In eutherian mammals, the first functional organ is the placenta, a transient structure that is
rapidly assembled in the extraembryonic compartment. By necessity the placenta develops
in advance of the embryo, which it supports in utero by performing many of the same
functions that the lungs, gastrointestinal tract, and urinary system carry out after birth.
Specialized epithelial cells that arise from the placenta, termed cytotrophoblasts (CTBs), are
responsible for redirecting maternal blood to the developing conceptus, which occurs as a …