Calcitonin receptor expression and its regulation by 1 alpha-25-dihydroxyvitamin D3 during de novo osteoclast formation in organ cultures of fetal mouse metatarsals.

Organ cultures of 15-day embryonic mouse metatarsals cultured in serumless, chemically-defined medium were used to investigate the influence of 1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) on calcitonin receptor (CTR) expression in osteoclasts formed from in situ progenitors. CTR expression was demonstrated in tartrate-resistant acid phosphatase-positive (TRAP+) osteoclasts and their precursors. The expression of TRAP preceded that of CTR and was first observed in cells identified as osteoclast precursors. Not all TRAP+ precursors were CTR+ and from 40 to 60% of newly formed osteoclasts lacked CTR. Treatment with 1,25(OH)2D3 increased precursor numbers without affecting CTR expression, whereas both the numbers and the percentage of newly formed osteoclasts expressing CTR was increased by 1,25(OH)2D3. Grain count analysis revealed that this increased percentage of newly formed CTR+ osteoclasts were cells with low and medium levels of CTR expression. This apparent down-regulation of CTR expression in newly formed osteoclasts may help to explain the 'escape' of osteoclastic bone resorption from the inhibitory effects of calcitonin.