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.
Miller KF, Wesson JA, Ginther OJ.The effects of administration of equine follicular fluid (FF) on gonadotropin concentrations in the ovariectomized (OVX) mare were examined. The FF was obtained by aspiration of follicles in ovaries obtained during the summer at a slaughterhouse. Systemic administration of whole equine FF to OVX mares resulted in significant decreases in circulating concentrations of FSH (26.0–74.0% of the pretreatment concentrations depending on dosage). Treatment with FF from which steroids had been removed tended (P0.10) on FSH. Treatment with whole FF initially decreased and then increased the concentrat...
Woodley SL, Burns PJ, Douglas RH, Oxender WD.This study was designed to test if oestradiol treatment would prevent or delay luteolysis in mares. Mares (5/group) received 0, 0.1, 1.0 or 10.0 mg oestradiol-17 beta daily from the day of ovulation until the next ovulation or for a maximum of 32 days. This treatment did not prevent luteolysis which occurred 15.8, 16.8, 15.8 and 17.3 days after the previous ovulation for the mares treated with 0, 0.1, 1.0 and 10.0 mg oestradiol respectively. Although oestradiol treatment failed to alter oestrous behaviour after luteolysis, daily treatment with 10.0 mg oestradiol prevented follicular growth and...
Evans MJ, Irvine CH.Deeply acyclic (seasonally anovulatory) mares were treated with GnRH or a GnRH analogue to induce follicular development and ovulation. Courses of GnRH (3--4) were administered at approximately 10-day intervals to reproduce the gonadotrophin surges which precede ovulation in the normal cycle. Exogenous progesterone was administered in an attempt to reproduce the luteal phase pattern. Induced serum FSH concentrations were comparable to those causing follicular development in the normal cycle, but induced LH levels were lower and of shorter duration than those of the periovulatory surge. Three o...
Snyder DA, Turner DD, Miller KF, Garcia MC, Ginther OJ.Observations of follicular and gonadotrophic changes were conducted on 14 Pony mares for 31-day periods beginning at the penultimate (second last) ovulation and at the last ovulation of the season. There were no main effects or interaction of day or period on the numbers of small follicles (less than 20 mm). Concentrations of FSH, numbers of large follicles (greater than 20 mm), and oestrous behaviour each showed an effect of day (P less than 0.01), but no effect of period. Concentrations of LH and diameter of the largest follicle each showed an interaction (P less than 0.01) between day and p...
Loy RG, Buell JR, Stevenson W, Hamm D.Sixty-two non-cycling mares were classified according to the size of largest follicles at the time of treatment with Prostalene, an analogue of prostaglandin (PG) F-2 alpha. Although oestrus occurred in only 77.4% of mares, 98.4% ovulated at an average of 6.8 days after treatment. Greatest variance of interval to ovulation was observed in mares having follicles greater than or equal to 40 mm at the time of treatment. This was due to regression of large follicles about one-third of the time and later ovulation of a succeeding follicle. This resulted also in greatest uncertainty of prediction of...
Freedman LJ, Garcia MC, Ginther OJ.A 16 h daily photoperiod hastened the onset of the ovulatory season (first ovulation); gonadotrophin and follicular changes prior to the onset were similar in intact light-treated and control mares. A preovulatory decline in FSH concentrations before the onset of the ovulatory season preceded the decrease in number of follicles (15--25 mm) and the rise in LH concentrations which was temporally associated with the growth of an ovulatory follicle. Seasonal changes of FSH and LH concentrations were found in ovariectomized mares and were influenced by photoperiod. During the anovulatory season, th...
Stewart F, Allen WR, Moor RM.Rat testicular radioreceptor assays specific for FSH and LH were used to determine the FSH:LH ratio of PMSG produced by horse, donkey, mule and hinny conceptuses. Measurements of FSH and LH activities in PMSG produced both in vivo and in vitro by the four types of conceptuses showed that the genotype of the foetus markedly influences the FSH:LH ratio of PMSG. The FSH:LH ratio of PMSG produced by the horse conceptus was around unity whereas the ratio of PMSG produced by the donkey conceptus was as low as 0-2. Furthermore, the hybrid mule and hinny conceptuses both produced PMSG with an FSH:LH r...
