Histochemical demonstration of unsaturated hydrophilic lipids with palladium chloride

Compounds in which olefinic linkages are accessible to aqueous reagents reduce the chloropalladite ion [PdCl4]2-, to metallic palladium. This reaction is used in a histochemical method whereby hydrophilic unsaturated lipids are stained dark brown or black. The specificity of the new method has been confirmed by means of solvent-extraction and chemical blocking procedures and by comparison with other histochemical techniques. Yellow staining of collagen, keratin and cytoplasm is probably due to attachment of the chloropalladite anion to proteins. The yellow background can be largely decolorized by treating the sections with aqueous pyridine, which forms colorless complexes with divalent palladium. A standard technique for staining with palladium is presented and the method is discussed in relation to other histochemical procedures that demonstrate unsaturated lipids.