Equine placenta – A clinician's perspective. Part 1: Normal placenta – Physiology and evaluation

Abstract

SummaryA complex organ, the equine placenta is responsible for fetal nourishment, protection from external and internal insults, and the production and/or metabolisation of various hormones. The endometrial cups are unique structures of the equine placenta that are responsible for producing an essential hormone for equine pregnancy, equine chorionic gonadotropin. Since mares have epitheliochorial placentae, with 6 layers of tissue separating maternal from fetal circulation, almost the entire surface of the chorioallantois must be attached to the maternal endometrium in order to support a developing fetus adequately. The only avillous areas of the normal chorioallantois are: the cervical star, sites of the endometrial cups, areas facing oviductal papillae, folds overlying major allantoic vessels, and fetal foot PADs (placental areas of degeneration). There are characteristic differences between the gravid and the nongravid horn of the chorioallantois. Allantoic vesicles, allantoic pouches, hippomanes, amniotic plaques and yolk sac remnants are normal features of the equine placenta. The clinician should thoroughly examine the entire placenta immediately after its expulsion. The most important aspect of this evaluation is to check for completeness of the chorioallantois, along with identifying any pathological lesions on the fetal membranes or the umbilical cord.