Internal organs heal injuries with new connective tissue. However the cellular and molecular events, and the sources of this tissue, remain obscure. Here we tagged extracellular matrix around the mesothelium lining various mouse tissues: peritoneum, liver, and cecum, and applied various injury models. We discovered that preexisting matrix is transferred across organs into wounds. Using proteomics, genetic lineage-tracing and by selectively injuring juxtaposed organs, we demonstrate that the matrix tissue of origin likely dictates the final healing outcome: whether scarring or regeneration.