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 ...
Debate Over Stem Cell Origins Continues
In science, things are not always as they seem. So it is for transdifferentiation, the apparent interconvertibility of certain specialized cell types and an underlying theme at a symposium on stem cell biology and applications at the recent annual meeting of the American Association for Cancer Research (AACR) in San Francisco. "For the past three years, people have been saying that hematopoietic [blood-forming] stem cells can become just about any tissue, challenging the paradigm that there are
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