GOLGI-COLONNIER METHOD - CORRELATION OF THE DEGREE OF CHROMIUM REDUCTION AND PH CHANGE WITH QUALITY OF STAINING

We examined the role of chromium reduction in the Golgi-Colonnier meth od, correlating the quality of neuronal impregnation with the levels o f hexavalent (Cr-VI) and trivalent (Cr-III) chromium in the tissue and in the chromation fluid (CF). The concentrations of both chromium spe cies were assessed by measuring spectrophotometrically the Cr-VI befor e and after oxidizing the sample and by calculating the ratio of Cr-VI to total chromium (chromium ratio, CrR). The CrR was almost identical in the tissue and the CF decreasing exponentially during chromation d ue to a progressive consumption of Cr-VI to form Cr-III. Satisfactory cell impregnation was obtained only when the CrR was 0.45-0.7, regardl ess of other factors. The CrR values could be accurately predicted by the pH increase of the CF; this increase has proven to be a most relia ble criterion to decide the endpoint of the chromation process. The de pendence of cell staining on the [Cr-III], together with the well-know n ability of this species to bridge proteins, suggests that the key ev ent for cell impregnation is the cross-linking of neuronal proteins by Cr-III polymers.