Osteoblasts

Osteoblasts are specialized bone-forming cells derived from mesenchymal stem cells located in the periosteum and endosteum. They arise from osteoprogenitor cells and differentiate under the control of transcription factors such as Runx2 and Osterix, with signaling inputs from bone morphogenetic proteins, Wnt signaling, and various hormones.

Their main function is to synthesize and secrete osteoid, an organic extracellular matrix rich in type I collagen and non-collagenous proteins including osteocalcin, osteopontin, and bone sialoprotein. They express alkaline phosphatase, an enzyme that facilitates mineralization by increasing local phosphate concentrations.

Subsequently, mineral crystals of hydroxyapatite are deposited in the osteoid, forming mineralized bone. Some osteoblasts become embedded in the matrix and differentiate into osteocytes, while others become bone-lining cells on the surface.

Osteoblasts also regulate bone remodeling by producing RANKL and osteoprotegerin (OPG), mediating the formation and activity of osteoclasts in a process that couples bone formation to resorption. They typically form a single layer on bone surfaces and respond to systemic signals such as parathyroid hormone and vitamin D; intermittent PTH can stimulate an anabolic response in bone.

Overall, osteoblasts play a central role in skeletal growth, maintenance, and repair, coordinating the production of new bone with mineralization and remodeling.