THE NUMBER OF TARTRATE-RESISTANT ACID PHOSPHATASE-POSITIVE OSTEOCLASTS ON NEONATAL MOUSE PARIETAL BONES IS DECREASED WHEN PROSTAGLANDIN SYNTHESIS IS INHIBITED AND INCREASED IN RESPONSE TO PROSTAGLANDIN-E(2), PARATHYROID-HORMONE, AND 1,25-DIHYDROXYVITAMIN-D-3

The culture of parietal bones from 4-day old mice in indomethacin (Ind ) for 1 day caused a large reduction in the number of tartrate-resista nt acid phosphatase positive osteoclasts (TRAP + OC) relative to both control bones and to freshly isolated bones. This reduction did not oc cur if prostaglandin E(2) (PGE(2)) was present. When 5-bromo-2'-deoxyu ridine (BDU) was injected into 4-day old mice, newly formed TRAP + OC nuclei became labeled 1 day later; these bones were then cultured with Ind for 1 day. TRAP + OC and newly labeled TRAP + OC nuclei were comm ensurately decreased in number. This suggests an active down-regulatio n rather than merely the inhibition of new TRAP + OC formation. Incuba tion of bones with Ind and either PGE(2), parathyroid hormone, or 1,25 dihydroxyvitamin D-3 for 6 hours following a 1-day preincubation in I nd, resulted in an increase in TRAP + OC compared with Ind alone. Usin g BDU labeling in vitro and in vivo, we show that this increase in num ber of TRAP + OC is not the result of cell proliferation, but rather d ifferentiation of postmitotic precursors.