Lapin DR, Ginther OJ.A crude equine pituitary ethanol extract (EE) was used to induce single and miltiple ovulations in seasonally anovulatory pony mares 3-15 years of age. 12 mares were injected daily for 14 days with EE; 6 of the EE-treated mares were also treated with human chorionic gonadotropin (HCG), and 6 control mares received saline vehicle only. In a 2nd experiment designed to determine if EE treatment could induce multiple ovulations in seasonally ovulatory mares, 7 mares were treated during diestrus, 7 mares were treated beginning on Day 1 of estrus, and 7 remained untreated. The results of experiment ...
Sharp DC, Kooistra L, Ginther OJ.Two groups, each of seven pony mares, were maintained from 17 October to 15 February (120 days) in the University Biotron where temperature and daily photoperiod were regulated to simulate normal conditions for that period (control group) or those normally applicable from 1 March to 1 July (treated group). Follicular growth, ovulation rate and oestrous behaviour were determined daily by rectal palpation, and by teasing with a stallion. By Day 69 of treatment, all ovarian end-points (number of follicles greater than 10 mm, number of follicles greater than 20 mm, average follicle diameter and di...
Evans MJ, Irvine CH.Heterologous radioimmunoassays were developed for equine FSH and LH. Serum concentrations were measured in twenty-three mares throughout the oestrous cycle and early pregnancy. FSH concentrations were raised fivefold by 'surges' rather than 'spiked', occurring at 10- to 11-day intervals during the oestrous cycle and early pregnancy. The late oestrous/early dioestrous surge of FSH appeared to initiate development of up to twenty follicles. The mid-dioestrous surge may be important for the further development of follicles destined to ovulate 10 to 13 days later. Levels of LH were increasing by t...
Younglai EV.The research studied the effect of follicular fluid and plasma on the steroid-producing activity of granulosa cells isolated from the ovaries of horses in heat. The results could potentially provide […]
SHORT RV.A comparison has been made between the steroids present in equine follicular fluid and luteal tissue. Follicular fluid was found to contain progesterone, 17α-hydroxyprogesterone, androstenedione, epitestosterone, 19-norandrostenedione, oestrone, oestradiol-17β and 6α-hydroxyoestradiol-17β, in confirmation of previous studies. On the other hand, luteal tissue contained large amounts of progesterone and 20α-hydroxypregn-4-en-3-one, and a small quantity of 17α-hydroxyprogesterone. No C18 or C19 steroids could be detected.
These findings are therefore interpreted as evidence in favour of ...
van Niekerk FE, van Niekerk CH.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...
Driancourt MA, Mariana JC, Palmer E.In the middle of the breeding season, 16 pony mares (n = 4 per day) were slaughtered on four different days (days 6, 14, 17 and the preovulatory day) of the oestrous cycle, day 0 being the day of the last ovulation. All the ovaries were examined histologically; the number, size and atresia (defined by granulosa cell pyknosis) of all follicles larger than 1 mm in diameter were studied, using a Kryotome-video recorder-TV system. Follicular distribution of all the sizes studied (1-5 mm, 5-10 mm, greater than 10 mm in diameter) was very similar in the right and left ovaries. However, compared to t...
Ginther OJ, Bergfelt DR.Seasonally anovulatory mares were injected, i.m., twice daily with a GnRH analogue (GnRH-A), and hCG was given when the largest follicle reached 35 mm in diameter. In Exp. 1, treatment was initiated on 23 December when the largest follicle per mare was less than or equal to 17 mm. An ovulatory response (ovulation within 21 days) occurred in 17 of 30 (57%) GnRH-A-treated mares on a mean of 15.8 days. The shortest interval to ovulation in control mares (N = 10) was 57 days. The diameter of the largest follicle first increased significantly 6 days after start of treatment. In Exp. 2, treatment wa...
Hohenhaus MU, Bostedt H.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...
Brück I, Bézard J, Baltsen M, Synnestvedt B, Couty I, Greve T, Duchamp G.In mares, the shortage of oocytes and the variability in nuclear maturation at a certain time of the oestrous cycle hinders the optimization of methods for in vitro maturation and in vitro fertilization. Increasing the number of small-to-medium-sized follicles available for aspiration in vivo may increase the overall oocyte yield. The aims of the present study were to investigate whether administration of crude equine gonadotrophins affects follicular development, oocyte recovery rate, in vivo oocyte maturation and follicular concentrations of meiosis-activating sterols. During oestrus, all fo...
