Ultrastructural characterization of preosteoclasts derived from bone marrow progenitors stimulated by osteoclast...

Osteoclast colony stimulating factor (O-CSF) is an osteoclast-specific growth factor that stimulates the clonal growth of primitive osteoclast progenitors from bone marrow cells in culture. To characterize the morphology of immature osteoclasts (preosteoclasts) arising from complex hematopoietic tissues, progenies of O-CSF-responsive progenitors were cocultured with devitalized calvariae, and their cytochemical and ultrastructural features were examined.

Murine bone marrow cells were cultured in semisolid medium for 14 days in the presence of O-CSF. Colonies derived from osteoclast progenitors were then cocultured with devitalized mouse calvariae for 5 days. Cells attached to the calvariae were stained for tartrate resistant acid phosphatase (TRAPase), and the ultrastructure of these cells was examined by transmission electron microscopy. Bone marrow cells stimulated by macrophage colony stimulating factor (M-CSF) were similarly studied as a control.

O-CSF-induced preosteoclasts stained strongly positive for TRAPase when cocultured with calvariae. These cells showed single nuclei, and their cytoplasm contained numerous mitochondria, vacuoles, granules, and coated vesicles. The ruffled cell border consisted of short, blunt, fingerlike projections. The adjacent clear zone contained abundant microtubules, microfilaments, and long narrow channels. M-CSF-induced macrophages were TRAPase negative, with no ruffled borders or clear zones.

All the characteristic features of active osteoclasts were observed in the cells derived from O-CSF-responsive bone marrow progenitors except that these cells were mononucleated, and their ruffled borders were not fully convoluted, indicative of their immature nature. This study documents for the first time the ultrastructural characteristics of preosteoclasts derived from cultured bone marrow progenitors in early stages of development.