According to classical embryology, cells in the early three-layered embryo receive irreversible fates: The outer ectoderm begets the skin and nervous system, the inner endoderm the digestive tract, and the sandwiched mesoderm forms nearly everything else. But in the late 1990s, experiments began showing that bone marrow can become liver, brain can become bone marrow, and other developmental detours once thought impossible do occur.1 Insights into the human condition have come from sex-mismatched transplants, in which tracking the telltale Y chromosome reveals stem cells in action, and sometimes crossing those hallowed barriers. For example, Martin Körbling, professor of blood and marrow transplantation at the M.D. Anderson Cancer Center, Houston, Texas, and coworkers examined cells from six women who had received peripheral-blood stem cells from their brothers. The women had Y-bearing cells in the skin and liver, indicating that mesoderm (hematopoietic stem cells) can become ectoderm (skin) and endoderm (liver).2 Transdifferentiation occurs within the embryonic layers too, such as muscle respecializing as blood, or skin as neuron.
From the Bone Marrow—or Not?
As the M.D. Anderson study suggests, stem cells from the bone marrow can travel, via the bloodstream, to various sites and then differentiate into nonblood cell types. "Alternatively, there may be pluripotent cells residing in a variety of tissues that can be traced to bone marrow at some point. They might be in various states of quiescence and can transdifferentiate, or dedifferentiate and redifferentiate, whatever you want to call it," says Dennis Steindler, professor of neuroscience and neurosurgery and researcher at the University of Florida College of Medicine in Gainesville.
Evidence that hematopoietic stem cells occupy such outposts comes from assistant professor of pediatrics Margaret Goodell's group at the Center for Cell and Gene Therapy at Baylor College of Medicine in Houston. They isolated mouse muscle cells that could reconstitute bone marrow, but not muscle, which other cells in the tissue might do.3 Yet Steindler doesn't think that homing or resident bone marrow emissaries account for the "brain marrow" that his group studies. "In neuropoiesis, we trace our cells to the marrow-like subependymal area in the brain."4
Cell fusion is yet another explanation for transdifferentiation. When Naohiro Terada, Edward Scott, and colleagues at the University of Florida combined bone marrow stem cells with embryonic stem cells (ESCs), and Qi-Long Ying, Austin Smith, and colleagues at the University of Edinburgh and the University of Oxford did the same with neural stem cells, both groups working with mice, they expected the ESCs to stimulate the specialized cells to dedifferentiate. The investigators saw this and more—four sets of chromosomes. The cells had fused, and stem cell biologists are still debating what it all means.5,6
"These results raise the issue of whether transdifferentiation or stem cell plasticity is the result of in vivo cell fusion," said Grompe. Irving Weissman, the Karel and Avice Beekhuis professor of cancer biology, cell, and developmental biology at Stanford University Medical College, suggested that cell fusion might explain how, in his experiments, a few labeled hematopoietic stem cells in mice whose bone marrow had been destroyed wound up in the gut and brain. "It happened in only one in 13 million cells, and it could be the result of cell fusion. But there is no evidence of plasticity in animals that have not undergone injury. Still, this work is a warning to everyone who believes that cells in bone marrow can become brain, muscle, heart, or fat. Can these changes instead occur by cell fusion?" he asked at the symposium. Researchers do not yet know whether cell fusion is an artifact of cell culture, or a fleeting and rare in vivo event that may be an intermediate in one cell type's reinvention as another.
Culturing cells may also permit mutations to occur that can change developmental fate, and may account for reports of transdifferentiation that cannot be replicated. For example, when professor of surgery Cindi Morshead and colleagues at the University of Toronto injected neural stem cells into adult mice with destroyed bone marrow, the injected cells, which had been cultured extensively first, did not reconstitute the bone marrow, as at least three previous studies had reported. The Toronto researchers examined more than 12 million neural stem cells and 31,000 bone marrow colonies and found no sign of transdifferentiation.7
Stem Cells in the Breast, Heart, and Brain
Cell lineages are less well understood in the breast. Ole William Petersen and colleagues at the Panum Institute in Copenhagen and the Lawrence Berkeley National Laboratory have identified a type of progenitor cell in milk ducts in women undergoing reduction mammoplasty, a standard source of healthy breast tissue for study.8 The newly discovered, rare cells are nestled within the cell layer that faces the lumen, which is ringed by myoepithelial cells. Petersen points out that there may be other types of progenitor cells in the breast. "If our understanding of the cellular phenotypes in the breast expands to encompass several progenitors and differentiated cells, this may lead to a better understanding of the quite diverse histopathological phenotypes and biological behaviors of different breast cancers," he adds.
