Xylenol orange, a fluorochrome useful in polychrome sequential labeling of calcifying tissues.

Xylenol orange is fixed in newly formed calcified tissues where it remains until removal of the bone mineral. It may be visualized in undecalcified histological sections in a similar way to the tetracyclines by means of its fluorescence. The fluorescence contrasts to that of known fluorescent dyes. Intravital bone labeling by Xylenol orange is achieved by parenteral administration of the dye. A dose of 90 mg/kg given as a 3% aqueous solution was found suitable. The general toxicity, local effect on calcification and resistance of the fluorescence to histological chemicals and long term excitation were assessed. It is noteworthy that the fluorescence of Xylenol orange is found exactly at the same site as that of tetracyclines, fluoresceins and Calcein blue. Xylenol orange is thus suitable for single or for polychrome sequential labeling.