The prevailing model of the role of DKK1 and the mesenchymal cell lineage in bone development and bone mass homeostasis. Wnt 3a, 5a, and 7b and Ihh cooperatively promote osteoblast differentiation, whereas DKK1 inhibits osteoblastogenesis and shifts the developmental program toward adipogenesis. β-catenin/TCF1 promotes early osteoblast lineage commitment through induction of the master bone development protein Runx2.⁹⁷  Runx2 then promotes transcriptional activation of a second master bone development transcription factor Osterix (Osx), which uses DKK1 to repress Runx2 expression to further promote osteoblast progenitor maturation.⁹⁸  Finally, phenotypic lineage-specific markers for osteoblast maturation are illustrated.^99,100