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Structure
The fetal membranes surround the developing embryo and form the fetal-maternal interface. The fetal membranes are derived from the outer trophoblast layer of the implanting blastocyst. The trophoblast layer differentiates into amnion and the chorion, which then comprise the fetal membranes. The amnion is the innermost layer and, therefore, contacts the amniotic fluid, the fetus and the umbilical cord. The internal pressure of the amniotic fluid causes the amnion to be passively attached to the chorion. The chorion functions to separate the amnion from the maternal decidua and uterus.

Development of the fetal membranes
Initially, the amnion is separated from the chorion by chorionic fluid. The fusion of the amnion and chorion is completed at the 12th week of development.

Amnion
The amnion is avascular, meaning it does not contain its own blood vessels. Therefore, it must obtain necessary nutrients and oxygen from nearby chorionic and amniotic fluid, and fetal surface vessels. The amnion is characterised by cuboidal and columnar epithelial layers. The columnar cells are located in the vicinity of the placenta, whereas the cuboidal cells are found in the periphery. During early pregnancy, the amnionic epithelium is sparsely covered in microvilli, which increase in number throughout pregnancy. The function of this microvillous surface is associated with a densely-packed glycocalix with anionic binding sites; these are thought to be involved with intra-amnionic lipid synthesis. This amnionic epithelium is connected to a basement membrane, which is then attached by filaments to a connective tissue layer.

Chorion
The chorionic membrane is a fibrous tissue layer containing the fetal blood vessels. Villi form on the outer surface of the chorion, which maximise surface area for contact with maternal blood. The chorionic villi are involved in fetal-maternal exchange.