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Topic:Follicular Activity
Follicular activity in horses pertains to the cyclical development and maturation of ovarian follicles, which are structures that contain the developing eggs. This process is integral to the reproductive cycle in mares and involves various stages, including follicular growth, maturation, ovulation, and regression. The activity is regulated by a complex interplay of hormonal signals, primarily involving the hypothalamus, pituitary gland, and ovaries. Hormones such as follicle-stimulating hormone (FSH) and luteinizing hormone (LH) play significant roles in stimulating and controlling follicular development. Understanding follicular activity is essential for managing breeding programs, diagnosing reproductive disorders, and optimizing fertility in mares. This page compiles peer-reviewed research studies and scholarly articles that explore the mechanisms, regulation, and implications of follicular activity in equine reproduction.
Cumulus expansion, chromatin configuration and meiotic competence in horse oocytes: a new hypothesis. When recovered from the follicle, horse oocytes may be categorised as having either a compact or an expanded cumulus. Cumulus expansion is strongly associated with follicle atresia. Oocytes with expanded and compact cumuli have similar proportions in the germinal vesicle stage when recovered from the follicle. However, during in vitro culture, a higher proportion of oocytes with expanded cumuli mature, and they do so more quickly, than do oocytes with compact cumuli. Using Hoechst 33258 to label chromatin, in the germinal-vesicle stage horse oocytes can be divided into those in which the nucle...
A comparison of the biochemical composition of equine follicular fluid and serum at four different stages of the follicular cycle. Samples of blood and follicular fluid were recovered from 27 Welsh Pony mares at 4 distinct stages of follicular development. Eighteen biochemical parameters were measured in each sample, including sodium, potassium, chloride, glucose, urea, creatinine, calcium, inorganic phosphate, total bilirubin, total protein, albumin, magnesium, triglyceride, total cholesterol, nonesterified fatty acids, alkaline phosphatase, gamma-glutamyltransferase and aspartate aminotransferase. The concentrations of progesterone, 17beta oestradiol and testosterone, pH and osmolarity, were also measured in all the fol...
Chorionic gonadotropin secretion is associated with an inhibition of follicular growth and an improvement in oocyte competence for in vitro maturation in the mare. This study reports the follicular growth and oocyte competence for in vitro maturation and fertilization under the influence of circulating eCG. Three to 7 successive ultrasound-guided follicular punctures were performed on 4 pregnant mares from Day 23 until Day 75 of pregnancy and on 5 control mares whose embryonic vesicle was crushed on Day 22. All follicles larger than 5 mm were punctured 24 h after the largest follicle reached 18 mm. Expanded cumulus oocyte complexes (COCs) were stained at recovery to analyze the nuclear stage. Compact COCs were cultured in vitro for 46 h and either staine...
Effects of follicular fluid supplementation of in-vitro maturation medium on the fertilization and development of equine oocytes after in-vitro fertilization or intracytoplasmic sperm injection. The aim of this study was to compare the effect of the addition of follicular fluid (FF) collected from preovulatory follicles with that of oestrous mare serum (EMS) (acting as the control) to TCM-199 medium on the in-vitro maturation, fertilization and development of equine cumulus-enclosed oocytes. Oocytes (<30 mm in diameter) were obtained from the ovaries of slaughtered mares. After in-vitro maturation in the presence of the two supplements, their fertilization, cleavage and developmental potential were compared after conventional in-vitro fertilization (IVF) or intracytoplasmic sperm inje...
Role of diameter differences among follicles in selection of a future dominant follicle in mares. Follicles > or = 5 mm were ablated in pony mares by a transvaginal ultrasound-guided technique on Day 10 (ovulation = Day 0). Follicle emergence (at 15 mm, experiment 1; at 6 mm, experiment 2) and development of the new wave was monitored by transrectal ultrasound. Deviation was defined as the beginning of a marked difference in growth rates between the two largest follicles. In experiment 1, mares were grouped (n = 4 per group) into controls, ablation-controls (ablations at Day 10 only), and a two-follicle model (periodic ablation sessions so that only the two largest follicles developed)....
