OPBSMusculoskeletal

Must Know

Chondrocytes, osteoblasts and osteocytes come from mesenchymal stem cells (MSCs).
Osteoclasts come from the myeloid haematopoietic lineage, shared with macrophages.
MSCs can also form fibroblasts, adipocytes, striated muscle and endothelial cells.
Long bones form by endochondral ossification on a cartilage anlage, starting in limb buds from lateral plate mesoderm.
Flat bones such as calvaria, and radial growth of long bones, use intramembranous ossification with no cartilage stage.
In the anlage, hypertrophic chondrocytes calcify their matrix, then undergo apoptosis as vessels invade.
Osteoid is unmineralised type I collagen and proteoglycan matrix that binds calcium.
Osteoblasts buried in mineralising osteoid terminally differentiate into osteocytes.

Chondrocyte Differentiation

Endochondral pathway. Maturation, hypertrophy, apoptosis and matrix calcification. Used in physes and repeated in fracture repair.
Hyaline pathway. Quiescent articular chondrocytes with load-bearing roles that persist for life.

Condensing mesenchyme expresses aggrecan and the IIa splice variant of type 2 collagen.
Sox9 is the master transcription factor of chondrogenesis. Nkx3-2 maintains Sox9.
Notch, through RBP-Jκ, represses Sox9 and slows chondrogenesis.
FGFs, hypoxia with HIF-1α, TGFβ and BMPs drive Sox9 and chondrogenesis.
Noggin is a soluble BMP decoy. Overexpressing it blocks cartilage formation.
Indian hedgehog and BMPs promote chondrocyte maturation.
PTHrP slows maturation by inhibiting RUNX2 through CREBP.
TGF-β and Smad signalling stop articular chondrocytes becoming hypertrophic.
Loss of that brake leads to MMP-9, MMP-13 and ADAMTS release, Col10a1 expression, apoptosis and OA.

Articular zone	Cells	Markers
Superficial	One to two layers, flattened	Prg4 (lubricin), Sox9, Col2a1, aggrecan, tenascin C, low Cilp
Intermediate	Round	As superficial but no Prg4 and more Cilp
Radial and calcified	Hypertrophic phenotype	Type 10 collagen (Col10a1)

Cartilage Matrix

Component	Point to remember
Type II collagen	Main matrix protein. Triple helix of one chain type, cleaved outside the cell
Aggrecan	Main proteoglycan. Chondroitin and keratan sulphate chains. Aggregates with hyaluronic acid and link protein
Type VI collagen	Pericellular matrix
Type IX collagen	Coats type II fibrils, links them to proteoglycan
Type X collagen	Made only by hypertrophic chondrocytes
Type XI collagen	Inside type II fibrils, may set fibril diameter
Decorin, biglycan	Small proteoglycans

Proteoglycans let cartilage absorb repeated compression without damage.
Hypertrophic chondrocytes express high alkaline phosphatase.
In monolayer culture chondrocytes dedifferentiate into type I collagen-making fibroblast-like cells.
Suspension or 3-D gels (collagen, agar, alginate) preserve the chondrocyte phenotype.

Signalling Pathways

Pathway	Signal carrier	Chondrocytes	Osteoblasts
Hedgehog (Shh, Ihh)	Gli	Sox9, chondrogenesis, hypertrophy	Not described
Notch	NICD after γ-secretase cleavage, RBP-Jκ	Inhibits chondrogenesis	Inhibits differentiation
TGFβ	Smad2, 3	Chondrogenesis, blocks hypertrophy	Progenitor proliferation, commitment
BMP	Smad1, 5, 8 and p38 MAPK	Chondrogenesis, induces hypertrophy	RUNX2, differentiation
PTHrP and PTH	PTH1R, CREBP	Inhibits RUNX2 and hypertrophy	Bone matrix deposition
Wnt	Frizzled and LRP5/6, β-catenin	Not described	Activates RUNX2, differentiation

Osteoblasts

Two steps. MSC commitment to osteoprogenitor, then pre-osteoblast maturation.
RUNX2 is the master osteoblast transcription factor, present in condensations before chondroblasts or osteoblasts appear.
RUNX2 drives VEGF-A, needed for vascular invasion of calcified cartilage.
Without Wnt, β-catenin is degraded in the proteasome. With Wnt, it enters the nucleus.
LRP5 G171V gain of function raises osteoblast number and bone mass.
Notch keeps the progenitor pool. Losing Notch gives high bone mass early then severe osteopenia.

