Luteal deficiency and embryo mortality in the mare.
Abstract: Four separate components combine to produce the progesterone and biologically active 5 alpha-reduced pregnanes needed to maintain pregnancy in the mare. The primary corpus luteum (CL) is prolonged beyond its cyclical lifespan by the down-regulation of endometrial oxytocin receptors to prevent activation of the luteolytic pathway and its waning progesterone production is supplemented from day 40 of gestation by the formation of a series of accessory CL which develop in the maternal ovaries as a result of the gonadotrophic actions of pituitary FSH and the equine chorionic gonadotrophin (eCG). From around day 100 the allantochorion secretes progesterone and progestagens directly to the endometrium and underlying myometrium and, in the last month of gestation, the enlarging foetal adrenal gland secretes appreciable quantities of pregnenelone which is also utilized by the placenta to synthesize progestagens. Between 10 and 15% of mares undergo foetal death and abortion at some time in gestation and the majority of these losses occur during the first 40 days of gestation when the primary CL is the sole source of progesterone. Yet, all the available evidence suggests that untoward luteolysis is not common in this period and the losses that do occur have other underlying causes. Beyond day 40 the secondary CL receive powerful luteotrophic support from eCG and from day 80-100 until term the supply organ (placenta) and target tissues (endometrium and myometrium) are in direct contact with each other over their entire surface. In the face of this interlocking and failsafe system for progestagen production throughout pregnancy, and despite a paucity of evidence that a deficiency of progesterone production is a cause of pregnancy loss in the mare, it is surprising, and worrying, that annually many thousands of pregnant mares throughout the world are given exogenous progestagen therapy during part or all of their gestation as a form of preventative insurance against the possibility of pregnancy failure. Basic investigative research is required urgently to validate or debunk the practice.
Publication Date: 2001-09-14 PubMed ID: 11555357
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Summary
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The research article explores the hormonal process that maintains pregnancy in mares (female horses) and investigates potential causes of early pregnancy loss. Despite a lack of evidence showing progesterone deficiency as a cause for pregnancy failure, it finds that globally, a lot of pregnant mares are given exogenous progestagen therapy as a precaution. The paper calls for more investigative research to verify or debunk this practice.
Four Pillars of Pregnancy Maintenance in Mares
- The article focuses on the four separate elements that work together to produce progesterone and biologically-active 5 alpha-reduced pregnanes, essential for maintaining pregnancy in mares.
- The first component is the primary corpus luteum (CL), which has an extended lifespan during a mare’s pregnancy. This lifespan extension is facilitated by reducing endometrial oxytocin receptors to prevent activation of the luteolytic pathway.
- As the primary CL’s progesterone production decreases after the 40th day of gestation, a series of accessory CL develop in the maternal ovaries due to the gonadotrophic actions of pituitary FSH and equine chorionic gonadotrophin (eCG), supplementing the progesterone supply.
- The third element is the allantochorion, which from around day 100, secretes progesterone and progestagens directly to the endometrium and underlying myometrium.
- Lastly, in the final month of gestation, the growing foetal adrenal gland secretes pregnenelone, which the placenta utilizes to synthesize progestagens.
Early Pregnancy Loss in Mares
- Between 10% and 15% of mares experience foetal death and abortion, mostly happening within the first 40 days of gestation when the primary CL is the sole source of progesterone.
- Despite these losses, the article reveals that there is no substantial evidence to suggest that untimely luteolysis is common during this period, implying that other underlying causes exist.
Progestagen Therapy in Pregnant Mares
- Despite the interlocking and failsafe system for progestagen production throughout a mare’s pregnancy, and the lack of strong evidence linking progesterone deficiency to pregnancy loss, many pregnant mares worldwide are given exogenous progestagen therapy during part or all of their gestation as a form of preventative insurance.
- The paper calls this an alarming practise given that scientific-medicinal evidence to support it is lacking.
- It concludes by emphasizing the crucial need for investigative research to either validate or debunk this globally prevalent practise of administering progestagen therapy to pregnant mares.
Cite This Article
APA
Allen WR.
(2001).
Luteal deficiency and embryo mortality in the mare.
Reprod Domest Anim, 36(3-4), 121-131.
Publication
Researcher Affiliations
- Thoroughbred Breeders' Association, Equine Fertility Unit, Mertoun Paddocks, Newmarket, Suffolk, UK.
