Photomicrograph of a bone biopsy from a patient with osteopetrosis, stained with haematoxylin-eosin. The amount of bone is markedly increased and represented by primary spongiosa (calcified cartilage rimmed by osteoid tissue). Note the numerous osteoclasts (arrows), laying along the trabeculae surface, without evidence of bone resorption (inset). Lacunae are obviously narrowed and the hematopoetic tissue reduced (courtesy of Professor F. Facchetti, Department of Pathology, University of Brescia, Italy).