CorpusHuman Anatomy
SkeletalTissue

Bone matrix

Also known as: osteoid, hydroxyapatite, extracellular matrix of bone

The extracellular material of bone: an organic osteoid of type I collagen and non-collagenous proteins, mineralised by crystals of calcium hydroxyapatite.

Overview

Collagen in organic matrix≈90% (type I)
MineralHydroxyapatite, Ca/P molar ratio ≈1.67

The organic matrix (osteoid), secreted by osteoblasts, is ≈90% type I collagen, arranged in cross-linked fibrils that template mineral deposition. The remaining ≈10% comprises non-collagenous proteins — osteocalcin (vitamin K–dependent, binds calcium; a serum marker of bone formation), osteonectin, osteopontin and bone sialoprotein (anchor cells and mineral), proteoglycans and growth factors (TGF-β, BMPs, IGFs) that are released during resorption and help couple formation to resorption.

The inorganic phase consists of plate-like crystals of calcium phosphate in the form of hydroxyapatite [Ca10(PO4)6(OH)2], with carbonate, magnesium, sodium and fluoride substitutions. Mineralisation begins in the gaps between collagen molecules ("hole zones") and within matrix vesicles budded from osteoblasts, which concentrate calcium and phosphate and contain alkaline phosphatase (removes the mineralisation inhibitor pyrophosphate). Primary mineralisation reaches ≈70% within days; secondary mineralisation continues for months. Unmineralised osteoid seam width increases in osteomalacia.

Location

Surrounds osteocytes in all bone tissue; forms lamellae and trabeculae.

Parts & subdivisions

  1. Type I collagen
    Triple-helical fibrils providing tensile strength; defective in osteogenesis imperfecta.
  2. Ground substance
    Proteoglycans and glycoproteins (osteocalcin, osteonectin, osteopontin, bone sialoprotein).
  3. Hydroxyapatite
    Ca10(PO4)6(OH)2 crystals ≈50 × 25 × 2–4 nm aligned along collagen fibrils; provides hardness and compressive strength.

Function

  • Gives bone its combination of stiffness (mineral) and toughness (collagen)
  • Acts as the body reservoir of calcium and phosphate
  • Stores growth factors that are liberated during resorption
Physiology

Formation markers: serum bone-specific ALP, osteocalcin, P1NP. Resorption markers: CTX and NTX (collagen cross-link fragments). Vitamin C is required for collagen hydroxylation (scurvy causes defective osteoid); vitamin K for osteocalcin carboxylation.

Clinical significance

Scurvy

Vitamin C deficiency prevents prolyl and lysyl hydroxylation of collagen → defective osteoid, subperiosteal haemorrhage, poor wound healing and, in children, bone pain and growth disturbance.

Compiled from standard anatomy and physiology references — see the Skeletal System sources. For education only; not medical advice.