Biology of Bone

OPBSMusculoskeletal

Must Know

• Bone gives mechanical support and protection, stores calcium and phosphate, and houses stem cell niches.
• Skull and upper mandible come from neural crest by intramembranous ossification, mesenchyme turning straight into osteogenic cells.
• Vertebrae, ribs, pelvis and limbs come from mesoderm by endochondral ossification on a cartilage anlage.
• Diaphysis has thick cortex and little cancellous bone. Metaphysis and epiphysis have thin cortex and more cancellous bone.
• Periosteum has an outer fibrous layer and an inner cambium layer holding skeletal stem cells and osteoprogenitors.
• Periosteal vessels supply the outer third of cortex. Marrow vessels supply the inner two thirds.
• Trabeculae contain no vessels and rely on the marrow circulation.
• Woven bone has random collagen, forms fast (development, fracture callus, osteosarcoma) and is later replaced by lamellar bone.
• Lamellae are 3 to 5 µm thick, with fibre direction shifting up to 90 degrees between layers.
• Haversian canals run along the long axis. Volkmann canals run across it.
• A cement line borders each osteon, laid down after resorption and before new matrix.

More on development and gross structure

• Long bones lengthen through growth plate cartilage from the first trimester until late adolescence.
• Many malformations, such as polydactyly and dwarfism, arise from faulty cartilage with bone itself normal.
• The adult skeleton has 80 axial and 126 appendicular bones.
• Marrow stroma houses haematopoietic stem cells and marrow stromal stem cells that can form bone, cartilage, tendon and fat.
• Whether bone has lymphatic drainage is unresolved.
• Lamellae are outer circumferential, inner circumferential, osteonal (around a vascular channel) or interstitial (fragments between osteons).

Bone Cells

Cell	Origin	Job	Marker or product
Osteoblast	Mesenchymal skeletal stem cell	Makes organic matrix, controls mineralisation. Postmitotic	ALP, osteocalcin
Osteocyte	Osteoblast buried in matrix	Mechanosensor and signaller. Most abundant bone cell	Sclerostin, RANKL, FGF-23, DMP-1, MEPE
Lining cell	Last osteoblasts, flattened	Covers quiescent surfaces. Can reactivate into osteoblasts	Not described
Osteoclast	Haematopoietic monocyte	Multinucleated. Resorbs mineral and matrix together	TRAP, cathepsins B, L, K

• New osteoblasts come from progenitors sitting behind the surface layer.
• Osteocyte canaliculi are about 0.3 µm wide, joined by gap junctions.
• Dying osteocytes raise RANKL after microdamage, oestrogen loss or unloading.
• Sclerostin is a Wnt antagonist that suppresses osteoblasts.
• Osteoclast carbonic anhydrase makes protons, pumped across the ruffled border to dissolve apatite.
• RANKL and M-CSF are the obligate factors for osteoclast differentiation.

More on cell biology

• Osteocyte lacunae measure 300 to 500 cubic µm.
• Fluid flow and small deformations in the lacunar canalicular system are thought to let osteocytes sense load.
• Osteocytes act on the kidney through FGF-23 and PHEX.
• Osteocytes can remodel their lacunae during mineral exchange, most obviously in lactation.
• Osteoprogenitors can become adipocytes instead, possibly more so with age.
• The osteoclast lineage shows few branches. The osteoblast lineage is plastic and can switch phenotype.

Matrix and Collagen

• Bone mass is about 30 percent organic and 70 percent mineral.
• About 90 percent of matrix protein is type I collagen.
• Tropocollagen holds two α1(I) chains (COL1A1) and one α2(I) chain (COL1A2) in a triple helix.
• Repeating Gly-X-Y, with glycine every third residue, X proline and Y often hydroxyproline.
• Molecules are about 300 nm long. A quarter stagger leaves hole zones where mineral crystals sit.
• Hydroxylation happens inside the cell. Cross-linking happens outside it, started by lysyl oxidase.
• Osteogenesis imperfecta comes from COL1A1 or COL1A2 mutations or faulty collagen modifying enzymes.

