.73）h；活动量平均升高（18.28±18.61）倍，持续（11.00±1.68）h；排卵时阴道温度平均下降（0.20±0.10）℃，持续（11.00±1.68）h。自动化发情监测显示，同期排卵处理母牛7头发情并排卵；预同期排卵母牛GnRH处理前全部发情排卵。两种同期排卵处理，虽可改变母牛性周期进程，促进母牛性周期同步化，但均难以使母牛性周期完全同步。因此，将同期排卵-定时输精和发情鉴定技术科学结合才能取得更好的繁殖效果。     

Effect of P.G. 600 on the timing of ovulation in gilts treated with altrenogest

We previously reported that ovulation rate, but not pregnancy rate or litter size at d 30 after mating, was enhanced by treatment with P.G. 600 (400 IU of PMSG and 200 IU of hCG, Intervet America, Inc., Millsboro, DE) in gilts fed the orally active progestin, altrenogest (Matrix, Intervet America, Inc.) to synchronize estrus. We hypothesized that in addition to increasing ovulation rate, P.G. 600 may have altered the timing of ovulation. Therefore, mating gilts 12 and 24 h after first detection of estrus, as is common in the swine industry, may not have been the optimal breeding regimen, and as a consequence, pregnancy rate and litter size were not altered. The objective of the present study was to determine the effect of P.G. 600 on the timing of ovulation in gilts treated with altrenogest. Randomly cycling, crossbred gilts (5.5 mo old, 117 kg BW, and 14.7 mm of backfat) were fed a diet containing altrenogest (15 mg/d) for 18 d. Twenty-four hours after altrenogest withdrawal, gilts received i.m. injections of P.G. 600 (n = 25) or saline (n = 25). Gilts were checked for estrus at 8-h intervals. After first detection of estrus, transrectal ultrasonography was performed at 8-h intervals to determine the time of ovulation. Gilts were killed 9 to 11 d after the onset of estrus to determine ovulation rate. All gilts displayed estrus by 7 d after treatment with P.G. 600 or saline. Compared with saline, P.G. 600 increased (P = 0.07) ovulation rate (14.8 vs. 17.5, respectively; SE = 1.1). The intervals from injection to estrus (110.9 vs. 98.4; SE = 2.7 h; P < 0.01) and injection to ovulation (141.9 vs. 128.6; SE = 3.2 h; P < 0.01) were greater in gilts treated with saline than in gilts treated with P.G. 600. Duration of estrus (54.4 vs. 53.7; SE = 2.5 h), the estrus-to-ovulation interval (30.2 vs. 31.7; SE = 2.2 h), and the time of ovulation as a percentage of estrus duration (55.8 vs. 57.5; SE = 3.0%) did not differ for the P.G. 600 and saline-injected gilts, respectively. In summary, P.G. 600 advanced the onset of estrus and ovulation following termination of altrenogest treatment and increased ovulation rate; however, treatment of gilts with P.G. 600 had no effect on the timing of ovulation relative to the onset of estrus.     

The Effect of Sulpiride Treatment During the Periovulatory Period on Prolactin Concentration and Ovulation in the Mare

Sixteen estrous cycles from 10 cyclic mares were randomly assigned to a control or sulpiride group (n = 8 each). All mares received 1,500 IU of human chorionic gonadotropin (hCG) (hour 0) during estrus with a follicular diameter ≥32 mm. Mares were scanned every 12 hours until ovulation. In the treatment group, beginning at hour 0, each mare received 1.5 mg/kg of sulpiride every 12 hours intra-muscularly until ovulation or formation of a luteinized unruptured follicle (LUF). Concentrations of luteinizing hormone (LH) and prolactin (PRL) were measured by radioimmunoassay. In each group, there were 10 preovulatory follicles for the eight cycles. The ovulation rate (9/10, 90%) was similar in the control and sulpiride groups. Two mares formed an LUF, which was first detected at hours 48 and 72 for the sulpiride and control mares, respectively. The interval from hCG to ovulation was 49.5 ± 11.1 and 43.5 ± 5.8 hours, for the control and sulpiride groups, respectively (P > .5). LH followed the typical preovulatory surge pattern, with no difference between groups (P > .5). Sulpiride administration increased PRL concentration in treated mares at 24 (P < .1), 36, and 48 hours (P < .05) after treatment. In conclusion, sulpiride administration every 12 hours increased PRL concentration in treated mares after 24 hours of the beginning of treatment. However, at this time window and concentration, PRL did not have any effect on ovulation. The control mare that developed an LUF had a PRL concentration similar to other ovulatory control mares (always ≤10 ng/mL).     

