osteoblast

Osteoblasts are bone-forming cells of mesenchymal origin responsible for synthesis and mineralization of the bone matrix. They differentiate from osteoprogenitor cells derived from mesenchymal stem cells located in the periosteum and endosteum; differentiation is driven by signaling pathways including Wnt/β-catenin, bone morphogenetic proteins, parathyroid hormone, estrogen, and mechanical cues.

On bone surfaces, osteoblasts are typically cuboidal to columnar and form a single layer lining growing surfaces in cortical and trabecular bone. They synthesize osteoid, an organic matrix rich in type I collagen and non-collagenous proteins such as osteocalcin, osteopontin, and bone sialoprotein, and secrete alkaline phosphatase, which facilitates mineralization.

Osteoid is subsequently mineralized as hydroxyapatite crystals are deposited, a process supported by alkaline phosphatase and matrix vesicles. As mineralization proceeds, osteoblasts either become embedded in the matrix to become osteocytes, or remain on the surface as bone-lining cells when inactive.

In remodeling, osteoblasts regulate osteoclastogenesis through expression of RANKL and its decoy receptor OPG, coupling bone formation with resorption. They respond to endocrine signals such as parathyroid hormone, vitamin D, calcitonin, and sex hormones, and participate in fracture repair by producing new bone tissue.

Osteoblast dysfunction or imbalance with osteoclast activity contributes to metabolic bone diseases such as osteoporosis; they are also essential for bone growth and healing.