The topic of stem cells is, at its core, a topic about regeneration.
As described in the section on "The 'Bank Account' Analogy", salamanders are the ultimate model of cellular regeneration. In "bank account" terminology, salamanders are stem cell billionaires - while most members of the human species, by contrast, are stem cell paupers. As previously described, all of the red blood cells (RBCs) in a salamander are nucleated, thereby acting as functional stem cells throughout the entire body. This is vastly different from human RBCs, which are no longer nucleated or capable of replicating once they migrate outside of the bone marrow. Because of their nucleated RBCs, salamanders can regenerate entire limbs. Humans, however, cannot. At least, not yet.
Theoretically, there are no limits to the possibilities for stem cell regeneration. As the NIH has stated, "This research has the potential to revolutionize the practice of medicine." (From http://stemcells.nih.gov). While humans do not innately possess the same natural regenerative ability of salamanders, we may nevertheless be able to harness the biological mechanisms that regulate these processes of regeneration. In fact, we are already well on the way to doing so.
According to James Andrew Lee, M.D., chief resident in general surgery at the Columbia Campus of New York Presbyterian Hospital, "The 20 year goal would be to create things like artificial limbs. You could potentially use this [stem cell] technology to create an arm, leg or finger, for example."
Dr. Lee has made numerous breakthroughs with adult stem cells derived from fat. He was the first researcher to successfully demonstrate in vivo that exposing adult stem cells derived from fat to various growth factors can generate bone, cartilage and other types of cells. "People have been showing that you can do this in vitro for a few years now, but no group had yet shown that you could do it in vivo, i.e., grow the cells in tissue culture and then transplant them into a living organism," said Lee.
He continues, "For the people who say it's too farfetched, adult stem cell research is one of the most viable avenues of stem cell research. The ethical problems with embryonic stem cell research are essentially obviated by using adult stem cells. And everyone's got plenty of fat just waiting to be used. So this has almost unlimited potential."
Freezing fat has also been shown to preserve stem cells. Researchers at the University of Kentucky have demonstrated that "cryopreserved adipose aspirates can serve as a viable source" of stem cells, according to Lee L.Q. Pu, M.D., Ph.D.. Companies such as StemSource (now part of MacroPore Biosurgery, of San Diego) already offer patients the option of storing their own stem cells for future use. The adipose aspirates are collected via liposuction, and selectively processed for the harvesting of useful stem cells.
Fat derived stem cells have also been used to generate bone grafts. It is believed that osteogenically differentiated fat derived stem cells (FDSCs) may one day allow physicians to repair bony defects without the limitations of autologous bone grafts.
The regeneration of specific tissue and bone is merely one piece of the regeneration puzzle. While the precise chemical signals by which the transdifferentiation of stem cells is directed remain unknown, one of the main goals of research today is to identify and describe these mechanisms. If the specific processes of transdifferentiation may be understood and controlled, it may ultimately be entirely realistic to grow replacement organs and limbs from adult stem cells.
Although such a goal may still be 20 years away, many milestones have already been reached. One such example is with cardiac tissue regeneration. While stem cells have not yet been found in the heart itself, stem cells from bone marrow have been shown to transdifferentiate into cardiac cells.
When hematopoietic stem cells (from bone marrow) were injected into the damaged walls of heart muscle in mice, the cells formed new cardiomyocytes, as well as vascular endothelium (which form the inner lining of new blood vessels), in addition to smooth muscle cells (which form the walls of blood vessels), thus generating "de novo" myocardium, including coronary arteries, arterioles, and capillaries. (Orlic et al.) As a source of replacement tissue for damaged hearts, this approach has immense advantages over heart transplant, especially since those in need outnumber donors.
Although it has been well established that human embryonic stem cells may become any of the more than 200 types of cells in the body, the most promising discoveries have focused on the ability of adult stem cells to exhibit such diverse transdifferentiation.
In January of 2002, a group from the University of Minnesota announced "the ultimate adult stem cell", after they created an immortal cell line from bone marrow stem cells. Early tests showed that these stem cells could become either of the 3 early tissues in an embryo that eventually lead to all the cell types of the body. This demonstrates that adult stem cells are far more versatile than previously believed, exhibiting not merely pluripotency, but potential totipotency as well. (Bohlin RG, "The Continuing Controversy Over Stem Cells," 2005).
The ability of embryonic stem cells to differentiate into cardiac tissue is well established. Dr. Itzhak Kehat and Dr. Loir Gepstein were the first researchers to grow human heart cells in the laboratory from embryonic stem cells. They reported that "40,000 human heart cells were produced, to form tissue about one square millimeter in size," and that these cells "contract like a pulsing heart." (J. of Clinical Investigation, 2004).