The heart has recently revealed its stash of stem cells too. Piero Anversa, professor of medicine and director of the Cardiovascular Research Institute at New York Medical College in Valhalla, pinpointed stem cells in healthy cardiac tissue in eight men who had received hearts from women.9 The researchers checked for the Y chromosome and surface antigens for stem cells and connective tissue cells, cardiac myocytes, and epithelial cells that make up the heart. The men had died at varying times posttransplant; their hearts revealed that as time went on, the Y-marked cells in the new heart were differentiating. A man who died just four days after the transplant had mostly stem cells, whereas a man who lived for several months had Y-bearing differentiated cells.
The trauma of the transplant may have triggered the repair, Anversa suggests. "The small heart had to adapt to these conditions suddenly, under tremendous stress. The host contributed help to the new heart in adapting to the new hemodynamic condition." The experiments did not reveal whether the cells with stem cell markers were in the recipient hearts all along or if they had migrated from the bone marrow as needed. Anversa foresees clinical applications once they confirm that the cells are stem cells, track their origin, and identify their responses to various cytokines and growth factors. "In the future, we hope to mobilize the heart to regenerate and induce cardiac repair. The goal of our research is to see whether, and how, the heart can heal itself."
Unlike the renewable epithelium of skin, breast, and heart, neurons were long thought to be incapable of dividing, despite evidence of new neurons in rats learning tasks and birds mastering songs. But neural stem cells—more accurately termed committed progenitor cells because their fate is restricted—can give rise to neurons or two types of glia. In the late 1990s, Fred Gage, chair of the AACR symposium and professor of biology at the Salk Institute for Biological Studies in La Jolla, Calif., and his colleagues identified and isolated neural stem cells in brain tissue of deceased cancer patients who had been treated with bromode-oxyuridine (BrDU), which labels dividing cells. At the AACR meeting, Gage showed spectacular videos of neural stem cells flowering into full-fledged neurons. The cells acted the part too. "These cells have the electrophysiological properties of newly born neurons. They are functionally integrated into a circuit," said Gage. The researchers have also harvested neural stem cells from newly deceased individuals and then coaxed the cells to differentiate in vitro.10
Now the question is not whether the central nervous system (CNS) can regenerate, but under what conditions it does so. "There are dividing cells throughout the nervous system. In the spinal cord and outside the hippocampus, [these dividing cells] give rise to only glia, [but] when exposed to a permissive environment, [these cells] can become neurons. This suggests that there are reserves of stem cells throughout the CNS with equal potentials, but with differentiation depending on local environments. There must be something different about the regions where neurogenesis occurs," hypothesized Gage.
Although Gage agrees with Steindler that neural stem cells probably originate in the embryonic brain, the fact that BrDU-marked brain cells are much more likely to be located near a blood vessel than not suggests a bone marrow origin, too. And this uncertainty reflects the current state of stem cell biology in general—that is, the science is not nearly as far along as patients hope and politicians fear. Summed up Gage, "Both the public and scientific literature have been paying significant attention to the relevance of stem cell biology for health care. But we are finding surprises almost daily. We need a better understanding of the biology of these cells and their relationships to each other and organs."
1. R. Lewis, "A paradigm shift in stem cell research?" The Scientist, 14:1, March 6, 2000.
2. M. Korbling et al., "Hepatocytes and epithelial cells of donor origin in recipients of peripheral-blood stem cells," New England Journal of Medicine (NEJM), 346:738-46, March 7, 2002.
3. S.L. McKinney-Freeman et al., "Muscle-derived hematopoietic stem cells are hematopoietic in origin," Proceedings of the National Academy of Sciences, 99:1341-6, Feb. 5, 2002.
4. D.A. Steindler, D.W. Pincus, "Stem cells and neuropoiesis in the adult human brain," The Lancet, 359:1047-54, March 23, 2002.
5. N. Terada et al., "Bone marrow cells adopt the phenotype of other cells by spontaneous cell fusion," Nature, 416:542-5, April 4, 2002.
6. Q. Ying et al., "Changing potency by spontaneous fusion," Nature, 416:545-8, April 4, 2002.
7. C.M. Morshead et al., "Hematopoietic competence is a rare property of neural stem cells that may depend on genetic and epigenetic alterations," Nature Medicine, 8:268-3, March 2002.
8. T. Gudjonsson et al., "Isolation, immortalization, and characterization of a human breast epithelial cell line with stem cell properties," Genes & Development, 16:693-706, April 2002.
9. F. Quaini et al., "Chimerism of the transplanted heart," NEJM, 346:5-15, Jan. 3, 2002.
10. T.D. Palmer et al., "Progenitor cells from human brain after death," Nature, 411:42-3, 2001.