MeSH Terms
- Abortion, Veterinary / etiology
- Abortion, Veterinary / prevention & control
- Animals
- Chorionic Gonadotropin / blood
- Chorionic Gonadotropin / metabolism
- Corpus Luteum / metabolism
- Corpus Luteum / physiology
- Female
- Fetal Death / etiology
- Fetal Death / prevention & control
- Fetal Death / veterinary
- Gonadotropins, Equine / blood
- Gonadotropins, Equine / metabolism
- Horses / physiology
- Pregnancy
- Pregnancy, Animal / drug effects
- Pregnancy, Animal / physiology
- Progesterone / blood
- Progesterone / deficiency
- Progesterone / metabolism
- Progestins / administration & dosage
- Progestins / adverse effects
- Progestins / metabolism
- Time Factors
Citations
This article has been cited 18 times.- Ashraf R, Rashid S, Rasheed I, Asif S. Early embryonic death in equines and camelids. Open Vet J 2022 Nov-Dec;12(6):903-909.
- Grabowska A, Kozdrowski R. Relationship between estrus endometrial edema and progesterone production in pregnant mares two weeks after ovulation. BMC Vet Res 2022 Nov 21;18(1):414.
- López-Gatius F, Garcia-Ispierto I. Clinical Overview of Luteal Deficiency in Dairy Cattle. Animals (Basel) 2022 Jul 22;12(15).
- Lawson EF, Grupen CG, Baker MA, Aitken RJ, Swegen A, Pollard CL, Gibb Z. Conception and early pregnancy in the mare: lipidomics the unexplored frontier. Reprod Fertil 2022 Jan 1;3(1):R1-R18.
- Segabinazzi LGTM, Andrade LRP Jr, Alvarenga MA, Dell'Aqua JA Jr, Canisso IF. Use of Intravaginal Progesterone-Releasing Device Results in Similar Pregnancy Rates and Losses to Long-Acting Progesterone to Synchronize Acyclic Embryo Recipient Mares. Vet Sci 2021 Sep 10;8(9).
- Alonso MA, Silva LA, Affonso FJ, Lemes KM, Celeghini ECC, Lançoni R, Carvalho HF, de Arruda RP. Effect of hCG application at different moments of the estrous cycle on corpus luteum and uterine vascularization and serum progesterone concentration in mares. Anim Reprod 2019 Oct 24;16(2):317-327.
- Hannan MA, Haneda S, Murata K, Takeuchi S, Cheong SH, Nambo Y. Birth of first foals through embryo transfer after artificial insemination using frozen semen in Japan. J Reprod Dev 2020 Apr 10;66(2):193-197.
- Aurich J, Köhne M, Wulf M, Nagel C, Beythien E, Gautier C, Zentek J, Aurich C. Effects of dietary L-arginine supplementation to early pregnant mares on conceptus diameter-Preliminary findings. Reprod Domest Anim 2019 May;54(5):772-778.
- Krakowski L, Bartoszek P, Krakowska I, Stachurska A, Piech T, Brodzki P, Wrona Z. Changes in Blood Lymphocyte Subpopulations and Expression of MHC-II Molecules in Wild Mares Before and After Parturition. J Vet Res 2017 Jun;61(2):217-221.
- Park JJ, Seong HK, Kim JS, Munkhzaya B, Kang MH, Min KS. Internalization of Rat FSH and LH/CG Receptors by rec-eCG in CHO-K1 Cells. Dev Reprod 2017 Jun;21(2):111-120.
- Mohammed BT, Sontakke SD, Ioannidis J, Duncan WC, Donadeu FX. The Adequate Corpus Luteum: miR-96 Promotes Luteal Cell Survival and Progesterone Production. J Clin Endocrinol Metab 2017 Jul 1;102(7):2188-2198.
- Cohen L, Bousfield GR, Ben-Menahem D. The recombinant equine LHβ subunit combines divergent intracellular traits of human LHβ and CGβ subunits. Theriogenology 2015 Jun;83(9):1469-76.
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- Lösle M, Lin CW, Beil-Wagner J, Aebi M, Buch T. Comparison of pregnant mare serum gonadotropin products with surprising differences in protein content. Sci Rep 2025 Feb 25;15(1):6824.
- Li L, Li S, Ma H, Akhtar MF, Tan Y, Wang T, Liu W, Khan A, Khan MZ, Wang C. An Overview of Infectious and Non-Infectious Causes of Pregnancy Losses in Equine. Animals (Basel) 2024 Jul 2;14(13).
- Bazer FW, Johnson GA. Early Embryonic Development in Agriculturally Important Species. Animals (Basel) 2024 Jun 26;14(13).
- Martínez-Boví R, Gaber JTH, Sala-Ayala L, Plaza-Dávila M, Cuervo-Arango J. Pregnancy and Luteal Responses to Embryo Reinsertion following Embryo Flushing in Donor Mares. Animals (Basel) 2024 May 29;14(11).
- Porto ACRC, Redoan MA, Massoco CO, Furtado PV, Oliveira CA. Additional effects using progestins in mares on levels of thyroid hormones and steroids in neonates. Anim Reprod 2023;20(4):e20230029.
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