The effect of dietary protein on reproduction in the mare. III. Ovarian and uterine changes during the anovulatory, transitional and ovulatory periods in the non-pregnant mare. In the main experiment the total daily protein intake and quality (essential amino-acids) was varied in 4 groups of mares. The incidence of oestrus in mares during the transitional period was unaffected by protein nutrition. Ovarian activity, as evaluated by follicular development and size of the ovaries, was affected. Mares that received low-quality protein (Groups 1 and 2) had a higher number of smaller follicles (< 10 mm) that developed during the transitional period compared to mares on a high-quality protein intake (Groups 3 and 4). The mares that received the high quality protein ovulate...
Relationships among oocyte-cumulus morphology, follicular atresia, initial chromatin configuration, and oocyte meiotic competence in the horse. Horse oocytes with expanded (EX) cumuli appear to have greater meiotic competence than do horse oocytes with compact (CP) cumuli but are thought to come from atretic follicles. We evaluated the relationships among cumulus expansion, follicle viability, initial chromatin configuration, and meiotic competence of horse oocytes. Follicle walls were sectioned for histological examination, and the follicles were scraped to obtain the oocytes. Half of the oocytes were evaluated immediately and half were matured for 24 h in vitro. Cumulus expansion was significantly associated with follicle atresia. I...
Dopaminergic regulation of gonadotrophin secretion in seasonally anoestrous mares. We have previously demonstrated that daily administration of the dopamine D2 antagonist, sulpiride, during seasonal anoestrus, effectively advances the mean time of onset of the breeding season in mares. The purpose of this study was to examine the effect of sulpiride administration on pulsatile FSH and LH secretion in seasonally anoestrous mares, follicular development, time of first ovulation and the fertility at the first ovulation. Fourteen anoestrous mares were selected based on progesterone concentrations < 1 ng ml-1 for 3 weeks and largest follicle diameter < 20 mm. Starting 30 Ja...
Effects of administration of N-methyl-D,L-aspartate (NMA) on gonadotropin secretion in untreated and steroid-treated ovariectomized mares during the breeding season and in intact and ovariectomized mares during anestrus. The purpose of this experiment was to investigate whether N-methyl-D,L-aspartate stimulated gonadotropin secretion in mares and to determine the response in two experimental paradigms where gonadotropin secretion is low or elevated. In Experiment 1, conducted during the breeding season (summer), eight long-term ovariectomized mares were treated daily for 21 d with progesterone plus estradiol (n = 4) or oil vehicle. Beginning on Day 14, each mare received, in a randomized design on alternate days, an intravenous injection of either 0, 0.5, 1.0, or 5.0 mg/kg NMA. Treatment with NMA was not accom...
Ultrasound-guided intrafollicular treatment in mares. A technique for intrafollicular treatment with a transvaginal ultrasound-guided injection needle was developed using equine chorionic gonadotropin (eCG) as the test substance. An injection was made into one growing follicle of a wave when the follicles were 20 to 23 mm. The treated follicles were injected with 1000 iu of eCG in 0.2 ml saline solution and control follicles were injected with 0.2 ml of the saline vehicle (10 mares per group, 1 follicle per mare). The injection system used an inner 25-gauge needle and an outer 20-gauge needle inserted together through the needle-guide channel of ...
Effect of follicular components on meiotic arrest and resumption in horse oocytes. Two experiments were conducted to evaluate the effect of follicular components on the maintenance of meiotic arrest in horse oocytes. In Expt 1, oocytes were incubated for 24 h with follicular fluid, or with granulosa cells suspended either in medium or in follicular fluid at 25 x 10(6) cells ml-1. None of the treatments resulted in significant maintenance of the germinal vesicle stage over that of non-suppressive control. Culture with follicular fluid plus granulosa cells resulted in a significantly higher proportion of oocytes at metaphase I compared with controls. In Expt 2, oocytes were di...