Silvia PJ, Squires EL, Nett TM.Four groups of mares, representing anestrus (AN; n = 8), early transition (ET; n = 7), late transition (LT; n = 8) and estrus (EST; n = 12) were used to examine release of luteinizing hormone (LH) and follicle stimulating hormone (FSH) after a bolus injection of gonadotropin releasing hormone (GnRH) during the transition from anestrus into the breeding season. Estrous mares received GnRH on d 2 or 3 of estrus in the cycle immediately preceding slaughter. Anestrous, ET and LT mares received GnRH exactly 1 wk prior to slaughter. A single injection of GnRH (Sigma LHRH, L-0507, 2.0 micrograms/kg b...
Nequin LG, King SS, Matt KS, Jurak RC.The transition from anoestrus to oestrus in mares is controlled by photoperiod. The present study examined whether additional daylength would accelerate the mares' response to gonadotrophin-releasing-hormone (GnRH). Nine anoestrous mares were placed under ambient or artificial long lighting on 7th January. The four month experimental period was divided into a three-day sequence which was repeated at 21 day intervals. Ovaries were palpated rectally on Day 1; saline was injected (1 ml intravenously [iv]) on Day 2; GnRH was administered (0.59 microgram/kg bodyweight iv) on Day 3. Blood was taken ...
Loy RG, Buell JR, Stevenson W, Hamm D.Sixty-two non-cycling mares were classified according to the size of largest follicles at the time of treatment with Prostalene, an analogue of prostaglandin (PG) F-2 alpha. Although oestrus occurred in only 77.4% of mares, 98.4% ovulated at an average of 6.8 days after treatment. Greatest variance of interval to ovulation was observed in mares having follicles greater than or equal to 40 mm at the time of treatment. This was due to regression of large follicles about one-third of the time and later ovulation of a succeeding follicle. This resulted also in greatest uncertainty of prediction of...
Melchert M, Aurich J, Ertl R, Reichart U, Walter I, Gautier C, Kaps M, Aurich C.Mares resume ovarian activity rapidly after foaling. Besides follicle-stimulating hormone (FSH) and luteinizing hormone (LH), the pituitary synthesizes prolactin and growth hormone which stimulate insulin-like growth factor (IGF) synthesis in the liver. We tested the hypothesis that follicular growth is initiated already antepartum, mares with early and delayed ovulation differ in IGF-1 release and that there is an additional IGF-1 synthesis in the placenta. Plasma concentrations of LH, FSH, IGF-1, IGF-2, activin and prolactin. IGF-1, IGF-2, prolactin and their receptors in placental tissues w...
Mahmood K, Hassan M, Channa AA, Ghafoor A, Riaz A.The present study aimed to evaluate and compare the overall and breed-specific seasonal breeding patterns, fertility rates, cyclicity, and follicular dynamics of Arab, Thoroughbred, and Percheron mares under the subtropical conditions of Pakistan. Methods: A retrospective analysis of climatic data and breeding records of eleven breeding studs spanning four years (2020-2023) was made to find out the overall seasonality in the breeding pattern of mares. Fifty mares of each breed (n = 150 in total) were scanned by ultrasonography for a calendar year to find the cyclicity pattern and follicular ...
Li X, Du M, Liu Y, Wang M, Shen Y, Xing J, Zhang L, Zhao Y, Bou G, Bai D, Dugarjaviin M, Xia W.During follicular development, changes in the composition of the follicular fluid are synchronized with the development of oocytes. Our aim was to screen the key factors affecting oocyte maturation and optimize the in vitro culture protocol by understanding the changes of proteins and metabolites in follicular fluid. Follicles are divided into three groups according to their diameter (small follicle fluid (SFF): 10 mm < d < 20 mm; medium follicle fluid (MFF): 20 mm < d < 30 mm; large follicle fluid (LFF): 30 mm < d). Proteins and metabolites from the follicular fluid w...
