Dr. Andy

Reflections on medicine and biology among other things

Wednesday, March 02, 2005

Stem Cell Primer II: What is all the fuss about?

What are "embryonic" stem cells? To get around the problems with using "adult" stem cells (those derived from adult humans) some scientists have been working with cell lines derived from early human embryos. Since these cells would, under "normal" circumstances, eventually create a whole human, they are likely to be better at producing large numbers of differentiated cells. These cells are generally easier to obtain and work with than adult stem cells. As I'll discuss later, there are of course some moral and ethical issues with using human embryos, which many would consider human life, as a source of cells.

One potential issue with stem cells derived from human embryos is that they are genetically distinct from the patient they will be used to treat, so when the differentiated stem cells are used, they may generate a strong immune response that could potentially kill of the introduced stem cells

What is the fuss about "cloning? First, let's define cloning. Cloning is basically creating genetically identical offspring, be they cells or a whole animal.

To get around the problem of genetic differences between embryonic stem cells (derived from human embryos) and potential patients, a technique called somatic cell nuclear transfer (SCNT)was developed. Before talking about what that is, let's remember how a "normal" embryo develops. An egg cell from the mother, containing 1/2 the normal number of human chromosomes (chromosomes are pieces of DNA) fuses with a sperm cell from the father, also containing 1/2 the normal number of human chromosomes. This yields a single cell embryo with the normal number of chromosomes. Note that the sperm contributes effectively only its DNA, the egg provides all the other apparatus of the cell necessary for development

Somatic cell nuclear transfer (SCNT) utilizes this asymmetry in the sperm and eggs contribution. Basically, the chromosomes only are removed from an egg cell and an entire adult nucleus (the part of the cell containing the chromosomes) is inserted. This yields somethink close to a single cell embyro, an egg cell with a full number of chromosomes. This cloned embryo can then develop more or less like a normal embryo. The cells derived from this embryo can be cultured to provide a variety of types of different cell types, in large numbers, that can then be used to treat disease, just like regular old embryonic stem cells. The advantage of SCNT is the cells produced are genetically identical to the patient, greatly reducing the risk of an immune reaction when the cells are introduced.

But you said this was like a normal embryo. Couldn't this technology be used to make genetic copies of humans?

Yes, this is what is called "reproductive cloning." "Therapeutic cloning" is creating a cloned embryo for the purpose of treatment, for example by growing pancreatic beta-cells to treat diabetes.

But this cloned embryo isn't quite normal. The nucleus (from the patient) has undergone certain changes that make it different from the one found in a normally fertilized egg. Dolly, the cloned sheep was created by SCNT, but this was an unusual event. Reportedly, Dolly was created on the 277th attempt, meaning actually cloning a human would be difficult. The technology apparently works better for mice. In addition, the cloned animal would probably not be normal. Dolly suffered from a variety of pathologies and died young

So for now, cloning to generate a whole new organism (reproductive cloning) is not thought to be feasable. Of course, technology and science march inexorably forward, so some of these difficulties MIGHT be overcome making reproductive cloning possible. Whether this would be desirable is another question.


Post a Comment

<< Home