Antral follicle development and in-vitro maturation of oocytes from macaques stimulated with a single subcutaneous injection of pregnant mare’s serum gonadotrophin. A single s.c. injection of 1000 IU of pregnant mare's serum gonadotrophin (PMSG) stimulates the growth of multiple antral follicles in cynomolgus monkeys (Macaca fascicularis). The number of cumulus-enclosed oocytes (CEO) from six non-stimulated controls was 36 (mean = 6). In contrast, a total of 95 CEO (mean = 31.7) were recovered from three animals stimulated and ovariectomized 3 days later, while 385 CEO (mean = 128.3) were obtained from three animals stimulated and ovariectomized 4 days later. A comparison of the effects of highly purified human follicle-stimulating hormone (FSH), human lu...
Characteristics of cyclicity in maiden thoroughbred mares in the United Kingdom. The characteristics of the cyclicity of 12 maiden thoroughbred mares kept in two groups were studied over a total of 58 cycles. On average, oestrus lasted 5.3 days and in 60 per cent of the cycles ovulation occurred in the last two days of oestrus. Oestrus and ovulation tended to be synchronised in each group of mares. The mean diameter of single-ovulating preovulatory follicles on the day before ovulation was 41.5 mm and during the seven days before ovulation they grew 2.5 mm/day. More than one follicle ovulated in 19 (33 per cent) of the cycles (seven double ovulations and 12 dioestrous ovul...
Effects of neutralization of pregnant mares’ serum gonadotrophin (PMSG) shortly before or at the preovulatory LH surge in PMSG-superovulated heifers on follicular function and development. Normally cyclic heifers (n = 34) received 2500 iu pregnant mares' serum gonadotrophin (PMSG) i.m. at day 10 of oestrus, and 15 mg prostaglandin (PG) i.m. at day 12. Thereafter, a monoclonal antibody against PMSG was administered i.v. before (n = 24), at (n = 6) or shortly after (n = 4) the preovulatory LH surge. Peripheral blood concentrations of LH and oestradiol were compared; follicular development was monitored by daily ultrasound scanning; and the numbers of preovulatory-sized follicles and ovulations were counted 96 h after injection of PG following death. Anti-PMSG treatment before the ...
Follicular and FSH responses to parturition during the anovulatory season in mares. The ovaries of periparturient pony mares (n=9 to 16 parturitions per month for January to April) were scanned ultrasonically on the day of parturition, while those of postpartum and control mares (n=12) were examined at least twice weekly. Four mares had apparent lactational anovulation (incidence, 7%) that corrected spontaneously (1 mare) or within 14 d after the weaning of foals on August 10 (3 mares). All but 2 of the postpartum ovulations occurred after April 29; that is, parturition did not effectively stimulate ovulation in ponies foaling during the anovulatory season. Mean diameter of t...
Ovarian follicular growth and development in mammals. Evidence from several species indicates that the initial stages of follicular growth proceed very slowly. In contrast, the stages after antrum formation are much more rapid. Atresia seems to be most prevalent as follicles approach the size at which they could be recruited for potential ovulation. Although most follicles become atretic around that stage, a few are recruited into a cohort or wave of follicles that continue to grow beyond the stage at which atresia normally occurs. Next, a species-specific number of follicles is selected for dominance. In some species (e.g. rats, primates, pigs),...
Follicular fluid lipoproteins in the mare: evaluation of HDL transfer from plasma to follicular fluid. Using a density gradient ultracentrifugal procedure, we have separated equine plasma and follicular fluid high-density lipoproteins (HDL). The density distribution of the follicular fluid HDL was clearly displaced towards the highest densities in comparison with that of plasma HDL. Similarly, an analysis of size distributions showed a decrease in follicular fluid HDL diameters (4.2 to 9.2 nm) compared to plasma HDL (5.5 to 9.5 nm). HDL were isolated into three subfractions on the basis of the disposition of the Sudan Black stained bands in the centrifuge tubes. Concentrations of each subfracti...
