Figure 9.

TNF-α directly induces myeloid cells to differentiate into committed osteoclast precursors and mature osteoclasts in vivo, irrespective of the capacity of stromal cells to respond to the cytokine. Chimeric animals were created by bone marrow transplantation in which β-galactosidase–positive osteoclast precursors exist within the stromal environments of TNFR-deficient animals or their TNFR-heterozygous littermates. Following engraftment, the animals were depleted of T cells and administered 150 μg/kg/day TNF-α or vehicle for 5 days by subcutaneous injection. (a) Equal numbers of whole marrow cells were then cultured ex vivo for 7 days in osteoclastogenic conditions, and stained for TRAP and β-galactosidase activity in combination. (b) Decalcified sections of long bones, representative of data from six chimeric animals, were stained for TRAP activity (red reaction product) (left panels, ×100; right panels, ×250).