Enhanced visualization of weak colloidal iron signals with Bodian's protein silver for demonstration of perineuronal nets of proteoglycans in the central nervous system.

The present study aimed for a clear visualization of faintly deposited colloidal iron in tissue sections for light microscopy. Paraffin blocks containing paraformaldehyde-fixed brain tissue from healthy adult mice were cut into sections 10-15 microm thick. After deparaffinization, the sections were stained with fine cationic iron colloid at a pH value of 1.0-1.5, and treated with a mixture of potassium ferrocyanide and hydrochloride for Prussian blue reaction. Some sections were further treated with Bodian's protein silver after the Prussian blue reaction. This sensitized development of Prussian blue reaction with Bodian's protein silver more clearly visualized the faintly deposited cationic colloidal irons than the demonstration by Prussian blue reaction alone, and allowed an enhanced visualization of the perineuronal nets of sulfated proteoglycans in the brain. Thus, such fine perineuronal sulfated proteoglycans as those in the CA3 field of the hippocampus, which are weakly stained with cationic iron colloid and usually overlooked by a demonstration with only a Prussian blue reaction, could be clearly visualized with striking contrast by the sensitized development with Bodian's protein silver after the Prussian blue reaction. Preliminary hyaluronidase digestion erased Bodian's protein silver development of perineuronal sulfated proteoglycans. Though some axonal fibers were also additionally stained with Bodian's protein silver itself, this sensitized development is useful to enhance such weak colloidal iron signals as are hardly detectable by only Prussian blue reaction.