Reproduction Index Glossary

Placentation in Horses

Implantation and Fetal Membranes

Early in gestation, between roughly day 12 and 15, the equine embryo is round and moves freely throughout the lumen of both uterine horns in response to uterine contractions. Such movement and contact between embryo and endometrium is thought to be a significant part of maternal recognition of pregnancy in horses. At approximately day 16 of gestation, a combination of an enlarged embryo and increased uterine tone leads to the embryo becoming fixed in one spot within the uterus. This process, appropriately enough, is called fixation.

The equine conceptus develops a full complement of extraembryonic membranes. For the first 3-4 weeks of gestation, however, the yolk sac of the equine embryo is quite large relative that that seen in most other mammals. During this time, a large portion of the surface of the embryo is composed of a choriovitelline layer. The allantois can first be seen at about day 20, after which it expands rapidly and soon provides the dominant blood supply to the fused chorioallantois. Through day 35 there is no discernible attachment of fetal membranes to endometrium. By day 40, the "conceptus pours hesitantly from the incised uterus", indictating the onset of attachment. Between this time and day 150 of gestation, the placenta develops to full maturity.

Gross Structure of the Placenta

The equine placenta is classified as diffuse. It involves the entire surface of the chorioallantois except for a small area adjacent to the cervix called the "cervical star", where attachment cannot occur.

The image below is of an equine conceptus at approximately 9 months of gestation, dissected away from the uterus. On the left is shown the unopened chorioallantoic surface which serves as the placenta. In the image on the right, both the chorioallantois and allantoamnion have been opened to expose the fetus. Note the rich vascularity of the allantoamnion, which is typical of equine pregnancies.

Microscopic Structure of the Placenta

On casual examination, the mature chorioallantois appears uniform in structure, but a closer look reveals the chorionic surface to be composed of thousands of polygonal structures 1-2 mm in size. These are microcotyledons, the fundamental unit of the fetal-maternal interface in the equine placenta. Microcotyledons consist of a cluster of highly vascularized chorionic villi which extend into elaborate invaginations of the endometrium. In the image to the right, two microcotyledons are depicted: the leftmost one shows chorionic villi pulled out of the endometrium, while that on the right is intact. Note how this arrangement brings fetal and uterine blood vessels into close contact.

The epitheliochorial nature of this interface remains intact throughout gestation, but there is a progressive thinning and flattening of the connective tissue and epithelial layers which brings the fetal and maternal blood supplies into closer apposition.

The microcotyledonary areas of the placenta allow efficient transfer of small molecules between fetal and maternal blood. The areas surrounding microcotyledons receive copious secretions from endometrial glands (not shown in the image above), which are apparently absorbed by the distinctly columnar chorionic epithelial cells in those areas. These channels, filled with "uterine milk", are referred to as arcades.

A unique feature of equine placentation is the development and ultimate degeneration of "endometrial cups". These structures are derivatives of embryonic trophoblast which secrete equine chorionic gonadotropin.

Placental Transport

General aspects aspects of placental transport are similar to that seen in other species. Immunoglobulins are not transported across the placenta from dam to fetus, and therefore, barring fetal infections, the foal is born without circulating antibodies.

Placental Endocrinology

As is seen in all mammals, the equine fetoplacental unit elaborates both steroid and protein hormones. Except for primates, equids are the only animals that synthesize a placental gonadotropin. The relative levels of major hormones in a mare's serum during pregnancy are summarized in the following figure. Notice that hormones produced by both the ovaries and placenta are depicted. You can alter which hormones are depicted by checking or unchecking the boxes to the right of the figure.

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Equine chorionic gonadotropin (eCG): This glycoprotein hormone is secreted by a part of the fetal placenta called endometrial cups. It turns out that eCG is actually equine luteinizing hormone, and is encoded by the same gene responsible for pituitary LH. In contrast to LH from most other species, equine LH/eCG possesses considerable follicle stimulating hormone-like bioactivity.

In pregnant mares, eCG is usually first detectable in blood between 35 and 40 days of gestation, rise rapidly to peak about 60 days, then decline slowly to become undetectable at about 120 days of gestation. The high blood levels of eCG during this time stimulate development of ovarian follicles. These follicles ovulate or luteinize, resulting in development of so-called secondary and accessory corpora lutea. In concert with the primary corpus luteum, these structures secrete sufficient progesterone to maintain pregnancy until placental progestin synthesis is adequate.

At one time, assays for eCG were popular for diagnosing pregnancy in mares, but this technique has been largely replaced by ultrasonic imaging, which allows much earlier diagnosis.

Equine CG is also known as pregnant mare's serum gonadotropin or PMSG. Due to its follicle-stimulating hormone-like activity, it has been widely used to induce superovulation in other animals, particularly in embryo transfer programs.

Progestins: The equine placenta appears not to synthesize progesterone. However, it secretes copious quantities of progestins (5-alpha-pregnanes), which serve the same function for maintainance of pregnancy. Toward the end of gestation, blood levels of these progestins are typically 100 times the maximal level of progesterone.

Estrogens: In has been known for many decades that mare urine contains high concentrations of estrogens during the second and third trimesters of pregnancy. Indeed, a large industry has developed for collection of pregnant mare urine, which is used to produce Premarin, an estrogen replacement therapy used widely by post-menopausal women.

The equine embryo begins to synthesize estrogens at roughly 12-14 days of gestation, well before development of the placenta. This early estrogen apparently does not escape the uterus and probably has only local effects. Estrogen levels in the serum and urine of pregnant mares begins to rise around day 60 of gestation, peaks at about day 200 and, in contrast to other species, declines during the remainder of gestation.

Estrogens are synthesized in the equine placenta from androgens that are produced by the fetal gonads. The gonads of both male and female fetuses synthesize and secrete into umbilical blood large quantities of the androgen dehydroepiandrosterone (DHA). Within the placenta, DHA is metabolized to a number of different estrogens, most prominently estrone, equilin and equilenin. Equilin and equilenin are estrogens that are apparently unique to pregnant equids.

Relaxin: Relaxin is a protein hormone that, in various species, is thought to act synergistically with progesterone to maintain pregnancy and to promote loosening of pelvic ligaments at the time of parturation. In horses, relaxin appears to be produced by the fetoplacental unit rather than the corpus luteum. It is detected in mare serum starting at about day 80 of pregnancy, and remains at high levels until term. The role of relaxin in equine pregnancy is not known with certainty.

References and Reviews

  • Asbury AC and LeBlanc MM: The placenta. In: Equine Reproduction, McKinnon AO and Voss JL (eds). Lea and Febiger, 1993.
  • Ginther OJ. Reproductive Biology of the Mare. Second Edition, 1992, Equiservices, Cross Plains WI. [a bible for equine reproductive physiology]

Index of: Implantation and Development of the Placenta

Last updated on February 17, 2000
Author: R. Bowen
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