The lead line in bone---a lesion apparently due to chondroclastic indigestion.

The metaphyseal line of increased radiodensity which occurs in lead poisoning was studied in children and young monkeys with lead encephalopathy and in guinea pigs. The histologic lesion consists of impaired resorption of calcified metaphyseal cartilage, depressed bone deposition on cartilaginous surfaces, and the accumulation of numerous multinucleate giant cells, some containing lead inclusions. By electron microscopy, the giant cells appear to be osteoclasts and chondroclasts containing large amounts of mineralized cartilage matrix. We interpret the lead line to be the result of a lead-induced inability of cartilage-resorbing cells to degrade mineralized matrix, with a resultant impairment of metaphyseal cartilage resorption. The radiodensity of the lead line would thus be due to persistent mineralized metaphyseal cartilage and not to a primary osseous change. Some observations on lead inclusions in these cells suggest that the fibrillar component forms before the amorphous part.