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The ultrastructure of osteoclasts was examined in fetal rat bones after stimulation or inhibition of resorption in culture . A central ruffled border area completely encircled by a clear zone was considered to represent the resorbing system of the cell. The proportion of ruffled border and clear zone in osteoclast cross sections was compared with changes in bone resorption as measured by the release of previously incorporated radioactive calcium ("Ca) . In control cultures 55% of the osteoclast cross sections showed an area closely apposed to bone and this consisted mainly of clear zone ; only 11 % showed ruffled borders . Treatment with parathyroid hormone (PTH) increased 45Ca release, increased the frequency of finding areas closely apposed to bone (79%), and markedly increased the frequency of the ruffled border area (64%) .Colchicine given concurrently with PTH decreased the number of osteoclasts . Colchicine or calcitonin treatment after PTH stimulation decreased the proportion of ruffled border area significantly by 1 h ; this was followed by a decrease in 45Ca release . These inhibited osteoclasts resembled osteoclasts from control, unstimulated cultures, suggesting that the cells had returned to their inactive state .Colchicine-treated osteoclasts also showed a loss of microtubules and a massive accumulation of 100 Â filaments, suggesting that synthesis of microtubular subunits had increased .3 46

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We administered high doses of calcitriol (up to 32 micrograms per day) to an infant with malignant osteopetrosis, in an attempt to stimulate bone resorption. The patient was placed on a low-calcium diet to prevent hypercalcemia. Measures of bone turnover increased during calcitriol therapy; hydroxyproline excretion rose from 140 to 1358 micrograms per milligram of creatinine per 24 hours, with parallel increases in the ratio of calcium to creatinine in the urine, urinary gamma-carboxyglutamic acid, serum osteocalcin, and serum alkaline phosphatase. A pretreatment bone-biopsy specimen contained no osteoclasts with ruffled borders, a feature of active osteoclasts. After 11 days of calcitriol, ruffled borders were noted. After three months, numerous osteoclasts with ruffled borders and associated bony disruption were evident. Before therapy, the patient's monocytes were incapable of in vitro bone resorption, but after calcitriol, their resorptive capacity was increased to 3.3 times control levels. These data demonstrate that calcitriol increased bone mineral and matrix turnover in our patient. However, during the three months of calcitriol therapy there was only slight clinical improvement in her severe disease. Early and sustained treatment with calcitriol may be useful in osteopetrosis.

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