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Interferon γ and calcitonin induce differential changes in cellular kinetics and morphology of osteoclasts in cultured neonatal mouse calvaria

The present study compares the effects of calcitonin (CT) and interferon γ (IFN‐γ) on the size, distribution, and ultrastructure of osteoclasts in cultured neonatal mouse calvaria. The number and cross‐sectional area of osteoclasts in cultured bones was increased by the addition of parathyroid hormone (PTH) to the culture medium for 24–48 h. Prolonged treatment (up to 72 h) with PTH led to extensive rarefication and formation of holes in the mineralized matrix. PTH‐activated osteoclasts exhibited an elaborate ruffled border and showed a typical zonal arrangement of intracellular organelles with the outer cytoplasmic region containing numerous membrane‐bound vesicles. CT (20 mU/ml) within 90 minutes caused a complete loss of the ruffled border in PTH‐activated osteoclasts. The typical zonal architecture disappeared with intracellular vesicles spread through the entire cytoplasm. Prolongation of CT treatment to 24–48 h led to the appearance of vesicles with dark granular content in inactivated osteoclasts. This morphologically distinct vesicle population allowed us to identify the latter cells as “postosteoclasts” and thereby to distinguish them from osteoclasts precursors. Small active osteoclasts that had originated very likely from these precursors appeared in calvarial bones coincidentally with escape from CT inhibition of bone resorption and resulted in increased total number of osteoclasts. In contrast, IFN‐γ (500 U/ml) acting as a proliferation inhibitor, reduced the total number of osteoclasts. In the presence of PTH it caused no immediate (90 minutes) change in the ultrastructure of PTH‐induced osteoclasts apart from an increase in the number of autophagic vacuoles. After prolonged exposure (24–48 h) in the presence of PTH, osteoclasts with low resorbing activity exhibited intermediate borders at their contact zone with the mineralized matrix. In the absence of PTH, the short‐term effect of the immune interferon on osteoclast morphology was almost comparable to that of CT. After prolonged treatment in the absence of PTH, postosteoclasts comparable in vesicle population and size to those after CT treatment were found.