A 2-year-old child has a history of multiple bone fractures with minor trauma. On examination, he has hepatosplenomegaly and palsies involving cranial nerves II, VII, and VIII. Laboratory studies show pancytopenia. Radiographs reveal diffusely and symmetrically sclerotic bones with poorly formed metaphyses. Molecular analysis of his bone reveals a defect in the production of carbonic anhydrase to solubilize hydroxyapatite crystal. He is treated with hematopoietic stem cell transplantation. Which of the following cells in his bones was most likely functionally deficient and replaced following transplantation?

Osteopetrosis (Albers-Schonberg disease) is a rare bone disease resulting from mutations in genes that regulate osteoclast activity and bone resorption. Carbonic anhydrase generates the protons used by the H+-ATPase proton pump located on the osteoclast ruffled border. Marrow is reduced within the sclerotic bone, with subsequent extramedullary hematopoiesis in the spleen and liver. Nerve compression in narrowed bony foramina leads to the palsies. The other cells listed are derived from mesenchymal stem cells, not hematopoietic stem cells. Osteopetrosis is the opposite of osteoporosis. Remember that in osteoporosis the osteoclast formation and action are enhanced when osteoblasts produce less osteoprotegerin and M-CSF, and increased RANK and RANKL interaction promote osteoclast differentiation and survival.