Osteopetrosis and storage disease of bone
- Lesion
When osteoclasts cannot remove bone, bones become dense but brittle and squeeze out the marrow. Storage diseases deform bones another way.
What it is
Osteopetrosis is a group of rare inherited disorders in which osteoclasts fail to resorb bone. Bone keeps being laid down but is never remodelled, so the skeleton becomes diffusely dense yet brittle, the marrow cavity shrinks and nerve foramina narrow. The causes include loss of carbonic anhydrase II, of the CLCN7 chloride channel, or of RANKL or RANK signaling. A different group, the lysosomal storage diseases such as the mucopolysaccharidoses, fill chondrocytes and macrophages with undegraded material and deform the skeleton (dysostosis multiplex).
Structure
- Gross: bones lack a medullary canal; long-bone ends are widened (Erlenmeyer flask) and vertebrae may show sandwich-like dense endplates.
- Micro: primary spongiosa and cores of calcified cartilage persist inside trabeculae, because they were never resorbed.
- Osteoclasts may be numerous or absent depending on the gene, but they lack resorption pits and ruffled borders.
- Mucopolysaccharidoses: chondrocytes swollen with clear vacuoles of stored glycosaminoglycans; the growth plate is disorganized.
Step by step
- Osteoclasts seal onto bone and pump acid through a proton pump, fed by carbonic anhydrase II, with chloride leaving through CLCN7.
- Acid dissolves mineral and cathepsin K digests the matrix.
- If RANKL or RANK signaling, carbonic anhydrase II or CLCN7 fails, bone is laid down but not removed.
- The marrow space fills with bone; blood cell production moves to the liver and spleen.
- Narrow foramina compress cranial nerves (optic, facial, vestibulocochlear) and the brittle bone fractures.
Two very different reasons for abnormal bone in inherited disease
| Feature | Osteopetrosis | Mucopolysaccharidoses |
|---|---|---|
| Basic defect | Osteoclasts cannot resorb bone | Lysosomal enzyme cannot degrade glycosaminoglycans |
| Bone density | Diffusely increased | Normal or decreased; bones misshapen |
| Marrow | Crowded out: anaemia, infections | Normal; macrophages may store material |
| Skeletal picture | Erlenmeyer flask ends, no medullary cavity | Dysostosis multiplex, short stature, joint stiffness |
| Example | Infantile osteopetrosis; carbonic anhydrase II deficiency | Hurler, Hunter and Morquio syndromes |
Clinical notes
- Infant with dense bones on X-ray, pancytopenia, hepatosplenomegaly and blindness: think of infantile osteopetrosis.
- Adult osteopetrosis is often found by chance on X-ray or after a fracture.
- Morquio syndrome mostly affects the skeleton and spine (odontoid hypoplasia with atlantoaxial instability) with normal intelligence.
Test yourself
Think of the answer, then tap the question to see it.
Which cell fails in osteopetrosis?
The osteoclast, which cannot resorb bone.
Why do infants with severe osteopetrosis have anaemia and hepatosplenomegaly?
The marrow cavity is filled with unresorbed bone, so hematopoiesis moves to the liver and spleen.
Which enzyme deficiency causes osteopetrosis with renal tubular acidosis?
Carbonic anhydrase II deficiency.
Why can a bone marrow transplant cure severe osteopetrosis?
Osteoclasts derive from monocyte-macrophage precursors, so donor stem cells supply working osteoclasts.
Elsewhere in DurgiPedia
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Osteopetrosis: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2025); Mucopolysaccharidosis type I: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2025); Gaucher disease: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2025).
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Representative model, for educational purposes only.