More on collagen

• The organic fraction is 98 percent protein and 2 percent cells.
• Collagen gives tensile strength and toughness. Mineral gives stiffness and compressive strength.
• Other collagens in bone are types II and X (cartilage remnants), III (periosteum, marrow), IV (vessels) and V.
• Cleaved propeptides in serum and urine mark synthesis. Pyridinoline cross-link fragments mark breakdown. Neither is bone specific.
• Glycine has no side chain, avoiding steric clash in the tight helix.
• Divalent cross-links mature into stable trivalent ones.

Noncollagenous Proteins

Group	Members	Point to remember
SIBLING	BSP, osteopontin, DMP-1, MEPE	RGD sequence binds integrin αVβ3
Gla proteins	Osteocalcin, matrix gla protein, periostin	Vitamin K dependent carboxylation lets them bind calcium
Serum derived	Albumin, α2HS-glycoprotein	Made in liver
Enzymes	ALP, MMPs, PHEX	PHEX loss causes X-linked hypophosphataemic rickets

• Osteocalcin is the most abundant gla protein in bone.
• DMP-1 and MEPE come mainly from osteocytes. Osteopontin and BSP come from osteoblasts.

Osteoblast marker	ALP	Osteocalcin
Timing	Early, persists in mature osteoblasts	Later, mature osteoblasts
In osteocytes	Lost	Not described
Specificity	Low, also in some leukocytes	Specific to osteoblast lineage

More on other matrix proteins

• Proteoglycans in bone are decorin (one GAG chain), biglycan (two) and perlecan (in the lacunar canalicular space).
• Growth factors stored in matrix include BMPs (first found in demineralised bone), TGF-β, IGFs, FGFs and sclerostin.
• α2HS-glycoprotein binds calcium and inhibits ectopic calcification.
• DMP-1 deficiency causes defective mineralisation.
• Fibronectin knockout mice die before the skeleton forms.
• PHEX regulates release of the mineralisation inhibiting ASARM peptide from MEPE.

Mineralisation

• Mineral approximates hydroxyapatite, substituted by magnesium and carbonate.
• Control is local, mainly by osteoblasts and likely osteocytes.
• Pyrophosphate (PPi), made from ATP, inhibits mineralisation. ANKH exports it from cytoplasm to matrix.
• ALP and PHOSPHO1 split PPi into two phosphates, promoting mineralisation.
• Osteomalacia leaves wide osteoid seams. Rickets widens unmineralised growth plates.

More on crystal formation

• Hydroxyapatite is Ca10(PO4)6(OH)2.
• Trace aluminium, lead, strontium and fluoride can enter mineral from diet or environment.
• Nucleation needs enough ion concentration. Crystal growth afterwards costs less energy.
• The ank mouse mutation causes progressive ankylosis.

Assessing Bone

Method	Strength	Limit
Histomorphometry	Shows cell activity	Two dimensional, needs biopsy
CT	Three dimensional microstructure	Needs defined volume of interest
DEXA	Clinical BMD and BMC	Areal density, affected by bone size

• Dynamic histology uses fluorochrome (tetracycline) double labels.
• Mineral apposition rate equals distance between labels divided by time between them.
• T score compares with young adult mean BMD. Z score compares with age matched mean.
• BMD explains only 50 to 70 percent of variance in vertebral strength.

More on histomorphometry and CT indices

• Mineralising surface is double labelled surface plus half the single labelled surface.
• Bone formation rate is mineralising surface times mineral apposition rate.
• Activation frequency is bone formation rate divided by wall width.
• Structure model index is 0 for parallel plates and 3 for rods.
• Nomenclature is standardised by ASBMR consensus committees.

Cortical Versus Cancellous Bone

Index	Cortical	Cancellous
Share of skeleton	80 percent	20 percent
Share of turnover	50 percent	50 percent
Surface to volume ratio	2.5	20

Site	Cortical to cancellous	Share of fractures	Trend from age 50
Vertebra	25 to 75	27 percent	Rises
Wrist	75 to 25	19 percent	Falls
Hip	34 to 66	14 percent	Rises
Pelvis	60 to 40	7 percent	Rises then plateaus

• Up to 65 percent of women over 65 with a fragility fracture go untreated at 6 months.