Endocrine and ovarian changes in response to the ram effect in medroxyprogesterone acetate-primed Corriedale ewes during the breeding and nonbreeding season

Two experiments were performed to determine the endocrine and ovarian changes in medroxyprogesterone acetate (MAP)-primed ewes after ram introduction. Experiment 1 was performed during the mid-breeding season with 71 ewes primed with an intravaginal MAP sponge for 12 days. While the control (C) ewes (n = 35) were in permanent contact with rams, the ram effect (RE) ewes (n = 36) were isolated for 34 days prior to contact with rams. At sponge withdrawal, all ewes were joined with eight sexually experienced marking Corriedale rams and estrus was recorded over the next 4 days. The ovaries were observed by laparoscopy 4-6 days after estrus. Four weeks later, pregnancy was determined by transrectal ultrasonography. In eight ewes from each group, ovaries were ultrasonographically scanned; FSH, LH, and estradiol-17beta were measured every 12 hours until ovulation or 96 hours after estrus. The response to the rams was not affected by the fact that ewes had been kept or not in close contact with males before teasing. No differences were found in FSH, LH, estradiol-17beta concentrations, growth of the ovulatory follicle, onset of estrus, ovulation rate, or pregnancy rate. Experiment 2 was performed with 14 ewes during the nonbreeding season. Ewes were isolated from rams for 1 month, and received a 6-day MAP priming. Ovaries were ultrasonographically scanned every 12 hours, and FSH, LH, estradiol-17beta, and progesterone were measured. Ewes that ovulated and came into estrus had higher FSH and estradiol-17beta levels before introduction of the rams than did ewes that had a silent ovulation. The endocrine pattern of the induced follicular phase of ewes that came into estrus was more similar to a normal follicular phase, than in ewes that had a silent ovulation. The follicle that finally ovulated tended to emerge earlier and in a more synchronized fashion in those ewes that did come into estrus. All ewes that ovulated had an LH surge and reached higher maximum FSH levels than ewes that did not ovulate, none of which had an LH surge. We conclude that (a) the effect of ram introduction in cyclic ewes treated with MAP may vary depending on the time of the breeding season at which teasing is performed; (b) patterns of FSH, and estradiol-17beta concentrations, as indicators of activity of the reproductive axis, may be used to classify depth of anestrus; and (c) the endocrine pattern of the induced follicular phase, which is related to the depth of anestrus, may be reflected in the behavioral responses to MAP priming and the ram effect.     

Endocrine and Ovarian Changes in Response to the Ram Effect in Medroxyprogesterone Acetate-primed Corriedale Ewes During the Breeding and Nonbreeding Season