Developmental changes in the gonadotropin releasing hormone neuron of the female rabbit: effects of tamoxifen citrate and pregnant mare serum gonadotropin. Developmental changes in immunostained gonadotropin releasing hormone neurons were demonstrated in female rabbits assigned to the following treatment groups: (i) tamoxifen citrate, 10 mg.kg-1 x day-1, in sesame seed oil (vehicle) (n = 24) or (ii) vehicle alone (control, n = 24) for 108 days; and (iii) 50 IU of pregnant mare serum gonadotropin on postnatal days 22 and 25 (n = 24) or vehicle on nontreatment days. Treatments had no effect on the total number of immunostained cells, but there was a significant (p = 0.0160) developmental shift from cells with smooth processes to rough. Group compar...
Effect of constant administration of a gonadotropin-releasing hormone agonist on reproductive activity in mares: induction of ovulation during seasonal anestrus. The potential of a gonadotropin-releasing hormone (GnRH) agonist (goserelin acetate), delivered constantly for 28 days via a subcutaneous depot, to induce ovulation in seasonally anestrous mares, was investigated. Two experiments were conducted, in which a range of doses (30 to 240 micrograms/mare/d) was examined. Mares were selected on the basis of lack of substantial follicular development (follicle diameter < 20 mm determined ultrasonically) and low serum concentrations of luteinizing hormone (LH) and progesterone. Constant administration of the GnRH agonist-induced ovulation in anestrou...
Growth of small follicles and concentrations of FSH during the equine oestrous cycle. The temporal associations between increases in FSH and growth of small follicles (2-20 mm) were studied during one oestrous cycle (ovulation to ovulation) in 15 horse mares. Follicular diameters were obtained ultrasonically. For each day, follicles were combined for both ovaries, grouped from largest to smallest (excluding dominant follicles), and divided into three to five tiers of six follicles for each mare (for example: tier 1, six largest follicles; tier 2, next six largest follicles). A significant increase in mean diameters followed by a significant decrease was used to define a follicu...
Induction of ovulation and superovulation in mares using equine LH and FSH separated by hydrophobic interaction chromatography. Pharmacological control of reproduction in mares requires the use of equine gonadotrophins to avoid induced immunological resistance. Crude equine gonadotrophins (CEG) have been used but the presence of equine luteinizing hormone (eLH) and follicle-stimulating hormone (eFSH) in CEG has led to disappointing results in superovulation studies. Separation of eLH and eFSH activities from CEG is necessary to overcome this problem. The hydrophobic properties of the two hormones were sufficiently different to permit their separation by hydrophobic interaction chromatography (HIC) on a phenyl Sepharose...
Secretory patterns and rates of gonadotropin-releasing hormone, follicle-stimulating hormone, and luteinizing hormone revealed by intensive sampling of pituitary venous blood in the luteal phase mare. We used our unique nonsurgical technique for collecting pituitary venous (pit) blood to study GnRH, FSH, and LH secretion patterns in midluteal phase mares. This method does not perturb endocrine function and allows continuous monitoring of GnRH and gonadotropin (Gn) secretion, determination of the amount of GnRH perfusing gonadotropes, and direct measurements of the amounts of Gn secreted. In a total of 80 h of 5-min sampling in four mares, eight Gn peaks occurred; however, more frequent sampling was needed to define secretory events precisely. Therefore, pit blood was collected continuously ...
Changes in the hypothalamic-hypophyseal axis of mares in relation to the winter solstice. In mares, the amount of gonadotrophin-releasing hormone (GnRH) is low in the hypothalamus during seasonal anoestrus, but by early spring, concentrations of GnRH are high. The timing of this response was characterized more precisely by determining concentrations of GnRH in hypothalamic tissue collected immediately before and at various times after the winter solstice (22 December 1986). Ovaries, pituitary gland, hypothalamus and a blood sample were collected from six groups of mares (6-12 mares per group) at death, 1 week before day of the winter solstice and 1, 2, 3 and 12 weeks afterwards. No...
[Double ovulation in mares. Clinical, hormone analytical and sonographic studies]. 108 mares with a total of 135 oestrous cycles were examined. 30% of the mares showed development of double follicles, found by palpation and ultrasonography. Eight cases of double ovulation, four of them synchronous and four asynchronous, were examined closely. These cases of double ovulation showed different growth and different development of the follicular wall. This occurred at the same time in cases of synchronous double ovulation whereas it differed in case of asynchronous double ovulation. The later ovulating follicle was still growing while the first one had already burst. With both fo...
