Pregnancy Recognition and Implantation of the Conceptus in the Mare.

This article discusses the complex process of pregnancy establishment in horses or mares, covering the various stages from fertilization to the formation of secondary corpora lutea, which is crucial for the production of progesterone that maintains pregnancy.

Early Pregnancy and Conceptus Transport

• The research starts by identifying the unique nature of the early stages of pregnancy in mares, focusing on their maternal recognition of pregnancy. This begins with fertilization and initial development in the mare oviduct, followed by the selective transport of the embryo through the uterotubal junction – a process driven by embryo-derived PGE2.

Formation of Acellular Glycoprotein Capsule

• Upon arriving at the uterus, an acellular glycoprotein capsule is created, which envelops the embryo, blastocyst, and conceptus (the embryo and extraembryonic membranes) within the second and third weeks of pregnancy.

Conceptus Mobility and Fixation

• The conceptus undergoes a phase of extended mobility between the 9th and 15th/16th days of pregnancy. This mobility is facilitated by uterine contractions activated by PGF2α and PGE2, both of which originate from the conceptus. This process propels the conceptus within the uterine lumen.
• The mobility ceases in a process referred to as “fixation,” which appears to be caused by an increase in the size of the conceptus, an increase in the endometrium’s thickness near the mesometrial attachment (a process known as encroachment), and reduced sialic acid content in the capsule.

Maternal Recognition of Pregnancy

• During maternal recognition of pregnancy, the release of endometrial PGF2α lessens due to reduced expression of key enzymes involved in prostaglandin production.
• The reactivity to oxytocin changes in early pregnancy, with reduced expression of the oxytocin receptor mostly regulated at the post-transcriptional rather than transcriptional level.
• The release of prostaglandin only temporarily lowers during early pregnancy; during the third week, the endometrium regains its ability to secrete PGF2α.

Steroidogenesis and Chorionic Girdle Formation

• The equine conceptus initiates steroidogenesis as early as Day 6, manufacturing estrogens, androgens, and progesterone. The metabolization of these estrogens is believed to occur locally, which might control their bioavailability and actions.
• Though some experiments aimed at proving whether conceptus-derived estrogens extend corpus luteum function yielded inconclusive results, it is known that by the fourth week of pregnancy, the chorionic girdle forms on the trophoblast. This leads to the formation of endometrial cups, which secrete equine chorionic gonadotropin.
• Equine chorionic gonadotropin possesses luteinizing hormone functions in the mare, leading to the luteinization of follicles that result in the creation of secondary corpora lutea. These are essential for progesterone production and, consequently, the maintenance of pregnancy.