This simulation explores advanced regenerative medicine at the level of an individual healing wound: mesenchymal stem cells seeded at the margin of a circular tissue defect proliferate along a growth-factor gradient that itself obeys a reaction-diffusion equation, then differentiate into mature tissue once their cumulative exposure to the signal crosses a threshold. Tune the growth-factor secretion rate and diffusion coefficient to shape the signalling field, or the proliferation rate and differentiation threshold to control how quickly stem cells divide versus commit to mature tissue, and watch live counts of proliferating stem cells, differentiated tissue and overall wound-closure percentage as the defect fills in — the same cell-therapy and growth-factor manipulation principles used in real tissue-engineering scaffolds.