Abstract
PURPOSE OF REVIEW: Periosteal apposition and endosteal remodeling regulate cortical bone expansion and thickness, both critical determinants of bone strength. Yet, the cellular characteristics and local or paracrine factors that regulate the periosteum and endosteum remain largely elusive. Here we discuss novel insights in cortical bone growth, expansion, and homeostasis, provided by the study of Secreted Frizzled Receptor Protein 4 (Sfrp4), a decoy receptor for Wnt ligands. RECENT FINDINGS: SFRP4 loss-of function mutations cause Pyle disease, a rare skeletal disorder characterized by cortical bone thinning and increased fragility fractures despite increased trabecular bone density. On the endosteal surface, Sfrp4-mediated repression of non-canonical Wnt signaling regulates endosteal resorption. On the periosteum, Sfrp4 identifies as a critical functional mediator of periosteal stem cell/progenitor expansion and differentiation. Analysis of signaling pathways regulating skeletal stem cells/progenitors provides an opportunity to advance our understanding of the mechanisms involved in cortical bone biology.