290 SIDMAN
Brookhaven showed that in the days after a single injection of this
agent into human subjects, radioactive white blood cells began to appear
in the circulating blood. Clearly the tritium had been incorporated into
dividing cells in the bone marrow, and those cells then matured and
entered the circulation. After passage of more days, the radioactivity per
blood cell decreased progressively because their precursors in the bone
marrow (stem cells, in today’s terminology) were diluting the radioactiv
ity that had been incorporated into DNA about 50 percent with each
new cell division that was taking place in the absence of further radio
active precursor.
Here, then, was a visualizable reagent that could target specifically on
dividing cells or be rapidly degraded and the tritium excreted as tritiated
water. It occurred to me that most cells in the developing nervous system,
unlike those in the bone marrow, ceased dividing early and permanently,
and therefore should not go on synthesizing new DNA. The radioactivity
would be expected to remain indefinitely in those brain cells undergoing
their final or penultimate round of cell division, and since tritium has a
12.5-year half-life, should serve to trace where and when cells are divid
ing in a fetal mammal’s brain, where they will reside in the adult brain,
and what those cells are destined to become.
Making arrangements at the science level, as opposed to the building-
renovation level, was marvelously efficient at the NIH. We found a newly
established commercial source of tritiated thymidine–the New England
Nuclear Company–and chose the mouse as the experimental animal, not
because of some clairvoyant recognition that the mouse would be the
animal of central importance in the medical research world of the future,
but simply because it was small and would need less of the expensive re
agent than a larger animal. We obtained permission to do our experiment
across the NIH campus in Building 14, a site that had been designated as
the only place on campus where radioactive compounds could be inject
ed into experimental animals. There were three of us working together
at the laboratory bench: Ned Feder, a medical school classmate, Irene
Miale, a postdoctoral fellow, and me. Miale’s mentor, Mac Edds, had
sent her from Brown University to his friend, Windle, because Miale’s
husband was just being assigned to duty in Washington, D.C., in the
U.S. Diplomatic Corps. Windle then assigned Irene to me.