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REVIEW
Contents Vol 36(2)

Understanding placentation in ruminants: a review focusing on cows and sheep

Gregory A. Johnson https://orcid.org/0000-0003-1949-1797 A * , Fuller W. Bazer B , Heewon Seo A , Robert C. Burghardt A , Guoyao Wu B , Ky G. Pohler https://orcid.org/0000-0003-2429-0893 B and Joe W. Cain A
+ Author Affiliations
- Author Affiliations

A Department of Veterinary Integrative Biosciences, Texas A&M University, College Station, TX 77843, USA.

B Department of Animal Science, Texas A&M University, College Station, TX 77843, USA.

* Correspondence to: gjohnson@cvm.tamu.edu

Reproduction, Fertility and Development 36(2) 93-111 https://doi.org/10.1071/RD23119

© 2024 The Author(s) (or their employer(s)). Published by CSIRO Publishing on behalf of the IETS

Abstract

Mammals differ regarding their placentae, but in all species placental trophoblasts interact intimately with the uterine endometrium to mediate the transfer of nutrients from the mother to the embryo/fetus through the closely juxtaposed microcirculatory systems of the uterus and placenta. Placentation in ruminants is intermediate between the non-invasive type, as observed in the epitheliochorial placenta of pigs, and the invasive type, as observed in the haemochorial placentae of mice and humans. In ruminants, placental trophoblast cells invade uterine endometrial tissue, but invasion is believed to be limited to the endometrial luminal epithelium (LE). In the LE there are varying degrees of syncytialisation among species, with syncytialisation being more extensive in sheep than cows. The hallmarks of placentation in ruminants include: (1) an extended period in which conceptuses (embryos and associated placental membranes) elongate and must be supported by secretions (histotroph) from the uterus; (2) a cascade involving an array of adhesion molecules that includes integrin-mediated attachment of the conceptus trophoblast to the endometrial LE for implantation; (3) syncytialisation of the developing early placenta, a process for which there is currently limited understanding; and (4) development of placentomes that define the cotyledonary placentae of cows and sheep, and provide haemotrophic support of fetal development.

Keywords: conceptus elongation, conceptus implantation, cows, endometrium, placentation, placentomes, pregnancy, sheep, syncytialisation, trophoblast.

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