"In a study published in Nature Communications, the researchers reveal an elegant molecular mechanism that acts like a GPS coordinate system for regenerating cells… the puzzle was how the cells in the regenerating limb-stump controlled their levels so precisely to know exactly where they were on the axis from shoulder to hand.”
The Nature paper:
Now’s a good time to point out that the first organ transplant happened in the 1950s, and it hasn’t been even 80 years since, meaning there are not only people alive who remember a time when that didn’t exist, but also there are surgeons who may only be in their early 40s that became transplant surgeons while the first ever transplant surgeon was still alive, and may have even gotten tutelage from him.