Chorion and Chorionic Villi

Chorion is outermost fetal membrane around embryo.

After implantation trophoblast differentiated into; 

1. Outer layer Syncytitrophoblast

2. Inner layer Cytotrophoblast (Langhan’s layer)

Chorion layer made up of trophoblast and mesoderm.

Chorion villi surround developing ovum.

Villi overlying decidua basalis continue to grow and expand called chorion frondosum and form placenta.

Mesodermal tissue (connecting stalk) connects inner cell mass to chorion and will form umbilical cord later on.