Palladium chloride as a stain for elastin at the ultrastructural level.

Palladium chloride in aqueous solution stains elastic fibers in thin sections of Epon-embedded tissues. When palladium chloride is used with a lead citrate counterstain, high contrast sections with gray to black elastic fibers are obtained. The stain was tested on newborn and adult mammalian tissues and on adult tissues from lower animals. Sections were mounted on stainless steel grids, stained with 1% palladium chloride solution for 5 to 15 min, rinsed thoroughly, and counterstained with lead citrate for 7 min. Palladium chloride staining solution is stable for several months at room temperature and if the stain is filtered immediately before use, contamination of sections is not a problem. Chemical studies indicate that palladium binds directly to purified bovine ligamentum nuchae elastin and that this binding is not affected by glutaraldehyde fixation or by sodium borohydride reduction of elastin. Osmium post-fixation of glutaraldehyde-fixed elastin did significantly lower the amount of palladium bound. Palladium was shown to be chemically bound to sites on the elastin and not weakly associated. The nature of these sites is discussed.