The chorion bilayer is a structure composed of an outer trophectoderm-derived epithelial layer of extraembryonic ectoderm and an inner stromal layer of epiblast-derived extraembryonic mesoderm and blood vessels. The bilayer forms by 7.5 dpc and fuses with the allantois at 8.5 dpc, forming the chorioallantoic placenta.
The chorionic bilayer lines the chorionic cavity which is one of three cavities in the embryo: chorion, amnion and yolk sac.
The chorionic cavity replaces the transient acellular extraembryonic reticulum, which resides between the cytotrophoblast layer and the Heuser's membrane. The extraembryonic reticulum is invaded by extraembryonic mesoderm (which lines the cytotrophoblast layer and Heuser's membrane), and is then broken down and replaced by the chorionic cavity. The Reichert's membrane is the mouse equivalent of the extraembryonic.
Multiple AncestorsSingle AncestorNo DescendantsDevelops fromPart of Parent