Osteoclasts are hematopoietic cells which play important roles in bone remodeling and resorption.
In this review we first describe the phylogeny of osteoclasts. Osteoclast generation is closely linked to the presence of bone tissues. The formation of bone cavities in aquatic animals is underdeveloped, even though they have cells which have the potential to differentiate into osteoclasts. Next we describe recent advances in our understanding of osteoclastogenesis that have resulted from the identification of critical molecules and mutated genes of osteopetrotic mice. We are now able to induce mature osteoclasts from hematopoietic stem cells and even from totipotent embryonic stem cells.