Bone formation niche dysfunction in osteoporosis: insights from single-cell and spatial transcriptomic studies.

Bone formation is spatially organized, yet the molecular architecture of the bone formation niche has only recently become accessible. Osteoporosis is a major skeletal disorder characterized by bone loss, microarchitectural deterioration, and increased fracture risk. Although antiresorptive and anabolic therapies are effective, a key conceptual gap remains: the cellular components of bone formation are increasingly well defined, but how these cells are spatially arranged and coordinated in health and disease is less understood. Recent advances in single-cell RNA sequencing and spatial transcriptomics now allow transcriptome-wide mapping of bone-forming microenvironments in intact human and mouse tissues. This review synthesizes emerging findings into a spatial framework for understanding bone formation and osteoporosis. We trace the conceptual lineage from the bone-remodeling compartment canopy hypothesis to its molecular refinement through recent single-cell and spatial atlases. We integrate evidence for mesenchymal stromal cell heterogeneity and spatial niche organization to propose a three-zone model of the bone formation niche, comprising a canopy or marrow-side stromal zone, a reversal or transition zone, and a bone-surface formation zone. We further propose "spatial de-zonation" as a hypothesis-generating framework for osteoporosis, referring to the potential erosion of spatial gradients that normally separate osteogenic, stromal, vascular, and adipogenic domains. Importantly, the available human spatial atlases were generated predominantly from osteoarthritic femoral heads and therefore provide reference maps rather than direct evidence of osteoporosis-specific niche disorganization. Finally, we consider niche-restorative therapeutic strategies that may move beyond targeting individual cell types toward restoring the architectural integrity of the bone formation niche.