Osteopetrosis, literally "stone bone", also known as marble bone disease and Albers-Schönberg disease, is an extremely rare inheriteddisorder whereby the bones harden, becoming denser, in contrast to more prevalent conditions like osteoporosis, in which the bones become less dense and more brittle, or osteomalacia, in which the bones soften. Osteopetrosis can cause bones to dissolve and break.
It can cause osteosclerosis. The cause of the disease is understood to be malfunctioning osteoclasts. Radiological findings will show a bone-in-bone appearance.
Normal bone growth is achieved by a balance between bone formation by osteoblasts and bone resorption (breakdown of bone matrix) by osteoclasts. In osteopetrosis, the number of osteoclasts may be reduced, normal, or increased. Most importantly, osteoclast dysfunction mediates the pathogenesis of this disease.
The underlying cause of osteopetrosis is mutations that interfere with acidification of osteoclast resorption pit. Example of this is deficiency of carbonic anhydrase encoded by the gene CA2. The carbonic anhydrase enzyme is important in for osteoclasts to produce protons. Absence of this enzyme causes defective hydrogen ion pumping by osteoclasts, and this, in turn, causes defective bone resorption by osteoclasts, as an acidic environment is needed for dissociation of calcium hydroxyapatite from bone matrix. Hence, bone resorption fails while its formation persists. The result is excessive bone formation.