Nevertheless, many scientists agree that media coverage has been biased toward embryonic stem cells and is factually incomplete as well as, in many cases, inaccurate. Indeed, both the numerous disadvantages of embryonic stem cells, as well as the numerous advantages of adult stem cells, are commonly overlooked in media reports. The mere fact that there is increasing evidence for the pluripotency of adult stem cells is seldom if ever reported in layman's terms. Despite the emphasis in media coverage on possible embryonic stem cell therapy, the greatest realistic clinical hope comes from adult stem cell therapy.
One particularly promising avenue of adult stem cell research involves bone marrow stem cells. Bone marrow stem cells have already shown great versatility in treating a wide range of diseases, from damaged cardiac tissue, to damaged neurological tissue, to diseased lung tissue in cystic fibrosis patients, to a wide variety of other diseases.
Dr. Shim of the National Heart Centre in Singapore has demonstrated "Ex vivo differentiation of human adult bone marrow stem cells into cardiomyocyte like cells." (Biochem. Biophys. Res. Commun., 2004). Similarly, Dr. Stamm has also described the formation of new cardiac tissue from adult bone marrow stem cells. ("Autologous bone marrow stem cell transplantation for myocardial regeneration," Lancet, 2003).
Dr. Mezey, among others, has described the formation of new neurons in the brain from adult bone marrow stem cells. ("Transplanted bone marrow generates new neurons in human brains," Proceedings of the National Academy of Sciences, 2003).
Additionally, Dr. Korbling has described the formation of liver, skin and digestive tract tissue from adult bone marrow stem cells. ("Hepatocytes and epithelial cells of donor origin in recipients of peripheral blood stem cells", New England J. of Medicine, 2002). Dr. Kraus has described the formation of functional marrow, blood, liver, lung, gastrointestinal tract, skin, heart and skeletal muscle cells, all from a single adult mouse bone marrow stem cell line. ("Multi-organ, multi-lineage engraftment by a single bone marrow derived stem cell," Cell, 2001). Dr. Jian has taken this possibility several steps farther, by describing how adult stem cells taken from bone marrow are capable of forming "all types" of body tissue and are, therefore, pluripotent rather than simply multipotent or monopotent, as was previously thought. ("Pluripotency of mesenchymal stem cells derived from adult marrow," Nature, 2002).
Stem cells derived from bone marrow are not the only cells to exhibit pluripotency. Dr. Clark has reported that "adult stem cells from the brain can grow into a wide variety of organs, including heart, lung, intestine, kidney, liver, nervous system, muscle, and other tissues." ("Generalized potential of adult neural stem cells," Science, 2000).
Additionally, numerous skeletal muscular injuries have been quickly healed using a direct injection of adult stem cells into the injured site.
Progress has already been made in identifying certain agents responsible for the chemical modification of stem cell differentiation. For example, it has been found that "ascorbic acid enhances differentiation of embryonic stem cells into cardiac myocytes", as reported by Takahashi, et al., of Harvard Medical School. "The study demonstrates the potential for chemically modifying the cardiac differentiation program," and is applicable to adult stem cells as well. Similarly, ascorbic acid has been found to enhance the differentiation of stem cells into neuronal cells, according to Shin et al., of the Seoul National University. Ascorbic acid has also been found to enhance the dopaminergic differentiation in proliferating midbrain neuroblasts, according to Volpicelli, et al., at the Institute of Genetics and Biophysics in Naples, Italy. "Adding ascorbic acid to the cultures resulted in a five to seven fold enhancement of viable DA neurons," Volpicelli reported.
In growing specialized tissue from autologous stem cells, one of the main obstacles that has been encountered is "an inability to provide transplantable vasculature to stem cells growing in tissue culture," according to Dr. Michael W. Findlay, microsurgery fellow at The Bernard O'Brien Institute of Microsurgery at St. Vincent's Hospital in Melbourne, Australia. On the opposite side of the world, at NYU Medical Center, "The project for the transplantable vascularization of stem cells" was thus begun, utilizing a bioreactor to solve this problem. With multipotent stem cells growing in vitro, "their growth stops once they get to be more than about 7 cell layers thick," according to Drs. Judah Folkman (the discoverer of angiogenesis) and M. Hochberg. "At this thickness, the diffusion of nutrients is insufficient to support further growth." The use of a bioreactor was thus developed in order to facilitate the in vitro vascularization of stem cells.
Although much today is still unknown, and many aspects of stem cells still await discovery, great progress has already been made. Whether it is more or less than 20 years away, the day will eventually come when human beings will be able to rival even the mysterious salamander in our regeneration capabilities.