Retained placenta is one of the most common peripartum complications in mares. It delays the recovery of the uterus, decreases fertility, and can be life-threatening. The mechanism of normal placenta release is unknown. In addition to systemic hormonal changes affecting the process of placenta separation, it is supposed that local mechanisms at the cellular level may play an important role in this process. It is known that the incidence of retained placenta correlates with reduced expression of classic class I major histocompatibility complex protein (classic MHC I) in cows’ placentas. In mares, classic MHC I is expressed in early pregnancy, but it is unknown if classic MHC I is expressed again in peripartum and if reduced expression correlates with retained placenta in mares. Both early and late expression seem likely, because early expression would prepare mares to reject placenta tissues if MHC is expressed peripartum. This article discusses how MHC I is expressed in placental tissues; how it affects lymphocyte migration, metalloproteinase activation, and extra-cellular matrix remodelling in those tissues; and how various factors can affect MHC I activation. The paper also describes a hypothesis for the mechanism of placenta separation in mares based on the similarity of these processes in other species that have been more extensively studied.
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