More on site variation

• Cancellous surfaces show 4 percent resorbing and 16 percent forming, against 2 and 8 percent in cortical bone.
• Genetics is estimated to account for 20 to 40 percent of skeletal function.
• Women have smaller, lighter bones. Ageing widens bones, offsetting cortical thinning.
• Up to 50 percent of women and 20 percent of men over 50 sustain a fragility fracture.

Osteoid and the Mineralisation Front

• About 20 percent of normal trabecular surface carries osteoid, roughly 10 µm thick and 2 percent of bone volume.
• Osteoid starts to mineralise about 10 days after it is laid down.
• The mineralisation front is a thin basophilic line between osteoid and mineralised bone on H&E.
• Tetracycline, technetium diphosphonate, aluminium and iron all localise to the mineralisation front.
• A hot bone scan reflects increased vascularity and active mineralisation. Sensitive but nonspecific.
• Aluminium from phosphate binders blocks mineralisation in dialysis osteomalacia. Iron overload can do the same.
• Fluoride leaves a basophilic line at the front. Fluoride without calcium can cause hyperosteoidosis that mimics osteomalacia.
• Matrix vesicles about 100 nm across bud from chondrocytes and osteoblasts and seed the first apatite crystals.
• Annexins II, V and VI let matrix vesicles take up calcium. Hypertrophic chondrocytes release MMP-13 within them.

More on collagen and matrix in disease

• Early fracture callus makes types II and III collagen, later replaced by type I.
• Faulty type III collagen production has been linked with delayed union and nonunion.
• Woven bone appears in the embryo, fracture callus, tumour bone and high turnover states such as Paget disease.
• Sharpey fibres anchor periosteum, tendon and ligament into bone. Weak attachment underlies tibial spine avulsion and Osgood-Schlatter disease.
• Lathyrism and osteogenesis imperfecta show abnormal collagen cross-linking.
• 1,25 dihydroxyvitamin D, PTH, IL-1 and glucocorticoids suppress type I collagen synthesis. IGFs and TGF-β stimulate it.
• COMP mutation causes pseudoachondroplasia and a form of multiple epiphyseal dysplasia, with COMP trapped in chondrocyte endoplasmic reticulum.
• Mice lacking matrix gla protein develop excess calcification of bone and soft tissue.
• Osteonectin stains osteosarcoma and osteoblastoma strongly.
• Serum osteocalcin is raised in every stage of Paget disease and in blastic metastases, and lowered by glucocorticoid excess.
• Vitamin K antagonists in the first trimester can cause fetal bone defects.

MMP group	Members	Substrate
Collagenases	MMP-1, 8, 13	Collagen types I, II, III
Gelatinases	MMP-2, 9	Type IV collagen, gelatin
Stromelysins	MMP-3, 10	Proteoglycan, laminin, fibronectin

• MMP-9 dominates bone degradation. MMP-1, 3 and 13 dominate arthritic joints.

Cement Lines

• Cement lines mark where resorption stopped and formation began, seen as thin basophilic lines on H&E.
• Reversal lines are scalloped, tracing old Howship lacunae.
• Resting or arrest lines are smooth, laid on unresorbed surfaces after a pause in formation.
• They are 1 to 2 µm thick, poor in collagen, rich in osteopontin and act as planes of weakness.
• Paget disease shows irregular wavy cement lines in a mosaic.
• Fluoride treated bone shows a prominent line separating new from host bone.

