Experimental evidence accumulated in recent years suggests that the osteoclast is derived from the fusion of mononuclear precursors which are of hematopoietic origin. Mononuclear cells were isolated from the spleen and bone marrow of young rats in order to examine osteoclast formation. The isolated cells were placed in diffusion chambers containing devitalized bone fragments freed of soft tissue, and the chambers sealed and placed in the peritoneal cavity of host rats. The host animals were killed after 4 days, and the bone removed from the chambers for examination. Light-microscopic examination demonstrated two types of cells adjacent to the bone surface, one a flattened and elongated mononuclear cell, and the other a larger and frequently multinucleated cell which had the morphological appearance of an osteoclast. Scanning electron microscopy demonstrated numerous flattened and elongated cells adjacent to the bone surface, as well as a second cell type which had dorsal membrane specializations and numerous lateral microprojections attaching to the bone surface. The second cell type was thought to correspond to the osetoclast-like cells seen with light microscopy. The observations suggest that osteoclast-like cells differentiate from mononuclear precursor cells of hematopoietic tissue.
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