The origin of the osteoclast.

The origin of the osteoclast has long been controversial. Various approaches such as traditional light microscopy, electron microscopy, microcinephotography, autoradiography, parabiosis, quail-chick nuclear marker, etc, have been used in attempts to resolve the problem. There are 2 major concepts: the view of skeletal (local) origin vs, that of extraskeletal origin (from blood-borne cells), and recently the latter view appears to have prevailed. Electron micrographic studies revealed that osteocytes and preosteoblasts can merge with pre-existing osteoclasts. Several studies by autoradiography and by quail-chick nuclear marker strongly supported the concept of extraskeletal origin. Although an alternative interpretation of the experimental results of Crelin and Koch supports the concept of skeletal origin, the majority of various experimental results by other authors would be in accordance with the following supposition: the osteoclast originates from a hematopoietic cell line (mononuclear phagocyte system) of the bone marrow which is essentially of extrinsic origin, but in the process of increasing the number of nuclei of the osteoclast, osteoblastic cells as well as osteoclastic cells can merge with the preosteoclasts or osteoclast. However, several problems regarding the origin of the osteoclast including the relation between the osteoclast and other giant cells, still remain unsolved.