The growing embryo requires nutrition
and oxygen, and a disposal system for the waste
products of its own metabolism. All of this is
accomplished by the placenta, which allows the
growing embryo to eat and breathe while in the
mother's uterus.
Following implantation of the fertilized egg
into the uterine lining, the outer layer of the
embryo develops spaces called lacunae. The lacunae
filled up with blood from the mother's
uterine lining. Small projections from the embryo's
chorionic layer reached out into the uterine
lining. The chorionic layer is one of the membranes
that surround the embryo and help it implant.
Blood vessels begin to form beneath this chorionic
layer.
Around day 21, the embryo's bloodstream
and the mother's bloodstream are in such
close contact that nutrients and oxygen can cross
from mother to embryo. The two bloodstreams are
separated by a thin collection of tissues in
the placenta called the blood barrier. This barrier
permits small particles like nutrients and oxygen
to pass from the mother to the embryo and allows
waste products to pass from the embryo back to
the mother.
The blood barrier also prevents many large or
potentially harmful particles from entering the
embryo's bloodstream. The red blood cells
do not cross from the mother's bloodstream
to the embryo's bloodstream.
It's important to keep the two bloodstreams
separate since the blood type of the mother and
embryo could be different. If the mother's
blood type is positive, and her embryo's
blood type is negative, then the mother's
blood cells would treat the embryo as an invading
foreign organism, and try to destroy it.
The placenta and its blood barrier are important
for supplying the growing embryo with nutrition
and oxygen, removing its waste products, and
preventing harmful substances from getting into
the embryo's bloodstream.
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