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

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. The United States cattle industry is burdened by losses of pregnancies of up to 45% of cows bred. Many of these losses in ruminant species are the result of errors in the development of the placenta. This review describes and discusses what is currently known about some of the complex molecular and physiological events that underlie successful placentation in cows and sheep. Increased understanding of placentation is critical for developing advances to improve reproductive efficiency in animal agriculture enterprises that provide high-quality animal protein for human consumption. Photograph by Dr Robert C. Burghardt.