noun, plural: chorions
(entomology) The outer protective membrane of insectegg
(botany) The outer membrane of a seed
In embryology, the chorion pertains to one of the extraembryonic membranes of a developing embryo (or fetus). In higher vertebrates such as humans, the chorion is part of the placenta. It is comprised of four layers: (1) cellular layer, (2) reticular layer, (3) basement membrane, and (4) trophoblast. The trophoblast is the outermost layer. The chorion together with the amnion forms the amniotic sac. The chorion has projections called chorionic villi involved in the transfer of nutrients from maternal blood to fetal blood. As the embryo develops into a fetus, the chorionic villi develop as well and become a part of the placenta. As a result, the placenta becomes comprised of the fetal placenta and maternal placenta. The fetal placenta is also referred to as the chorion frondosum.
Word origin: Latin chorion, Ancient Greek khorion (“membrane surrounding the fetus, afterbirth”)