Severe, chronic anemias (such as thalassemias and sickle cell anemia) can increase the bone marrow response to produce more RBC's. This drive for increasing erythropoiesis may increase the mass of marrow and lead to increase in the amount of marrow in places, such as the skull seen here, that it is not normally found. Such an increase in marrow in skull may lead to "frontal bossing" or forehead prominence because of the skull shape change.

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