Friday, March 02, 2007

Stem Cells Part IX: Adult stem cells.

Up to this point, we've talked a lot about embryonic stem cells derived from murdered embryos. We've seen that, not only are they hopelessly mixed up with evil, but the miraculous cures we hear about on a daily basis which they are supposed to give rise to are, in fact, so much science fiction. We've also seen that attempts to obtain material for hES cell cultures, either by "embryo biopsy" or from "biological artifacts" aren't ethical either, even though that is the stated motive for pursuing those investigational lines. This brings us to adult stem cells which, at least on the face of things, appear moral.

The best known and most extensively studied adult stem cells are those found in the bone marrow. Recall that all of our red blood cells, platelets (which help form blood clots), and many of our infection fighting white blood cells (specifically granulocytes, eosinophils, basophils, and the monocyte/macrophage system) are formed in the bone marrow. These cells are known collectively as hematopoietic cells. Hematopoietic cells turn over rapidly, such that each one of us requires a hundred billion (100,000,000,000) new ones each day. These new hematopoietic cells come from adult stem cells found in the bone marrow known as hematopoietic stem cells (HSCs). The bone marrow is a complex structure, because it forms so many different kinds of cells. It is also fragile, and many things can harm or kill it, including radiation, drugs, and, specifically, chemotherapy. Fifty years ago it was shown that if bone marrow removed from healthy donors was injected into patients whose marrow had been destroyed, the donor HSCs (mixed in the donor marrow injections) would engraft and form new marrow in the patient. That was the birth of stem cell therapy, and research.[i] In addition to bone marrow, HSCs are found in peripheral blood and umbilical cord blood; this latter source underlies the recent interest in "banking" cord blood. All three of these sources - bone marrow, peripheral blood, and cord blood - are used more or less routinely in clinical practice, mostly for cancer patients (leukemia and lymphoma patients, usually) whose own marrow has been destroyed by high dose chemotherapy. Pure HSCs are not transplanted, because they are difficult to isolate in sufficient quantities. What is usually transplanted is mixtures of HSCs with other marrow or blood elements.

In some cases, patients are transplanted with their own marrow (autologous donation). In other cases, the marrow is from someone else (allogeneic donation). In the case of allogeneic donation, the marrow HSCs encounter the same problem as any other allogeneic donation: immunologic rejection.[ii] However, the type of immune rejection one encounters with bone marrow transplants is different. In regular donor tissue rejection, the recipient's immune system attacks the donor tissue or organ. In bone marrow rejection, it is the immune system generated by the donor bone marrow (which is genetically the same as the person who donated the marrow) which attacks the patient/recipient. This is known as graft vs. host disease, and is an uncommon, but profoundly serious, complication of marrow transplant.

So, the adult stem cells known as hematopoietic stem cells have significant technical problems: inability to purify them in sufficient quantities (which leads to the need to transplant mixtures of cells), and the ubiquitous immune rejection problem. Even so, bone marrow transplantation, as well as the numerous modern variants including peripheral blood and cord blood - derived transplants - are highly successful overall, and form a significant component of the hematologist-oncologist's clinical armamentarium. Further, HSCs represent the only stem cell type which currently has clinical utility.

A short while back, cells were recovered from amniotic fluid which, when cultured in vitro, showed many of the characteristics of human embryonic stem cells.[iii] This resulted in a brief blizzard of internet articles as various Catholic and Protestant figures waxed enthusiastically about this "new, ethical" source of stem cells,[iv] [v] although there had been earlier, similar reports.[vi] It is true that these cells are promising, and, in and of themselves, pose no ethical problems because no embryos are murdered to obtain them. But they would have the immune rejection problem just like all the other stem cell sources, adult and embryonic and, also like all the other stem cell sources (except HSCs), research on these is in its infancy.

Besides HSCs and cells from amniotic fluid, other sources of adult stem cells may include brain, muscle, skin, digestive tract, cornea, retina, liver and pancreas.[vii] These cells all share the same problems as HSCs, notably that they are rare in the body, and difficult to isolate in pure form. This is part of the reason there's such a push for hES cells: it's much easier to produce a bunch of embryos and murder them for their inner cell masses than it is to obtain adult stem cells. In addition, it's by no means clear that adult stem cells are as plastic as embryonic stem cells, which limits their theoretical usefulness, though it's worth keeping in mind that the only clinically useful stem cell out there currently is the relatively inflexible HSC. There are numerous other technical problems common to both adult stem cells and embryonic stem cells, and there are ethical problems common to both as well. It is the ethical problems that adult and embryonic stem cells share which, in my view, will limit research on both types from a moral point of view. However, we will take this issue up in the next, and final, installment of this stem cell series. We will also examine briefly the media myth that hES cell research is slowed due to President Bush's ban on Federal funding, and instead look at the real reason: intellectual property rights.

Endnotes.
[i] "Regenerative Medicine 2006", National Institutes of Health, Department of Health and Human Services August, 2006. See Chapter 2. Entire report downloadable in pdf format at http://stemcells.nih.gov/info/scireport/2006report.
[ii] We discussed this in Stem cells Part VII: Why is cloning necessary?
[iii] De Coppi P, et al. Isolation pf amniotic stem cell lines with potential for therapy. Nature Biotechnology 2007 Jan 7
[iv] "Christians hail ethical source for stem cells." WorldNetDaily 10 Jan 2007, at http://www.worldnetdaily.com/news/article.asp?ARTICLE_ID=53705
[v] "Amniotic Stem Cell Discovery Backed up by New Study." LifeSiteNews.com 8 Jan 2007 at http://www.lifesitenet/ldn/2007/jan/07010805.html
[vi] "Discarded placentas deliver researchers promising cells they report are much like embryonic stem cells." HypeandHope.com 5 August 2005 at http://www.hypeandhope.com/wt/page/index/it_1123267982
[vii] "Stem Cells and the Future of regenerative Medicine" National Research Council/Institute of Medicine, National Academy Press, Washington, D.C. 2002. Pg.23.

1 comment:

I. Segarra said...

Very usefull to have these series of articles. A compilation of news and articles in available at www.bioethicsforum.info