Body Parts: Connecting with the peculiar placenta

By: BRADLEY J. FIKES - Staff Writer | Saturday, April 21, 2007 9:12 PM PDT ∞

My choice for the strangest body part is the placenta. It's a vital organ to all, but present for less than a year. And it's the only body part shared by two people: a woman and her unborn child.

Joining two individuals into one is one of nature's most delicate tasks. Errors can occur at any point along the way. A successful pregnancy is a minor miracle.

Offspring get their genes from both mother and father. The father's genes are foreign to the mother. The body's immune system vigorously attacks anything foreign, and can't tell the difference between harmless paternal genes and dangerous invading microbes.

The placenta is a joint venture of fetus and mother, containing cells from both. Blood from both flows through the placenta, allowing nutrients to cross from mother to the fetal circulatory system, and waste products from fetus to the mother's circulatory system.

The placenta forms after the fertilized egg, or zygote, enters the uterus and implants itself into the uterine wall. Doctors say implantation marks the official beginning of pregnancy. Hormones cause blood vessels to proliferate in the uterine wall, colonized by the rapidly dividing embryonic cells. These join to form the placenta.

The placenta directs fetal blood into extremely tiny blood vessels. These are surrounded by many small pools of maternal blood, fed by the mother's blood vessels. The fetal blood vessels are leaky enough that some molecules such as sugars and small proteins can pass through from the mother's blood, and waste products can diffuse out. But nearly all blood cells are confined within the blood vessels. This semiporous system almost always keeps the mother's immune system from attacking the fetus.

A great exception is when the mother lacks the gene for a certain blood protein called the Rh factor, but the fetus does have the gene. This happens when the mother is Rh-negative and the father is Rh-positive. In that case, the fetus will have the gene. The few fetal blood cells that leak across the placental barrier are enough to provoke an immune response, causing Rh disease.

Antibodies from the mother pass through the placenta and attack the fetal blood cells, causing anemia and jaundice. Rh disease can cause brain damage or even death. This rarely happens in a first pregnancy with an Rh-positive child, but the odds greatly increase with each successive pregnancy.

Fortunately, Rh disease is treatable, especially if detected early. It's easy to test for Rh factor, so both mother and father should know their status. There's also a test for Rh antibodies that indicates if the mother's immune system has been activated.

Premature detachment, called placenta previa, is a danger to baby and mother alike. It's caused when the placenta forms over or near the cervix, the aperture between the vagina and the uterus. In this position, the placenta is not firmly attached to the uterine wall. Placenta previa often corrects itself earlier in the pregnancy; the placenta shifts upward in the uterus.

When it persists into the third trimester, placenta previa causes the placenta to partly detach from the uterus, producing a flow of bright red blood from the vagina. This demands a call to the doctor, and perhaps hospitalization. Mothers with a persistent placenta previa will require a Caesarean section. Vaginal delivery would cause too great a risk from bleeding, and the placenta is in the way of the baby getting out.

Genetic errors during conception can cause the growth of abnormal placental tissue instead of an embryo. One of the less severe kinds is called hydatidform mole. The worst is cancer of the placenta, or choriocarcinoma. It spreads fast, often metastasizing to the lungs.

Placental cancer mortality used to be nearly 100 percent before 1957. In that year, Dr. Roy Hertz and Dr. Min C. Li discovered that the chemotherapy drug methotrexate could completely cure the disease. That success was one of the factors leading President Nixon to declare a "war on cancer" in 1971.

Surprisingly, men can get cancer of the placenta. This is a rare form of adult testicular cancer in which the malignant cells start to malform into placental cells. It is highly metastatic and deadly, but also responds to chemotherapy.

The normal end to the placenta comes with a successful pregnancy. After the baby is born, the placenta is expelled, hence the common name "afterbirth." It usually weighs about a pound. If the placenta is not completely removed from the body, its remnants can cause complications such as infections or bleeding.

Contact staff writer Bradley J. Fikes at (760) 739-6641 or bfikes@nctimes.com.