Maternal recognition of pregnancy in the horse: a mystery still to be solved

Maternal recognition of pregnancy in the horse is the sum of events leading to maintenance of pregnancy; in a narrow sense, maternal recognition of pregnancy refers to the physiological process by which the lifespan of the corpus luteum is prolonged. The horse is one of the few domestic species in which the conceptus-derived pregnancy recognition signal has not been identified. The presence of the conceptus reduces pulsatile prostaglandin F-2 alpha secretion by the endometrium during early gestation in the mare, partly attributed to the reduced expression of cyclooxygenase-2. Cyclooxygenase-2 has therefore been suggested as one of the regulators of endometrial prostaglandin F-2 alpha release modified by the antiluteolytic factor secreted by the conceptus. In addition, altered oxytocin responsiveness has been implicated in the adjustment of prostaglandin release in pregnant mares. While conceptus mobility has proven to be essential for establishment of pregnancy, conceptus-derived oestrogens and prostaglandins, principally prostaglandin E-2, have not been confirmed as the critical antiluteolytic factor. Various ways to induce prolonged luteal function in the non-pregnant mare will be highlighted in the current review, specifically, how they may pertain to the process of maternal recognition of pregnancy. Furthermore, recently published microarray experiments comparing the transcriptome of pregnant and nonpregnant endometria and different stages of conceptus development will be reviewed. Findings include the prevention of conceptus adhesion, the provision of nutrients to the conceptus and the avoidance of immunological rejection, among others.