Pregnancy Recognition and Implantation of the Conceptus in the Mare

Abstract

Few, if any, biological processes are as diverse among domestic species as establishment of early pregnancy, in particular maternal recognition of pregnancy. Following fertilization and initial development in the mare oviduct, selective transport of the embryo through the uterotubal junction driven by embryo-derived PGE2 occurs. Upon arrival in the uterus, an acellular glycoprotein capsule is formed that covers the embryo, blastocyst, and conceptus (embryo and associated extraembryonic membranes) between the second and third weeks of pregnancy. Between Days 9 and 15/16 of pregnancy, the conceptus undergoes an extended phase of mobility. Conceptus mobility is driven by conceptus-derived PGF2α and PGE2 that stimulate uterine contractions which in turn propel migration of the conceptus within the uterine lumen. Cessation of conceptus mobility is referred to as fixation and appears to be attributable to increasing size of the conceptus, preferential thickening of the endometrium near the mesometrial attachment referred to as encroachment, and a reduction in sialic acid content of the capsule. During maternal recognition of pregnancy, endometrial PGF2α release is attenuated, a consequence of reduced expression of key enzymes involved in prostaglandin production. Oxytocin responsiveness is altered during early pregnancy, and reduced expression of the oxytocin receptor appears to be regulated at the posttranscriptional level rather than the transcriptional level. Prostaglandin release is attenuated temporarily only during early pregnancy; during the third week of pregnancy, the endometrium resumes the ability to secrete PGF2α. The equine conceptus initiates steroidogenesis as early as Day 6 and synthesizes estrogens, androgens, and progesterone. Estrogens are metabolized locally, presumably regulating their bioavailability and actions. Results of experiments attempting to prove that conceptus-derived estrogens are responsible for extension of corpus luteum function have been inconclusive. By the fourth week of pregnancy, the chorionic girdle becomes visible on the trophoblast. Subsequent invasion of chorionic girdle cells leads to formation of endometrial cups which secrete equine chorionic gonadotropin. Equine chorionic gonadotropin has luteinizing hormone functions in the mare, causing luteinization of follicles resulting in the formation of secondary corpora lutea essential to production of progesterone and maintenance of pregnancy.

Expression Profile of the Chromosome 14 MicroRNA Cluster (C14MC) Ortholog in Equine Maternal Circulation throughout Pregnancy and Its Potential Implications

“…Moreover, pathway analysis of the same set of miRNAs revealed that they are associated with Wnt signaling pathway and oxytocin receptor mediated signaling pathway, among others. Interestingly, it has been reported that oxytocin responsiveness is altered during early equine pregnancy, and reduced expression of the oxytocin receptor (OXTR) seems to be regulated at the posttranscriptional level rather than the transcriptional level [50]. Our prediction suggests that these miRNAs are potentially involved in oxytocin receptor mediated signaling pathway during early pregnancy in the mare.…”

Fertility restoration of racing mare with persistent corpus luteum

“…Embryo development can be hampered following improper progestin supply when the embryo reaches the uterus [21]. Adequate P4 concentrations are vital mediators of the appropriate embryo-maternal environment during early pre-implantation embryo development [28,29]. The 45.45% of mares may fail to become pregnant in this study because of inadequate P4 concentration during post-mating.…”

Conception and early pregnancy in the mare: lipidomics the unexplored frontier

“…This anti-luteolytic signal enables the continued production of ovarian progesterone ( Sharp et al 1997 ) and pregnancy maintenance. The downregulation of PGF 2α is due to the attenuation of intrauterine oxytocin receptor expression, and hence COX2 at a post-transcriptional level ( Klein 2015 ) ( Fig. 5 ).…”