Two experiments were performed to determine the endocrine and ovarian changes in medroxyprogesterone acetate (MAP)-primed ewes after ram introduction. Experiment 1 was performed during the mid-breeding season with 71 ewes primed with an intravaginal MAP sponge for 12 days. While the control (C) ewes (n = 35) were in permanent contact with rams, the ram effect (RE) ewes (n = 36) were isolated for 34 days prior to contact with rams. At sponge withdrawal, all ewes were joined with eight sexually experienced marking Corriedale rams and estrus was recorded over the next 4 days. The ovaries were observed by laparoscopy 4–6 days after estrus. Four weeks later, pregnancy was determined by transrectal ultrasonography. In eight ewes from each group, ovaries were ultrasonographically scanned; FSH, LH, and estradiol-17β were measured every 12 hours until ovulation or 96 hours after estrus. The response to the rams was not affected by the fact that ewes had been kept or not in close contact with males before teasing. No differences were found in FSH, LH, estradiol-17β concentrations, growth of the ovulatory follicle, onset of estrus, ovulation rate, or pregnancy rate. Experiment 2 was performed with 14 ewes during the nonbreeding season. Ewes were isolated from rams for 1 month, and received a 6-day MAP priming. Ovaries were ultrasonographically scanned every 12 hours, and FSH, LH, estradiol-17β, and progesterone were measured. Ewes that ovulated and came into estrus had higher FSH and estradiol-17β levels before introduction of the rams than did ewes that had a silent ovulation. The endocrine pattern of the induced follicular phase of ewes that came into estrus was more similar to a normal follicular phase, than in ewes that had a silent ovulation. The follicle that finally ovulated tended to emerge earlier and in a more synchronized fashion in those ewes that did come into estrus. All ewes that ovulated had an LH surge and reached higher maximum FSH levels than ewes that did not ovulate, none of which had an LH surge. We conclude that (a) the effect of ram introduction in cyclic ewes treated with MAP may vary depending on the time of the breeding season at which teasing is performed; (b) patterns of FSH, and estradiol-17β concentrations, as indicators of activity of the reproductive axis, may be used to classify depth of anestrus; and (c) the endocrine pattern of the induced follicular phase, which is related to the depth of anestrus, may be reflected in the behavioral responses to MAP priming and the ram effect.     

Endocrine pathogenesis of postweaning anestrus in swine: response of the persistently anestrous sow to hormonal stimuli

The endocrine function of the individual components of the hypothalamo-hypophyseal-ovarian axis of the postweaning anestrous sow was evaluated by monitoring the sow's response to exogenous estradiol, gonadotropin releasing hormone (GnRH), and gonadotropins. Sows (4 to 6/group) not returning to estrus by 42.8 +/- 3.1 days after weaning were assigned to 1 of the following treatments: 10 micrograms of estradiol benzoate (EB)/kg of body weight; 200 micrograms of GnRH, 1,000 IU of pregnant mare's serum gonadotropin (PMSG); 1,000 IU of human chorionic gonadotropin (HCG); or 4 ml of saline solution plus 2 ml of corn oil. A preovulatory-like surge of luteinizing hormone [(LH) greater than 12 hours in duration] was observed in all weaned sows treated with EB. All EB-treated sows exhibited estrus and ovulated but none conceived. Sows given GnRH had transiently increased (less than 3 hours) LH concentrations that were not associated with estrus or ovulation. Treatment with PMSG caused an increase in peripheral concentrations of 17 beta-estradiol that was followed by an LH surge, estrus, ovulation, and conception. Treatment with HCG caused an increase in circulating concentrations of 17 beta-estradiol that was accompanied by a surge of LH in some sows and ovulation in all sows. Not all sows treated with HCG exhibited estrous behavior, but conception occurred in 2 of 3 sows that were mated at estrus. None of the sows treated with saline plus corn oil had consistent changes in circulatory concentrations of 17 beta-estradiol or LH and none exhibited estrus or ovulated.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in plasma progesterone concentrations in mares treated with cloprostenol and human chorionic gonadotropin and inseminated during estrus

Daily changes in the plasma progesterone concentrations were determined in eight mares treated with intramuscular injections of 250 μg cloprostenol, a prostaglandin analogue, followed five days later by 2500 I.U. human chorionic gonadotropin. A second cloprostenol injection was given 14 days after the first; the mares were then inseminated on the third and fifth day of the subsequent estrus and a second injection of human chorionic gonadotropin was administered on the fifth day. The onset of estrus following the second cloprostenol treatment was synchronized beginning three to four days after treatment in all eight mares. All eight ovulated, five mares conceived and only four foaled. Evaluation of the progesterone profiles provided reliable indicators of luteolysis, ovulation and luteal function. Decreasing plasma progesterone concentrations were associated with cloprostenol induced luteolysis or preceded spontaneous onset of estrus. The plasma progesterone concentrations increased consistently after ovulation, and in the pregnant mares, the progesterone concentrations remained high during the first month after insemination.