Skeletal dysplasias may be classified according to the type of bone formation, endochondral, intramembranous or both, that is affected by the diminished osteoclastic activity.
These are further classified according to the stage of skeletal development, either primary or secondary.
Osteopetrosis, for example, involves a defect in endochondral bone formation during the initial development of the skeleton. It is therefore, the medullary bone that is affected.
Cortical or periosteal bone formation is intramembranous, without a preexisting cartilage template. A defect in intramembranous bone formation, therefore would result in cortical thickening as seen in progressive diaphyseal dysplasia (Engelmann�s disease).
Osteopetrosis
marble bone disease aka "stone bone" Albers-Schönberg disease
due to abnormal function of osteoclast (bone resorption cells)... so bone formation cannot match with bone resorption.. which ends up as no mature trabeculae meshwork and bone filled up by spongy tissue which tentatively break resulting in fractures...
Histologically, osteopetrosis is characterized by the persistence of primary, unmineralized spongiosa in the medullary canals. (In normal individuals, the primary spongiosa is normally replaced by the bone marrow.)
Types of Adult osteopetrosis
some of the cases are known to caused by genetic deficiency of carbonic anhydrase -2........----> The 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......
so there won't be enough space for hematopoiesis which eventually result in anemia,thrombocytopenia,leukopenia----leading to extramedullary hematopoeisis
Laboratory findings in infantile osteopetrosis include the following: