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Reproduction, fertility, and development2002; 13(7-8); 623-634; doi: 10.1071/rd01063

Equine placentation.

Abstract: A tough, elastic glycoprotein capsule envelops the equine blastocyst between Days 6 and 23 after ovulation. It maintains the spherical configuration of, and provides physical support for, the embryo as it traverses the entire uterine lumen during Days 6-17, propelled by myometrial contractions that are stimulated by pulsatile release of prostaglandin F2alpha and prostaglandin E2. The capsule also accumulates constituents of the exocrine secretions of the endometrial glands ('uterine milk') as nutrients for the mobile embryo as it releases its antiluteolytic maternal recognition-of-pregnancy signal to the whole of the surface of the endometrium. Mobility ceases abruptly on Day 17 with a sudden increase in uterine tonicity that 'fixes' the conceptus at the base of one of the uterine horns. At Day 35, the trophoblast of the spherical conceptus has separated into its invasive and non-invasive components. The former, distinguished as the thickened, annulate chorionic girdle, invades the maternal endometrium to form the unique endometrial cups. These secrete a chorionic gonadotrophin that synergizes with pituitary follicle-stimulating hormone to induce secondary luteal development in the maternal ovaries. The cup cells express foreign fetal antigens that stimulate strong maternal humoral and cell-mediated immune responses, which curtail their lifespan. The non-invasive trophoblast of the allantochorion establishes a stable microvillous contact with the endometrial epithelium around Day 40 and, over the next 100 days, develops a complex multibranched interdigitation with the endometrium to form the microcotyledonary haemotrophic exchange units that cover the entire surface of the diffuse epitheliochorial placenta. Reduction in the effective total area of fetomaternal contact at this placental interface, by competition between twin conceptuses for the limited area of available endometrium, by attachment of the allantochorion to an imperfect endometrium in a mare with endometrosis, or following cross-breeding or embryo transfer between a sire and dam of dissimilar size, will all induce intrauterine growth retardation of the fetus and runting of the foal, which persists into adult life. Over 40 years ago, Professor Roger Short and his colleagues determined that the high concentrations of conventional and unique ring B unsaturated oestrogens in the blood and urine of mares during the second half of pregnancy stem from placental aromatization of large quantities of C-19 precursor molecules secreted by the temporarily hypertrophic fetal gonads. Placental production of progesterone and 5alpha-reduced progestagens, on the other hand, depends on both maternal and fetal adrenal sources of pregnenelone.
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Publication Date: 2002-05-10 PubMed ID: 11999314DOI: 10.1071/rd01063Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

The research article essentially detailed the process of equine placentation, including the formation, function, and influences on the development of the equine placenta and its impact on fetal development and growth.

The role of the glycoprotein capsule

  • The paper described that a strong, elastic glycoprotein capsule enwraps the equine blastocyst, which includes the embryo and its early amniotic cavity, during the period between Day 6 and 23 after ovulation.
  • This capsule was seen to be vital for the embryo’s preservation, particularly when it traverses the full uterine lumen between Days 6 to 17.
  • Its movements are powered by contractions in the muscular wall of the uterus, stimulated by the cyclical release of certain substances called prostaglandin F2alpha and prostaglandin E2.
  • Furthermore, the capsule accumulates uterine milk, which provides nourishment for the mobile embryo once it starts to release a signal that prevents the destruction of the corpus luteum, necessary for maintaining the pregnancy.

Fixation and further separation of the trophoblast

  • On Day 17, the embryo ceases to be mobile due to a sudden rise in uterine tonicity. This cements the conceptus at the base of a uterine horn.
  • By Day 35, the covering of the growing embryo (the trophoblast) has been divided into invasive and non-invasive parts. The invasive part, the chorionic girdle, infiltrates the maternal endometrium forming unique endometrial cups.
  • These cups release a special hormone which, together with follicle-stimulating hormone, induces the growth of the secondary corpus luteum necessary for the maintenance of the pregnancy.
  • These cups also express antigens which trigger a maternal immune response that limits their lifespan.

Development of placental exchange units and related problems

  • The non-invasive part of the trophoblast contacts with the endometrial epithelium by Day 40 and over the next 100 days, forms a complex network of contact points, forming the haemotrophic exchange units that cover the whole of the diffuse epitheliochorial placenta.
  • The limited area of available endometrium for twinning, endometrosis, or when there is a significant difference in the size of the parents can all cause a reduction in the total area of contact, resulting in slowed intrauterine growth of the fetus and the stunting of the foal that can continue into adulthood.
  • The increased estrogen levels in mares’ blood and urine during the second half of the pregnancy originates from placental transformation of large quantities of C-19 precursor molecules secreted by the temporarily hypertrophic fetal gonads.
  • In contrast, the production of progesterone and other progestogens by the placenta requires both maternal and fetal adrenal sources of pregnenolone.

Cite This Article

APA
Allen WR, Stewart F. (2002). Equine placentation. Reprod Fertil Dev, 13(7-8), 623-634. https://doi.org/10.1071/rd01063

Publication

Reproduction, fertility, and development
ISSN: 1031-3613
NlmUniqueID: 8907465
Country: Australia
Language: English
Volume: 13
Issue: 7-8
Pages: 623-634

Researcher Affiliations

Allen, W R
  • University of Cambridge, Department of Clinical Veterinary Medicine Equine Fertility Unit, Mertoun Paddocks, Newmarket, Suffolk, UK.
Stewart, F

    MeSH Terms

    • Allantois / anatomy & histology
    • Animals
    • Chorion / anatomy & histology
    • Embryonic and Fetal Development
    • Endometrium / anatomy & histology
    • Endometrium / physiology
    • Extraembryonic Membranes / anatomy & histology
    • Extraembryonic Membranes / physiology
    • Female
    • Fetus / metabolism
    • Gestational Age
    • Horses / physiology
    • Placenta / metabolism
    • Placentation
    • Pregnancy
    • Steroids / biosynthesis

    Citations

    This article has been cited 26 times.
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