β3 integrin-null osteoclasts are dysfunctional, but their numbers are increased in vivo. In vitro, however, the number of β3-/- osteoclasts is reduced because of arrested differentiation. This paradox suggests cytokine regulation of β3-/- osteoclastogenesis differs in vitro and in vivo. In vitro, additional MCSF, but not receptor activator of NF-κB ligand (RANKL), completely rescues β3-/- osteoclastogenesis. Similarly, activation of extracellular signal-regulated kinases (ERKs) and expression of c-Fos, both essential for osteoclastogenesis, are attenuated in β3-/- preosteoclasts, but completely restored by additional MCSF. In fact, circulating and bone marrow cell membrane-bound MCSFs are enhanced in β3-/- mice, correlating with the increase in the osteoclast number. To identify components of the MCSF receptor that is critical for osteoclastogenesis in β3-/- cells, we retrovirally transduced authentic osteoclast precursors with chimeric c-Fms constructs containing various cytoplasmic domain mutations. Normalization of osteoclastogenesis and ERK activation, in β3-/- cells, uniquely requires c-Fms tyrosine 697. Finally, like high-dose MCSF, overexpression of c-Fos normalizes the number of β3-/- osteoclasts in vitro, but not their ability to resorb dentin. Thus, while c-Fms and αvβ3 collaborate in the osteoclastogenic process via shared activation of the ERK/c-Fos signaling pathway, the integrin is essential for matrix degradation.