This simple nutritive system, however, is only adequate
to provide for the embryo during the ﬁ rst few weeks, when it
is very small. The structure that takes over this function is the
a combination of interlocking fetal and maternal tis-
sues that serves as the organ of exchange between mother and
fetus for the remainder of the pregnancy.
The embryonic portion of the placenta is supplied by the
outermost layers of trophoblast cells, the
maternal portion by the endometrium underlying the chorion.
Fingerlike projections of the trophoblast cells, called
extend from the chorion into the endometrium
). The villi contain a rich network of capillaries
that are part of the embryo’s circulatory system. The endo-
metrium around the villi is altered by enzymes and paracrine
agents secreted from the cells of the invading villi so that each
villus becomes completely surrounded by a pool, or
maternal blood supplied by maternal arterioles.
The maternal blood enters these placental sinuses via
the uterine artery; the blood ﬂ ows through the sinuses and
then exits via the uterine veins. Simultaneously, blood ﬂ ows
from the fetus into the capillaries of the chorionic villi via the
and out of the capillaries back to the fetus
All of these umbilical vessels are con-
tained in the
a long, ropelike structure that
connects the fetus to the placenta.
Five weeks after implantation, the placenta has become well
established, the fetal heart has begun to pump blood, and the
entire mechanism for nutrition of the embryo and, subsequently,
fetus and excretion of its waste products is in operation. A layer
of epithelial cells in the villi and of endothelial cells in the fetal
capillaries separate the maternal and fetal blood. Waste products
move from blood in the fetal capillaries across these layers into
the maternal blood, and nutrients, hormones, and growth fac-
tors move in the opposite direction. Some substances, such as
oxygen and carbon dioxide, move by diffusion. Others, such as
glucose, use transport proteins in the plasma membranes of the
epithelial cells. Still other substances (e.g., several amino acids
and hormones) are produced by the trophoblast layers of the pla-
centa itself and added to the fetal and maternal blood. Note that
there is an exchange of materials between the two bloodstreams
but no actual mixing of the fetal and maternal blood. Umbilical
veins carry oxygen and nutrient-rich blood from the placenta
to the fetus, whereas umbilical arteries carry blood with waste
products and a low oxygen content to the placenta.
Meanwhile, a space called the
between the inner cell mass and the chorion (
Inner cell mass
Lumen of the uterus
Eleven-day human embryo completely embedded in the uterine lining.
From A. T. Hertig and J. Rock,
Carnegie Contrib. Embryol.