Identification of the Mineralization Front: Comparison of a Modified Toluidine Blue Stain with Tetracycline Fluorescence

A modified toluidine blue method for identification of the mineralization front at the zone of demarcation between bone and osteoid in undecalcified, plastic-embedded sections of bone is described. The intensity of staining is increased both by increasing the pH of the solution and by increasing the duration of staining. The method is reproducible, since measurements of the extent of mineralization front on two nonadjacent sections from the same biopsy in 20 cases had a correlation coefficient of 0.98. The identification of the mineralization front by toluidine blue agrees closely with identification by means of in vivo tetracycline uptake, with a correlation coefficient of 0.97 between measurements of its extent using the two methods on adjacent sections from the same biopsy. It is likely that both toluidine blue staining at pH 6.5 and tetracycline uptake depend on some property of the most recently deposited bone mineral. With either method we find low values for the extent of mineralization front as a fraction of osteoid surface in many patients with osteoporosis. The uniformly normal values for this quantity in osteoporosis reported by other investigators may reflect different criteria for distinguishing osteoid-covered from quiescent bone surfaces.