While previous models of OI were based on systemic Col1 mutation and/or deletion, our studies identity the precise Fsp1 cell lineage that is responsible for Col1 production in maturing bones starting about three weeks of age, and absence of such synthesis by the Fsp1+ cells leads to abnormal bones, resulting in easy fractures with minimal trauma. This evidence concerns the gene S100A4 and osteogenesis imperfecta.