Type I collagen is over 90 percent of bone organic matrix, two α1 and one α2 chains.
BMP-1 cleaves the C-propeptide but is not osteoinductive.
Serum C- and N-propeptides reflect bone formation rate.
Fibril direction turns 90 degrees between layers, a plywood structure maximising tensile strength.
Next most abundant products are osteocalcin, osteopontin and osteonectin.
Osteoblasts carry PTH1R and the vitamin D receptor.
Systemic glucocorticoids inhibit osteoblast proliferation and lower bone formation and mass.

Maturing osteoblasts switch	From	To
Collagen	Type III and I mix	Mainly type I
Alkaline phosphatase	Low	High
Matrix protein	Versican	Fibronectin
Attachment protein	Thrombospondin	Osteonectin, osteocalcin, osteopontin, sialoprotein

Osteocytes

DMP-1 drives osteocyte maturation. Deleting it gives immature osteocytes, osteomalacia and rickets.
Inactivating Osterix in early osteoblasts impairs osteocyte maturation and lowers bone mass.
Processes run in canaliculi and connect through connexin gap junctions.
Osteocytes sense canalicular fluid flow and tensile strain, the load sensors of bone.
Sclerostin (SOST gene) from osteocytes blocks Wnt signalling and reduces bone formation.

Osteoclasts

Multinucleated cells formed by fusion of myeloid precursors.
M-CSF and RANKL are the two required cytokines for osteoclast formation.
OPG is a RANKL decoy receptor that halts differentiation at any step.
Attachment is through integrin αvβ3 (vitronectin receptor) binding RGD sequences.
A proton pump acidifies the sealed zone, dissolving hydroxyapatite. Carbonic anhydrase II supplies protons.
Cathepsin from lysosomes digests collagen.
Osteoclasts resorb for about 2 weeks, then undergo apoptosis.
Only osteoblasts carry the PTH receptor, so PTH drives resorption indirectly.
TNF-α, IL-1 and IL-6 drive pathological resorption in tumours, infection, loosening, nonunion and osteoporosis.

Pro-osteoclastogenic	Anti-osteoclastogenic
RANKL, M-CSF, IL-1, IL-6, GM-CSF, PGE2, TNF-α	OPG, IFN-β, IFN-γ, IL-4

More on osteoclast coupling and immunity

TRAP and lysosomal enzymes stay on the resorbed surface. Their glycosylation lets them bind the IGF-II mannose-6-phosphate receptor on osteoblasts, a possible coupling signal.
c-src helps polarise the cell and form the ruffled border.
A calcium receptor may trigger pseudopodia so the cell creeps along the surface.
Old osteoclasts die in the deep centre of the lacuna while new cells join at the cutting edge.
RANKL, M-CSF, TNF, IL-1 and VEGF-A also block osteoclast apoptosis.
Osteoimmunology studies osteoclasts acting as immune regulators.

Gene Defects in Bone Cell Signalling

Gene or factor	Defect	Skeletal result
RUNX2 (Cbfa1)	Null mouse	No osteoblasts, cartilage-only skeleton, perinatal death
RUNX2	Heterozygous	Cleidocranial dysplasia
Osterix	Null mouse	No osteoblasts
Indian hedgehog	Null mouse	No osteoblasts
PTH1 receptor	Loss in humans	Blomstrand lethal osteochondrodysplasia
PTHrP	Null or overexpressed	Abnormal endochondral bone, or short limbs
SOX9	Human mutation	Campomelic dysplasia
FGF receptors	Human mutation	Craniosynostosis
LRP5	Loss of function	Osteoporosis pseudoglioma syndrome, fractures and blindness
LRP5	Gain of function	High bone mass
WNT1	Heterozygous missense	Dominant early onset osteoporosis
SOST (sclerostin)	Loss of function	Sclerosteosis with syndactyly, van Buchem disease
β-catenin	Deleted in progenitors	Progenitors become chondrocytes
Leptin	Deficient mouse	Obesity with high bone mass

Myeloma cells secrete sclerostin, suppressing Wnt signalling and osteoblasts in lytic lesions.
Generalised lipodystrophy causes osteosclerosis.
Aromatase deficiency and oestrogen receptor defects cause osteoporosis and unfused physes, with growth continuing into adulthood.
Pericytes can form bone. Circulating osteoblast lineage cells, under 1 in 100 million mononuclear cells, may explain some ectopic ossification.

