Histology, Osteoblasts

Excerpt

Osteoblasts are colloquially referred to as cells that "build" bone. These cells are directly responsible for osteogenesis (or ossification). Osteoblasts synthesize and deposit organic bone matrix (osteoid) proteins that will mineralize in both developing skeletons and during the process of bone remodeling that occurs continuously throughout an individual's life.

Bone is approximately 10% water, 30% organic, and 60% inorganic. The organic component is approximately 85 to 90% collagen (primarily type 1 -resisting tensile forces), proteoglycans (resisting compressive forces), non-collagenous proteins (osteocalcin and osteonectin) and glycoproteins (osteopontin). The inorganic component, or mineralized matrix, is composed of hydroxyapatite crystals [Ca10(PO4)6(OH)2] that provides protection and support while serving as the body's repository for calcium and phosphate. Osteoblasts also indirectly regulate osteoclast formation and bone remodeling by cell-cell contact, paracrine signaling, and cell-bone matrix interaction.

Osteoblasts derive from two embryonic populations of mesenchymal stromal cells (or mesenchymal stem cells, MSCs). MSCs originating from the neural ectoderm can directly differentiate into osteoprogenitor cells that will become osteoblasts and form bone through intramembranous ossification (i.e., squamous bones of the calvaria and clavicle). MSCs originating from the paraxial mesoderm differentiate into axial skeleton osteoblasts, while the MSCs of the lateral plate mesoderm form osteoblasts of the appendicular skeleton. The axial and appendicular skeleton develop by endochondral ossification, with these osteoblasts deriving from intermediate perichondral cells or hypertrophic chondrocytes. Both the indirect and direct processes converge at osteoprogenitor cells (or preosteoblasts). Osteoblasts are induced from the osteoprogenitor cells by numerous signals. Understanding of the osteogenic lineage remains incomplete.