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The multilayered, outermost fetal membrane consisting of extraembryonic somatic mesoderm, trophoblast, and, on the maternal surface, villi bathed by maternal blood; as pregnancy progresses, part of the c. becomes the definitive fetal placenta. SYN: chorionic sac, membrana serosa (1). [G. c., membrane enclosing the fetus]
- c. frondosum the part of the c. where the villi persist, forming the fetal part of the placenta. SYN: shaggy c..
- c. laeve the portion of the c. from which the villi disappear in the later stages of pregnancy. SYN: smooth c..
- previllous c. SYN: primitive c..
- shaggy c. SYN: c. frondosum.
- smooth c. SYN: c. laeve.
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cho·ri·on 'kōr-ē-.än, 'kȯr- n the highly vascular outer embryonic membrane that is associated with the allantois in the formation of the placenta
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n.
the embryonic membrane that totally surrounds the embryo from the time of implantation. It is formed from trophoblast lined with mesoderm and becomes closely associated with the allantois. The blood vessels (supplied by the allantois) are concentrated in the region of the chorion that is attached to the wall of the uterus and forms the placenta. See also villus.
• chorionic adj.
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cho·ri·on (korґe-on) [Gr. “membraneâ€] 1. in human embryology, the cellular, outermost extraembryonic membrane, composed of trophoblast lined with mesoderm; it develops chorionic villi about 2 weeks after fertilization, is vascularized by allantoic vessels a week later, and gives rise to the fetal part of the placenta. 2. in mammalian embryology, the cellular, outer extraembryonic membrane, not necessarily developing villi. 3. in biology, the noncellular membrane covering eggs of various animals, including fish and insects. chorial, chorionic adjMedical dictionary. 2011.