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.
Reproduction is one of the basic biological functions in animals and humans. Due to the high biological relevance of reproduction and energy investment in their rearing offspring need to be of the best genetic quality and fitness to ensure preservation of the species. Both males and females employ mating strategies that would promote reproduction success and survival of their offspring. Choosing a high-quality mating partner is considered to be the main strategy in the reproduction process. One of the factors influencing the partner’s attractiveness is Major Histocompatibility Complex class I (MHC I). The influence of MHC I on mate choice is well established in animals, whereas it is still questioned in humans, where the social status of a partner may strongly influence the mate choice. In this review the role of the MHC I on mate choice in animals and humans is discussed. The studies published so far show that all investigated mammalian species can detect fractions of the MHC I molecules in urine and other body fluids. The response to the signal carried by MHC I is context-dependent and varies not only between species, but also between genders and may be modulated by various socioecological factors in every phase of the reproduction process, until zygote formation. These results suggest that MHC plays an important role in the choice of a reproductive partner in all mammal species, including humans.