Hayashi inserted the PGCs into the
testes of mice that were incapable of producing
their own sperm, and waited to see whether
the cells would develop.Saitou thought that it
would work, but fretted. “It seemed like a 50/50
They repeated the experiment with induced
pluripotent stem (iPS) cells — mature cells that
have been reprogrammed to an embryo‑like
state. Again, the sperm were used to produce
pups, proving that they were functional — a
rare accomplishment in the field of stem‑cell
differentiation, where scientists often argue over
whether the cells that they create are truly what
they seem to be. “This is one of the few exam‑
ples in the entire field of pluripotent‑stem‑cell
research where a fully functional cell type has
been unequivocally generated starting from a
pluripotent stem cell in a dish,” says Clark.