Control of FSH, follicular development and estrus synchronization in the mare with steroid-free follicular fluid. Twenty-two pony mares were used in a project designed to determine the effectiveness of different treatments in controlling FSH, follicular development and synchronization of estrus and ovulation. Mares in Group 1 (n=8) received daily oral altrenogest (0.044 mg/kg); those in Group 2 (n=7) received daily altrenogest (0.044 g/kg) and, during the last 4 days of treatment they received steroid-free follicular fluid, (15 cc) intravenously (I.V.) two times a day; Mares in Group 3 (n=7) received daily intramuscular (I.M.) injections of progesterone (80 mg) and estradiol valerate (7 mg). All treatment...
Relationships among LH, FSH and prolactin secretion, storage and response to secretagogue and hypothalamic GnRH content in ovariectomized pony mares administered testosterone, dihydrotestosterone, estradiol, progesterone, dexamethasone or follicular fluid. Thirty-five ovariectomized pony mares were used to study the relationships among luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin (PRL) concentrations in blood (secretion), in pituitary (storage) and in blood after secretagogue administration, as well as the content of gonadotropin releasing hormone (GnRH) in hypothalamic areas, under various conditions of steroidal and nonsteroidal treatment. Five mares each were treated daily for 21 d with vegetable shortening (controls), testosterone (T; 150 micrograms/kg of body weight, BW), dihydrotestosterone (DHT; 150 microgram...
Follicle-stimulating hormone pulse amplitude decreases with the onset of the breeding season in the mare. The relationship between daily mean FSH concentrations in serum and the pattern of FSH detected by frequent sampling for 12-h periods (samples every 15 min) was examined in five mares during the transition into the breeding season. The five mature anestrous mares were exposed to a natural increase in daylength. Blood samples were collected daily from February 1 until the first ovulation of the breeding season (April 14 +/- 3.7 days, Mean +/- SEM). Periods of frequent blood collection were performed every two weeks. Blood samples were obtained daily by jugular venipuncture or jugular cannula (f...
Effect of aspiration of the preovulatory follicle on luteinization, corpus luteum function, and peripheral plasma gonadotropin concentrations in the mare. Follicular fluid from small- to medium-sized follicles has been shown to have an inhibiting effect on luteinization of granulosa cells in vitro. This study was conducted to investigate the effect of in vivo removal of follicular fluid on luteinization, peripheral gonadotropin concentrations, and ovulation of secondary follicles in the mare. Follicular fluid was aspirated from the preovulatory follicles of mares when the diameter of the follicle was 30-34 mm (Group A), 35-39 mm (Group B), or 40-44 mm (Group C). Mares in Group D served as controls and the preovulatory follicle was not aspirated....
Effect of sexual arousal on gonadotrophin-releasing hormone, luteinizing hormone and follicle-stimulating hormone secretion in the stallion. In an experiment conducted late in the physiological breeding season, 5 stallions were fitted with indwelling pituitary venous cannulae that permitted unobtrusive collection of blood coming from the pituitary and the hypothalamo-hypophyseal portal vessels. The next day, blood samples were collected at 5 min intervals for several hours while the stallions were resting. Pulses of gonadotrophin-releasing hormone (GnRH), follicle-stimulating hormone (FSH) and luteinizing hormone (LH) occurred approximately once per hour. After this, an oestrous mare was brought into contact with each stallion for ...
Control of onset of breeding season in the mare and its artificial regulation by progesterone treatment. Mean plasma luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels and their pulse frequencies and the size of the largest ovarian follicle increased during spring in 12 mares studied twice monthly from deep anoestrus to the occurrence of the first ovulation of the breeding season. Mean FSH levels were reduced significantly in deep anoestrus and when a pre-ovulatory follicle was present, whereas mean LH levels were highest close to ovulation. Five of these 12 research mares and 20 of 40 maiden or barren Standardbred mares at a commercial studfarm were given daily intramuscular ...
