The Equine Endometrial Cup Reaction: A Fetomaternal Signal of Significance

Abstract

A remarkable feature of equine pregnancy is the development of the invasive trophoblast of the chorionic girdle and its formation of the gonadotrophin-secreting endometrial cup cells in early gestation. The details of this process have been revealed only slowly over the past century, since the first description of the endometrial cups in 1912. This centennial presents an opportunity to review the characteristics of the cells and molecules involved in this early, critical phase of placentation in the mare. The invasiveness of the chorionic girdle trophoblast appears to represent an atavistic attribute more commonly associated with the hemochorial placentae of primates and rodents but not with the more recently derived epitheliochorial placentae of the odd-toed ungulates. The nature of and raison d’etre for the strong fetal signals transmitted to the mare by the endometrial cup reaction, and her responses to these messages, are the subject of the present review.

Paternally expressed genes predominate in the placenta

“…The eCG genes themselves are not imprinted; the imprinting phenotype is a result of differential cell division that determines the number of eCG-producing cells: high numbers in horse and hinny (horse father) conceptuses, and low numbers in donkey and mule (donkey father). The eCG system represents an example of fetal signaling to the mother during pregnancy that is determined by the paternal genome and thus is consistent with the paternal-maternal conflict theory (40).…”

Review of the reproductive endocrinology of the pregnant and parturient mare

“…Thus, revival of the primary CL and formation of the secondary CL are marked not only by increased progesterone secretion but also by the appearance of 17OHP and androstenedione, both metabolites of progesterone itself. Excellent reviews of the history of the discovery of the endometrial cups and of eCG secretion and biology have appeared previously [74,75]. It has been stated that among the mammals, only perissodactyls and primates express and secrete a chorionic gonadotrophin [76].…”

“…Secretion of eCG is extremely variable among mares and is influenced by a variety of factors [74] including nutrition [86], uterine environment [87], and genetics [88], but it is highly correlated with maternal progesterone concentrations and numbers of accessory corpora lutea between weeks 5 and 20 of gestation [47,62,79]. The decline in luteal tissue weight, that begins as early as GD 60 [62], occurs in concert with regression of the endometrial cups, decreasing eCG concentrations [89] and with concomitant development of the allantochorion [90].…”

Uterine Responses to the Preattachment Embryo in Domestic Ungulates: Recognition of Pregnancy and Preparation for Implantation

“…Another unusual feature is the formation of the equine-specific endometrial cups beginning on Day 35 (66). The function of these structures is the secretion of equine chorionic gonadotropin (eCG) into the maternal circulation to further increase the production of progesterone in the ovaries (72,73).…”