Heavy metal | Staining solution | Binds to | Staining considerations |
---|---|---|---|
Iodine * [38] | Iodine potassium iodide in water, ethanol or methanol. Various concentrations | Non-specific staining. Preferentially binds to lipids and glycogen. Good for muscle fibres, nerve myelin sheath, connective tissues and the eye lens | • Rapid and deep tissue penetration • Particularly suited to larger specimen (> 2 mm) • An effective stain for flower parts, allowing counting of pollen ovules [39] • Has been used to study water transport in plants [35] |
Tungsten and Molybdenum* [40] | Phosphotungstic acid Phosphomolibdic acid | Proteins including fibrin and collagen. Suited to connective tissues | • Moderate tissue penetration rate and depth • Can provide more detailed visualisation compared with iodine-based stains • Phosphotungstic acid also effective in plants [41] |
Osmium* [42] | Osmium tetroxide, as used in electron microscopy | Lipids including those in cell membranes, some proteins and nucleic acids | • Tissue penetration is slow and can be limited (not suited to samples requiring a penetration depth greater than 1–2 mm) • Highly toxic, requiring special safety considerations |
Indium [43] | Gallocyanin-chromalum | Cell nuclei. Can show cell density and individual cells | • A histology stain • Low contrast overall |
Iodine, aluminium and iron [44] | Verhoff’s Stain | Arterial walls of the vascular network | • A trichrome histology stain |