PAGE 2 OF 2 Adult stem cells are self-renewing and can in
principle be redirected to become multipotent—able to
differentiate into, among other things, organ-specific tissues,
such as cardiac cells and blood vessels, that are essential for
organ regeneration. Researchers typically turn to bone marrow
and sometimes to the heart or brain for adult stem cells. But
these cells are rarer than diamonds. “I would probably have to
remove the majority of a patient's bone marrow to get enough
stem cells to eventually regenerate just a small piece of the
heart,” says Rafii. In addition, the technology to grow enough
adult stem cells to use in regenerating human organs simply
doesn't exist. Instead, Rafii and his team turned to
spermatogonial progenitor stem cells in the testes. Usually,
these cells make only sperm. But Rafii's experience with
teratomas suggested to him that maybe the cells could be
directed toward tissue repair or regeneration. The researchers
began with mice. “The challenge was to isolate the spermatog