Figure 3. Schematic Representation of Osteoclast and Osteoblast LineagesSchematic representation of the osteoclast (top) and osteoblast (bottom) lineages. The two lineages are distinct, but there is regulatory interaction among the cells (vertical arrows). Osteoclasts originate from a hematopoietic stem cell that can also differentiate into a macrophage, granulocyte, erythrocyte, megakaryocyte, mast cell, B-cell, or T-cell. Osteoblasts originate from a mesenchymal stem cell that can also differentiate into a chondrocyte, myocyte, fibroblast, or adipocyte. The terminology for these lineages is still evolving and is herein [over] simplified. Many intermediate steps and regulatory factors are involved in lineage development. (see Acknowledgements)