Bone cells

Osteoprogenitor/Osteogenic cells

• Are stem cells that can differentiate into osteoblasts, fibroblasts, adipocytes, chondrocytes, muscle cells
• Found on periosteal and endosteal surfaces of bone and inner surface of central/Haversian canals

Osteoblasts

• Deposit bone matrix (for composition, see here)
• Mature when they get trapped in their secreted osteoid
• Typically on surfaces of matrix bound by integrins, forming a single layer of cuboidal cells
• Come from osteoprogenitor/osteogenic cells (mesenchymal stem cell origin)

Osteoclasts

• Resorb/breaks down bone
• Giant multinucleate cells with high surface area ruffled surface for bone breakdown
• Come from hematopoetic cells (monocytic stem cell origin)

Bone remodeling

1. Osteoblasts sense micro-cracks in bone → secrete RANKL
   • RANKL: receptor activator of nuclear factor-kB (kappa beta) ligand
2. RANKL binds to monocytes, inducing them to fuse, forming multinucleate osteoclasts
   • RANKL also facilitates osteoclast maturation
     • Binds to RANK → recruits TRAF6 →Nf-kB, MAPK pathway activation → translocation to nucleus → Nf-kB increases cFos expression → cFos induces FFATc1, stimulating osteoclast differentiation and maturation
3. Osteoclasts secrete enzymatic factors and metalloproteinases to digest bone (e.g., collagenase), forming howship’s lacunae, and HCL/H+ (dissolves hydroxyapatite into soluble Ca2+ and PO43-)
   • This is bone resorption

How is this pathway regulated?

• Osteoblasts concurrently secretes osteoprotegerin (binds to RANKL, preventing it from binding to RANK receptors), slowing osteoclast maturation
• Once bone resorption is finished…
  • Osteoclasts commit apoptosis
  • Osteoblasts lay collagenous osteoid seam which fills howship’s lacunae → new osteoid binds to Ca2+ ions, raising [Ca2] → stimulates release of alkaline phosphatase from osteoblasts → form calcium phosphate crystals
    • This is bone (matrix) deposition