Monday, March 16, 2009

Chimerical Complications

We talked about chimera earlier – individuals who are the result of the merger of two zygotes at a very early stage of development. Different parts of the body develop from the stem cells related to the two different zygotes so the individual ends up being their own sibling, with some organs and fluids from one zygote and others from the other.

It turns out that the chimera phenomenon is a little more complicated than this ‘simple’ picture. Let’s start out by talking about some good news for pregnant women – researchers have been able to verify that there is ‘leakage’ from their fetus into the mother’s blood stream. That is, cells from the fetus make their way across the placenta or through the wall of the uterus and are found in the mother’s blood. Why good news? This means that you can take a blood sample from the mother, identify cells from the fetus, culture them and find out things about the fetus – its gender, whether it carries specific genetic disorders that can be treated in utero and so on. This means that for many genetic tests the day will come when we no longer have to do invasive procedures such as amniocentesis or chorionic villi sampling.

Now let’s add a couple of other bits of information. First, in some women fetal cells are not eliminated from the mother’s bloodstream after birth but persist. Second, transfer of cells also occurs from mother to fetus. Suddenly we have the prospect of a lot of genetic material floating around that may or may not be incorporated into a developing fetus. Genetic material from prior pregnancies may persist and may become part of a new fetus, creating a chimera. Consider, for a moment, the implications for the children of surrogate mothers who have their own children or have carried surrogate pregnancies for multiple couples.

Before we get too disturbed by all this, let’s concede that standard chimera are relatively rare and the type of genetic transfer just discussed is probably rarer still. On the other hand, like babies with webbed feet and tails, it does occur and could result in apparent or actual NPEs.


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