The origin of osteoclasts: evidence, clinical implications and investigative challenges of an extra-skeletal source

Cell surface characterization of the human osteoclast: phenotypic relationship to other bone marrow-derived cell types

Among the differentiated progeny of stem cells transplantable by bone marrow are osteoclasts, the multinucleate cells which are the major agents of bone resorption. Although the osteoclast is well characterized from a structural and functional standpoint, its development and origin are still far from clear. We have used monoclonal antibodies to investigate the interrelationship between osteoclasts and other haemopoietic cells in man. We have analysed the distribution of 19 granulocyte-monocyte antigens in eight reactivity clusters on the non-neoplastic osteoclasts present within nine osteoclastomas (syn. giant cell tumours of bone) and a single example of aneurysmal bone cyst. We found that osteoclasts are antigenically effete, failing to express granulocyte-monocyte, common leucocyte or other haemopoietic determinants; the only monocyte antigens detected on osteoclasts are My-7 and two closely related specificities, MCS.2 and DüHL60.4, which are also expressed by tissues outside the haemopoietic system. Our findings, taken together with recent transplantation studies, cast further doubt on the view that osteoclasts are specialized bone-resorbing macrophage-derived giant cells, and support a hypothesis that they are the end product of fusion of a hitherto unidentified circulating mononuclear cell type, the preosteoclast, which constitutes a cell lineage separate from those originating from the conventional multipotential haemopoietic stem cell, although still of bone marrow origin.

Osteoclast formation from mononuclear phagocytes: role of bone-forming cells

In a previous study, using co-cultures of embryonic bone rudiments stripped of periosteum, and mononuclear phagocytes of various sources, we found that multinucleated mineral-resorbing osteoclasts developed in vitro from radiosensitive mouse bone marrow mononuclear phagocytes (BMMP). (Burger, E. H., J. W. M. van der Meer, J. S. van de Gevel, C. W. Thesingh, and R. van Furth, 1982, J. Exp. Med. 156:1604-1614). In the present study, this co-culture technique was used to analyze the influence of bone-forming cells on osteoclast formation and bone resorption by BMMP or peritoneal exudate cells (PEC). BMMP or PEC were co-cultured with liver or dead bone, i.e., in the presence or absence of liver bone-forming cells. Mineral resorption and osteoclast formation were monitored via 45Ca release from prelabeled live or dead bone followed by histology. Osteoclasts developed from precultured BMMP as indicated by [3H]thymidine labeling, but only in live and not in dead bone. They formed readily from BMMP but only erratically, and after a longer culture period, from PEC. Macrophages from BMMP and PEC invaded live and dead bone rudiments but did not resorb the intact mineralized matrix. In contrast, ground bone powder was resorbed avidly by both cell populations, without formation of osteoclasts. We conclude that live bone-forming cells are required for osteoclast formation from progenitors. Live bone is only resorbed by osteoclasts, and not by macrophages. Osteoclast progenitors are abundant in cultures of BMMP but scarce in PEC, which makes a direct descendance of osteoclasts from mature macrophages unlikely.

The cytology of the dental follicle and adjacent alveolar bone during tooth eruption in the dog

Previous studies from our laboratories have shown that premolar eruption in dogs depends upon the presence of the dental follicle and is independent of root or crown growth or attachment to the oral epithelium. The present study is an analysis of the cellular composition of the dental follicle and the cellular investment of the adjacent walls of the bony crypt before and during eruption of the third and fourth mandibular permanent premolars in young beagle dogs. Four premolar follicles and their adjacent bony crypts were examined at 2-week intervals over 12 weeks before and during eruption of these teeth. Tissues were removed, fixed, processed, and oriented so that each follicle and the adjacent crypt wall could be reproducibly examined in vertical and horizontal planes. Mononuclear cells with abundant cytoplasm, euchromatic nuclei, and prominent nucleoli were present in juxtavascular location in the coronal part of the dental follicle; and these cells increased in number immediately preceding and during tooth eruption in parallel with an increase in osteoclasts on the adjacent crypt wall. These data are interpreted to mean that the coronal part of the dental follicle may coordinate the alveolar bone resorption required for tooth eruption by attracting and directing to the crypt wall a population of mononuclear cells, which either become osteoclasts and/or direct osteoclastic activity during tooth eruption.