Skeletal fragility fractures kill as many women each year as breast cancer and arise from defects in both bone material properties and bone micro/macro architecture. Bone architecture and material properties are ultimately the product of bone-forming cells; however, the mechanism by which disease-induced changes in bone cellular composition ultimately result in skeletal fragility is unclear.  Insight into this issue comes from our recent discovery that bone contains multiple distinct pools of skeletal stem cells, each with different functions and mechanisms of bone formation.  We here hypothesize that each skeletal stem cell population will form bone with characteristic microarchitectural or material properties, providing the first cellular basis for these key determinants of bone resistance to fracture. To address this hypothesis, we have formed a cross-campus team of 3 Cornell University investigators with complementary primary expertise in bone stem cells, bone architecture and bone material properties. We will form in vivo bone organoids of defined cellular composition and will then determine how the material properties of bone matrix in these organoids differ (Aim 1). Next, we will examine whether different subsets of skeletal stem cells play distinct roles in the bone anabolic response to mechanical loading by using genetic methods to target and selectively delete these subsets of stem cells in mice (Aim 2). Lastly, we will utilize a novel transplantation-based strategy to determine the contribution of specific stem cell populations to local anatomic differences in bone architecture through physical relocation of those stem cells in vivo (Aim 3).