n, pl  -centas  or  -cen*tae   :  the vascular organ in mammals except monotremes and marsupials that unites the fetus to the maternal uterus and mediates its metabolic exchanges through a more or less intimate association of uterine mucosal with chorionic and usu. allantoic tissues permitting exchange of material by diffusion between the maternal and fetal vascular systems but without direct contact between maternal and fetal blood and typically involving the interlocking of fingerlike vascular chorionic villi with corresponding modified areas of the uterine mucosa - see ABRUPTIO PLACENTAE  .
 
 
 
Published under license with Merriam-Webster, Incorporated.  © 1997-2004.