What did Michael W. Young’s transplantation experiment prove about the master clock in mammals?

The master clock resided within individual cells.

Michael W. Young’s research at Rockefeller University provided critical evidence regarding the localization of the circadian pacemaker. By conducting cell transplantation experiments using mutant mice, his team demonstrated a definitive cellular basis for the rhythm. Specifically, they transplanted cells taken from a mouse exhibiting a short-period rhythm into a normal mouse. If the altered rhythm was successfully transferred to the recipient mouse’s overall pattern, it proved that the fundamental timing mechanism was intrinsic to the cells themselves rather than dependent on a central signaling hub requiring constant, external input across the whole organism. This supported the realization that nearly every cell in the body possesses its own autonomous timekeeper.