Remodelling in Tissue Section

• The basic multicellular unit (BMU) couples osteoclast resorption to osteoblast formation.
• Reversal cells cover about 80 percent of eroded surfaces, clean lacunae with MMP-13 and lay down the cement line.
• Coupled balanced remodelling refills the cavity completely. This is normal.
• Coupled unbalanced remodelling refills it only partly, as in postmenopausal osteoporosis.
• Uncoupled remodelling leaves the cavity empty because the reversal phase arrests.
• A cortical cutting cone is led by about 10 osteoclasts, is about 300 µm long and advances about 40 µm a day.
• A normal ×10 field shows no more than one BMU.
• Hyperparathyroidism shows intratrabecular tunnelling resorption.
• Only formation can be measured dynamically, by double tetracycline labels. Normal mineral apposition is about 0.6 µm a day.
• Flat inactive osteoblasts lay osteoid at about 0.4 µm a day. Plump active ones reach about 1.8 µm a day.
• Activation frequency rises in hyperparathyroidism and Paget disease and falls in adynamic renal bone disease.

More on remodelling

• Remodelling may take place inside a bone remodelling compartment, a canopy of lining cells over the site.
• Microcracks crossing canaliculi are thought to start remodelling through osteocyte apoptosis.
• Cortical bone is 80 to 90 percent mineralised by volume, against about 20 percent for trabecular bone.
• Primary tumours, metastases and early osteomyelitis mostly start in marrow-bathed cancellous bone. Fracture is the exception.

Blood, Lymphatic and Nerve Supply

• Arterial supply comes from nutrient, metaphyseal and epiphyseal, and periosteal vessels.
• Metaphyseal arteries are end arteries, which may explain metaphyseal localisation of haematogenous osteomyelitis.
• Epiphyseal arteries anastomose, but epiphyseal and metaphyseal circulations do not connect.
• Cortical flow is centrifugal. After osteotomy or nutrient artery loss it reverses and periosteal vessels take over.
• The marrow cavity acts as a noncollapsible vein, so intraosseous infusion works in shock.
• Lymphatic markers LYVE-1 and podoplanin show no lymphatics in normal cortical or cancellous bone, only in surrounding tissue.
• Lymphatic vessels within bone are the hallmark of Gorham disease.
• Metastases confined to bone lack lymphatics, arguing against lymphatic spread to bone.
• Periosteum is the most richly innervated part. Formalin and hot paraffin destroy fine nerve fibres in sections.

More on supply

• Blood flow rises in Paget disease and hyperparathyroidism and probably falls with ageing.
• Rheumatoid arthritis spares a paretic limb after hemiplegia or polio.
• Sympathectomy transiently raises limb blood flow and was once used for limb lengthening after polio.

Source

Vigorita VJ, Ghelman B, Mintz D. Basic Science of Bone. In. Vigorita VJ, Ghelman B, Mintz D. Orthopaedic Pathology. 3rd ed. Philadelphia, Wolters Kluwer, 2016. Chapter 1.

Numbers to Remember

• Blood supply. Periosteum outer third, marrow inner two thirds.
• Lamella. 3 to 5 µm.
• Canaliculus. About 0.3 µm.
• Tropocollagen. About 300 nm, offset by 25 percent.
• Mineral timing. 70 percent in 5 to 10 days, the remaining 30 percent over 3 to 6 months.

Memory Aids

• PPi brakes, ALP releases. Pyrophosphate blocks crystals. ALP and PHOSPHO1 cut it into phosphate fuel.
• Haversian long, Volkmann across. Picture a ladder with Volkmann canals as rungs.

Exam Traps

• Osteoblasts are postmitotic and do not divide.
• Lining cells and osteocytes belong to the mesenchymal osteoblast lineage.
• TRAP marks osteoclasts. ALP and osteocalcin mark osteoblasts.
• Cortical bone is 80 percent of skeletal mass yet only half of turnover.
• Wrist fractures decline with age because they follow falls.

Related pages. Bone as a biomaterial, Normal Skeletal Metabolism, Bones, Inhibitors of Bone Resorption, Bone Grafts and Substitutes, Fracture Healing

Source

Majeska RJ, Kennedy OD, Schaffler MB, Morgan EF, Gerstenfeld LC. Biology of Bone. In. Aaron RK, editor. Orthopaedic Basic Science. Foundations of Clinical Practice. 5th ed. Philadelphia, Wolters Kluwer, 2021.

Practise Type X questions on this topic

Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 17 October 2026.