Comparison of bone and parathyroid hormone as stimulators of osteoclast development and activity in calvarial cell cultures from normal and osteopetrotic (mi/mi) mice

Osteoclast development was studied in cell cultures prepared from calvaria of neonatal osteopetrotic (mi/mi) mice or their normal littermates, using tartrate‐resistant acid phosphatase (TRAPase), as an osteoclast marker. In cultures from normal mice, treatment with 10 nM PTH for 4–5 days stimulated the formation of osteoclasts. However in cultures from mi/mi mice, this response was only 7% ± 5% that of normal mice and they were significantly smaller than osteoclasts of normal mice. Mineralized bone particles elicited osteoclast development in cultures from both normal and mi/mi mice, and osteoclast size was identical for both genotypes. Seventy‐eight to 96% of the TRAPase‐positive cells bound 125 I‐CT, as demonstrated by autoradiography. 125 I‐CT binding characteristics were identical in cultures from both genotypes treated with bone particles, exhibiting a K d of 3.3–3.6 × 10 −10 M. Addition of PTH stimulated 45 Ca release from the added bone particles only in the case of cultures prepared from normal mice, and CT inhibited this response. Cells from normal mice were capable of excavating bone from the surface of smooth cortical bone wafers, but such excavations were rarely seen in the case of calvarial cells from mi/mi mice. Thus, PTH‐driven differentiation of osteoclasts is arrested in calvarial cell cultures from mi/mi mice, but mi/mi preosteoclasts retain the ability to express certain osteoclast markers in response to bone derived signals. We hypothesize that the lack of activity of mi/mi osteoclasts is due to the failure of mi/mi preosteoclasts to respond appropriately to resorptive agents, or to cytokines elicited by these agents.