Figure 1. Osteoclast progenitor cells are closely related to monocytes/macrophages, and initial proliferation/differentiation is controlled by common signaling pathways. However, several extra- and intracellular signaling molecules are specifically involved in differentiation, fusion, polarization, and activation of the osteoclast-specific lineage. Some of these molecules are expressed in osteoclasts or their progenitor cells (in italic) and some in stromal cells/osteoblasts (underscored).