Osteoclast Defects and Osteopetrosis

Defect	Site of fault	Osteoclasts	Marrow transplant
M-CSF (op/op mouse)	Extrinsic	Absent	Does not correct
RANKL	Extrinsic	Absent	Not known
RANK, c-fos or PU.1	Intrinsic	Absent	Corrects
c-src	Intrinsic	Increased, no ruffled border	Corrects
Mitf	Intrinsic	Immature, no ruffled border	Corrects
Cathepsin K or CLCN7	Intrinsic	Normal number, abnormal ruffled border	Not known
TRAP	Intrinsic	Increased, extensive ruffled border	Not known

Mice lacking αvβ3 integrin form no actin rings, have abnormal ruffled borders and do not resorb bone.
Calcitonin stops motility and disrupts the actin ring. Continuous dosing causes escape through receptor loss.
The sealed resorption space reaches about pH 4.5. A coupled chloride channel keeps electrical neutrality.
Carbonic anhydrase deficiency prevents acidification, so resorption fails.
Osteoclasts rarely sit on osteoid. Collagenase and stromelysin must strip it first.
Oestrogen and bisphosphonates promote osteoclast apoptosis. Resorptive stimuli delay it.
Heparin may cause osteoporosis by acting directly on osteoclasts.
Osteoclast-like giant cells occur in giant cell tumour and in some carcinomas of breast, lung, liver and thyroid.

Growth Plate and Ossification in Pathology

The groove of Ranvier and the perichondrial ring of Lacroix hold packed precursor cells that widen the physis.
Unstable fractures heal through cartilage then endochondral bone.
Rigidly stabilised fractures express no cartilage genes and heal by intramembranous bone.
Endochondral ossification persists in osteochondroma caps, osteophytes, callus and growing loose bodies.
Periosteal new bone in callus, tumour and infection is mostly membranous.
Distraction first expands the hyperplastic cell zone of the physis.
Harris lines are transverse trabeculae crossing the medulla, marking periods of growth stress.
Greulich and Pyle grades hand and wrist ossification centres to give skeletal age.

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

Type I collagen. Over 90 percent of bone organic matrix.
Osteoclast working life. About 2 weeks.
Superficial zone. One to two cell layers.

Memory Aids

Sox9 for cartilage, RUNX2 for bone. Two master switches, one per lineage.
Notch says not yet. It holds back both chondrocyte and osteoblast differentiation.
Ihh speeds, PTHrP brakes. Opposing control of chondrocyte maturation.

Exam Traps

Osteoclasts lack the PTH receptor. Osteoblasts relay the PTH signal.
TGFβ blocks hypertrophy, while BMP and hedgehog signalling induce it.
BMP-1 is a protease, not an osteoinductive BMP.
Type X collagen marks hypertrophic chondrocytes only.
Monolayer culture loses the chondrocyte phenotype. 3-D culture keeps it.

Recent Review. Skeletal Stem Cells

Skeletal stem cells (SSCs) self-renew, generate every cell in their lineage and persist long term in vivo.
The MSC label, coined by Caplan in 1991, describes mixed culture-adapted cells defined by plastic adherence, with poor engraftment.
MSCs lack proof of in vivo self-renewal and serial engraftment, so they fail stem cell criteria.
SSCs are defined by immunophenotyping and lineage tracing. Bianco and Robey introduced the term in the early 2000s.
Mouse SSCs are CD45, CD31 and Ter119 negative, and CD200 and CD51 positive.
Self-renewal is tested by serial transplantation, with repeated engraftment and reisolation forming bone organoids.
A human SSC was first isolated in 2018.
Separate regional SSC lineages exist in long bone, calvaria, vertebrae and growth plate, each keeping its identity when moved.
PTHrP-positive SSCs in the resting zone feed proliferative and hypertrophic chondrocytes, nearby osteoblasts and CAR cells.
Periosteal stem cells mainly form osteoblasts, so multipotency is not part of the strict SSC definition.

Source

Zuscik MJ, Xing L, Boyce BF, Lee FY. Cell Biology, Cell Differentiation, and Stem Cells of the Skeletal System. In. Aaron RK, editor. Orthopaedic Basic Science. Foundations of Clinical Practice. 5th ed. Philadelphia, Wolters Kluwer, 2021.

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