Malignant osteopetrosis: a disease of abnormal osteoclast proliferation.

Bone biopsies were obtained from two infants with the malignant form of congenital osteopetrosis. Non-decalcified histological sections of each biopsy contained vast numbers of osteoclasts occupying virtually the entire residual marrow space. Despite the apparent failure of normal bone resorption in both patients, the osteoclasts of one appeared histologically and ultrastructurally normal, exhibiting ruffled membrane formation and evidence of acid phosphatase synthesis and secretion. In contrast, the osteoclasts of the second patient were unusually shaped, abnormally large and contained no ruffled membranes. It is concluded that the bone marrow failure which appears in the malignant, as compared to the benign form of osteopetrosis, is at least in part due to replacement of hematopoietic precursors by osteoclasts. Furthermore, the different cytological appearance of the osteoclasts of each patient suggests that a spectrum of pathogenetic mechanisms may be responsible for the development of the disease. Finally, ruffled membrane formation by the osteoclasts of one patient calls into question the assumption that this morphological feature is invariably associated with bone resorption.