Placental Shape

Placental Shape

        In more primitive eutherian mammals, placentas are not leafy. The villi are irregularly scattered over the chorion surface in pigs. A placenta with scattered villi is known as a diffuse placenta. On the other hand, as in cattle, villi are present in patches on the chorion surface so that part of the chorion surface is plain.

        The patches or groups of villi are called as cotyledons and hence the placenta is cotyledonary. In some cases such as carnivores, the villi are formed in specific region so as to form a belt around the blastocyst. Here the villi are called zonal and, therefore, the placenta is called the zonary placenta. The chorion surface, in man and anthropoids, is first covered throughout with villi. The villi are developed only on the side and then this side turns away from the lumen of uterus. 

        The other side which comes in contact with maternal tissue is provided with less of villi and thus placenta is in the shape of a disc and that is Why is this type of placenta called discoidal placenta. Rats, mice, rabbits, and other rodents have developed discoidal placentas. Where there are two discs of placentae, that type of placenta is known as bidiscoidal placenta.