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Osteoclast progenitors are seeded via the blood stream in the mesenchyme surrounding embryonic long bone models long before the appearance of multinucleated osteoclasts. The proliferation and differentiation of these progenitors in embryonic mouse metatarsal bones was studied with acid phosphatase (AcP) histochemistry and 3H-thymidine autoradiography. In vivo, tartrate-resistant, acid phosphatase-positive, mononuclear cells appear in the periosteum (AcPP-P cells) at the age of 17 days (after conception). On day 18, AcP-positive, multinucleated osteoclasts invade the bone rudiment and start resorbing the calcified cartilage matrix, resulting in the formation of the marrow cavity. The kinetics of osteoclast formation in vitro was studied in metatarsal bones of embryonic mice of different ages cultured in the continuous presence of 3H-thymidine. In young bones (15 days), mainly proliferating, 3H-thymidine-incorporating progenitors gave rise to AcPP-P cell and osteoclast formation. In older bones (16 and 17 days) osteoclasts were progressively more derived from postmitotic, unlabeled precursors. Irradiation of the metatarsal bones with a radiation dose of 5.0 Gy prior to culture resulted in a selective elimination of the proliferating progenitors, whereas the contribution of postmitotic precursors in AcPP-P cell and osteoclast formation remained unchanged. The results demonstrate that in the periosteum of embryonic metatarsal bones a shift occurs from a population composed of proliferating osteoclast progenitors (15 days) to a population composed of postmitotic precursors (17 days) before multinucleated osteoclasts are formed (18 days). Obviously, postmitotic AcP-negative precursors, already present in 16-day-old bones, differentiate into precursors characterized by tartrate-resistant AcP activity, the preosteoclasts (17 days), which in their turn fuse into osteoclasts.