Adult human stem cells may offer the opportunity to use one of biomedical science's most promising technologies without the ethical dilemmas of embryonic cells. But whether the cells' plasticity-or ability to ignore germ-line heritage and differentiate into therapeutically useful tissues-warrants clinical application at this stage remains controversial.
"We're still debating it," says Amy Wagers, Harvard Medical School investigator and plasticity critic. "It's too early to tell which way things will fall." Biologists generally agree that even the most potent adult stem cells can't approach the therapeutic power of embryonic stem cells. Nevertheless, at least a dozen clinical trials based on adult-cell plasticity have commenced in patients with serious heart disease – prematurely, some contend.
In embryonic development, cells form three germ layers: ectoderm, mesoderm, or endoderm. Generally, biologists considered cell differentiation overwhelmingly unidirectional and progressively restrictive. A cell fated to make neurons could not make blood cells; a stem cell ...
