Silver nitrate and silver proteinate were both successfully used as electron stains for the demarcation of the aortic endothelial cell borders in the scanning electron microscope.

By energy-dispersive X-ray analysis it was demonstrated that silver was present in the demarcation lines around the endothelial cells. The presence of silver closely coincided with the places where secondary electrons were produced.

Judged by the quality of the aortic tissue in the transmission electron microscope, the method using silver proteinate was preferable to those using silver nitrate. The limited resolution of the method is demonstrated.

The role of the dextran solution in such procedures was investigated, and it was shown that the presence of such a dextran solution, or its particle size or charge, does not contribute to the formation of these silver lines around the endothelial cells. In ultrathin sections of such material the cell surface-contrasting capacity of colloidal thorium dioxide solutions was compared to the cell border-contrasting capacity of silver proteinate. It was found that prior aldehyde fixation abolished the cell border demarcation by silver proteinate, but only reduced the cell surface demarcation by thorium dioxide particles.

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