June 23, 2008
Cloning and stem cell research
(This series of posts reviews in detail Francis Collins's book The Language of God: A Scientist Presents Evidence for Belief, originally published in 2006. The page numbers cited are from the large print edition published in 2007. The complete set of these posts will be archived here.)
In the Appendix of his book The Language of God: A Scientist Presents Evidence for Belief (2006), Francis Collins gives a very clear and brief exposition of the issues involved in stem cell research and cloning, which are not the same thing despite popular impressions.
A human being starts out as a single cell formed by the union of an egg and a sperm. The nucleus of this cell contains the contributions of DNA from each of the two parents and thus all the genetic instructions, while the region outside the nucleus, called the cytoplasm, contains the nutrients and signaling mechanisms that enable the cell to do whatever it is meant to do.
The single cell starts multiplying by copying itself, a process known as mitosis. In the very early stages, all the cells are identical and capable of eventually becoming any specialized cell like a liver cell, blood cell, etc. Such cells are called 'pluripotent' because of their ability to become any of the tissues that make up the body and it is these cells that are called embryonic stem cells and the center of the ethical debate.
Soon these embryonic cells begin to specialize and differentiate into cells that begin to form different organ tissues. They do this by having the DNA start turning switches on and off in its genes. Some of these specialized cells, such as those found in limited amounts in bone marrow, become what are known as adult stem cells in that while they still have the ability to differentiate further, they can do so only into a much more limited variety of adult tissues. Such stem cells are called 'multipotent'.
The promise of stem cell research is that one can use a person's own stem cells to regenerate tissues lost or damaged by all kinds of diseases. Since these cells are not perceived as foreign matter, this would not trigger the body's immune mechanism that rejects foreign tissues, as occurs currently with transplants. At present, this immune response has to be suppressed with powerful drugs, leaving the patient vulnerable to other infections.
The ethical problem is that although adult stem cells can be obtained and used from an adult without harming that person, they have only a very limited flexibility. Pluripotent cells are preferred but at present using such cells results in the loss of the embryos from which they are taken, and this immediately raises the ethical issue of whether by destroying an embryo, we are destroying life.
Currently pluripotent stem cell lines are created during the process of in-vitro fertilization, by taking an egg from a woman, fertilizing it in a petri dish with sperm from a man, and growing the resulting cell in solution containing the necessary nutrients for its growth. After about five days, what is called a 'blastocyst' is formed which consists of about 70-100 cells. This consists of an outer wall of cells encompassing a hollow cavity, and an inner clump of about 30 cells (called the inner cell mass) at one end of the cavity. It is the inner cell mass that eventually turns into the tissues that make up the growing fetus, while the outer wall becomes the placenta.
In-vitro fertilization is done to assist childless couples. The selected blastocyst is implanted in the uterus of either the person who donated the egg (the biological mother) or a surrogate, and once it adheres to the wall of the uterus, it receives oxygen and other nutrients from the mother and develops as any other fetus.
The ethical dilemma arises because the process is not 100% certain, and thus many more fertilized eggs and blastocysts are created this way than are currently used to generate actual pregnancies, and this has resulted in hundreds of thousands of unused fertilized eggs. They are currently kept frozen.
Researchers suggest that these fertilized eggs be used (with the donors' permission) to generate embryonic stem cell lines that can be used for research purposes. To do this, the inner cell mass is extracted from the blastocyst and transferred into a dish containing a culture that enables it to grow. When this is done, the blastocyst is effectively destroyed and cannot be used to create a human.
Opponents of embryonic stem cell research say that even a single fertilized egg cell is a human life and thus the blastocyst created this way should never be destroyed. Others argue that a blastocyst has none of the qualities that we associate with being human and thus destroying it not taking a life.
This dilemma created by scientific advances may be resolved by further scientific advances.
One possible compromise arises from the discovery of the process by which animals have been cloned, starting with the famous cloned sheep Dolly. This process is known as somatic cell nuclear transfer (SCNT). What happened with Dolly is that a single cell was taken from the udder of an adult sheep and its nucleus (containing all the genetic information) was extracted. Then an egg cell was taken and its nucleus removed and replaced with the nucleus that had been extracted from the udder cell.
What one might have expected to have created was a cell that was specialized for udders since one had taken a cell from the udder of an adult and by that time the cell should have become specialized for just that purpose. It was once thought that this process of specilization was irreversible. i.e., once a pluripotent embryonic stem cell becomes an adult stem cell or an adult specialized cell, there was no going back to its unspecialized state.
What researchers found to their amazement was that when the udder cell nucleus was inserted into the egg cell that had had its nucleus removed, the nucleus seemed to effectively go back in time and become like the original embryonic cell that had eventually resulted in the sheep from which the udder cell was obtained. When this was then implanted in a sheep, it grew as if from a single fertilized egg and gave rise to a new sheep (Dolly) that had genes identical to those of the sheep from which the original udder cell was taken.
This process has now been repeated with other mammals like horses, cows, dogs, and cats. Although the Raelians made the spectacular claim that they had used this technique to clone a human being, that seems like a hoax.
As a result of this research, it looks like it should be possible to take a nucleus from (say) the skin cell of an adult human and insert it into an egg cell that has had its nucleus removed and thus create cells that have all the properties of embryonic stem cells. Thus it should be possible to create blastocysts in the laboratory without having them originate in the fusion of sperm and egg, the traditional way in which children are conceived. These stem cells would have DNA identical to those of the adult whose skin cell the nucleus was taken from, and not a fusion of mother and father DNA information, the way an embryo is normally formed.
Of course, if this cell is implanted in a uterus, one could potentially create a cloned human being but no one is suggesting that that be done. In fact, there is strong worldwide opposition to such an act. But if the cell is grown in a petri dish, then it could generate the equivalent of embryonic stem cells for both research and therapeutic purposes.
Would the process of SCNT be considered sufficiently different from the usual process of creating a fertilized egg to be considered not a potential human and thus overcome the ethical problems of stem cell research? That remains to be seen.
POST SCRIPT: Tough times
We know that the troubled economy is hurting many people. The Daily Show looks at how it is affecting the people of Beverly Hills.