Definition of placental

The term "placental" refers to a characteristic reproductive adaptation that occurs in certain mammals during fetal development. The word itself comes from the Latin prefix "placa" meaning "flat" or "board," and the Greek suffix "enton" meaning "vessel" or "pouch." In placental mammals, the embryo develops inside a specialized structure called the placenta, which is attached to the uterine wall of the mother. This unique organ serves as a temporary organ that allows for the exchange of nutrients, oxygen, and waste products between the mother's bloodstream and that of the embryo. The placenta is formed through a complex developmental process, during which the fertilized egg undergoes several cellular divisions and differentiations. The embryonic portion of the placenta, called the chorion, grows directly onto the uterine lining, while the fetal portion, called the amnion, houses the developing fetus in a fluid-filled sac called the amniotic cavity. The functional connection between mother and fetus that the placenta provides is essential for the proper development of the embryo in placental mammals, allowing the fetus to grow and thrive until it is ready for birth. The word "placental" thus highlights the importance of this crucial adaptive feature, as it has evolved in a variety of mammalian lineages across the globe.