Le Breton A, Lewis N.Assisted reproductive technologies (ARTs) are performed worldwide in the equine industry to produce genetically valuable foals. Among them, ovum pick up (OPU) combined with intra-cytoplasmic sperm injection (ICSI) can now be more efficient than embryo transfer (ET) under optimal conditions. However, OPU is not a benign procedure for the mare and the process is costly. Improved efficiency is therefore in the interest of everyone, maximizing mare welfare and optimizing economics for the client. One of the key factors of success is the antral follicle count (AFC) at the time of OPU and subsequent...
Feugang JM, Gad A, Menjivar NG, Ishak GM, Gebremedhn S, Gastal MO, Dlamini NH, Prochazka R, Gastal EL, Tesfaye D.Ovarian follicular fluid (FF) is a dynamic environment that changes with the seasons, affecting follicle development, ovulation, and oocyte quality. Cells in the follicles release tiny particles called extracellular vesicles (EVs) containing vital regulatory molecules, such as microRNAs (miRNAs). These miRNAs are pivotal in facilitating communication within the follicles through diverse signaling and information transfer forms. EV-coupled miRNA signaling is implicated to be associated with ovarian function, follicle and oocyte growth and response to various environmental insults. Herein, we in...
Márquez-Moya A, Sala-Ayala L, Carreras-Vico N, Martínez-Boví R, Cuervo-Arango J.The objectives of this study were to investigate the influence of different OPU variables (vacuum and injection pressures during follicle aspiration and flushing, needle twisting to scrape follicles, number of follicle flushes and type of flushing media) on oocyte recovery rate (ORR) and morphology. Overall, 120 postmortem excised ovaries were processed in 62 replicates (1.9 ± 0.7 ovaries per replicate), with a total of 1336 follicles punctured and aspirated (11.1 ± 6.2 follicles per ovary) by ultrasound-guided follicle aspiration resembling the OPU procedure from live mares. The aspirat...
Hinrichs K, Schnobrich M, Fernandes CB, Fleury P, Barillari V, Bruggeworth S.Transvaginal ultrasound-guided follicle aspiration (TVA) is being performed commonly in clinical equine practice. There is limited information in the literature about the type and incidence of complications related to TVA. We conducted a survey (Study 1), completed by 23 practicing veterinarians, soliciting information about the types and incidence of minor and major complications and of fatalities they had experienced associated with performance of TVA. In Study 2, data from a large clinical practice in which six veterinarians performed over 14,000 TVAs over a period of 7 years was reviewed t...
Hinrichs K, Gleason K, Dobbie T, Felix MR.Transvaginal ultrasound-guided follicle aspiration (TVA) is being performed frequently in equine practice, to recover oocytes for in vitro embryo production via intracytoplasmic sperm injection. While complications from TVA are rare, one of the most prevalent major complications is formation of an ovarian abscess, which can result in the necessity for euthanasia. The acute-phase serum proteins, fibrinogen and serum amyloid A (SAA), are markers of inflammation that might be of use in diagnosis of ovarian abscess. However, no information is available on the effect of standard TVA on these acute ...
Bukhari SSUH, Urooj S.In winter anestrous, prolonged melatonin secretion inhibits gonadotropin-releasing hormone (GnRH). However, synthetic GnRH analogues such as buserelin can stimulate follicular development. We aimed to investigate clinical relationship between retinol, cholecalciferol, and buserelin for inducing estrous in anestrous mares (Equus caballus). We used a total of twenty-one anestrous mares, randomly divided into three groups of seven animals. Group A received retinol, cholecalciferol, and buserelin; group B received buserelin; and group C was control. Groups A, B, and C had 71.42% (n = 5; 95% CI...
Cuervo-Arango J, Sala-Ayala L, Márquez-Moya A, Martínez-Boví R.The objective of this study was to determine the effect of two aspiration pressures (75 vs. 150 mmHg), the follicle flushing method (injection pump controlled by a foot pedal vs. a plastic syringe) and the twisting of the OPU needle on oocyte recovery and in vitro embryo production. OPU data from a total of 104 warmblood sport mares belonging to a commercial OPU-ICSI program were collected as part of a prospective study split into three experiments. Each mare was used only once for OPU. In Experiment 1, the mares' follicles were aspirated using either a high aspiration pressure (flow rate of 1...
