Drexel University College of Medicine

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Placenta formation

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 negative, and her embryo’s blood type is positive, 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.

Physician Reference

International Classification of Diseases, 9^th Revision (ICD9)

Current Procedural Terminology (CPT)

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