The Placenta

Abstract

The human placenta is a discoid organ that has a hemochorial relationship with the mother. That is to say, the trophoblast that covers the villi is in direct contact with the maternal blood. Transfer of substances has to traverse the syncytiotrophoblast, cytotrophoblast, villous basement membrane, fetal capillary bed, and its endothelium. Although the placenta is the most available tissue, it is the least thoroughly investigated organ, and many aspects of its control still are unknown. Especially deficient is our knowledge of the genetic control of its many functions. The placenta also has a different imprinting pattern (paternal imprinting) from that of the embryo. The prevailing sentiment is that once delivery has occurred, it is a superfluous tissue that only needs to be discarded. There are many reasons, however, that knowledge of placental pathology can aid materially in a better understanding of newborns who have anomalies or neonatal problems. Further, a detailed examination of the placenta at birth can provide a better insight into prenatal life and its possible disturbances than is possible from notes in the history of the patient. The shape of the placenta is extremely variable (Fig. 1), probably due to placental location, its interaction with the endometrium, and uterine shape. The placenta implants anywhere in the uterus, but most commonly, it is in an anterior or posterior location. Much less often it is on the fundus. The shape often can be understood best by considering the history of its implantation. The reason for a specific shape can be derived further from the frequently aberrant shapes of twin placentas where competition for space in the uterus, a phenomenon that has been termed “trophotropism,” can be inferred to have existed. This term was used initially in attempts to explain the placental features in the uncommon event of a placenta …