Silver, usually as the nitrate, is an invaluable material in histological technique. Many structures could hardly be demonstrated without it. From alkaline, ammoniacal solution it is used to impregnate nerve cell bodies, neurones, glia cells and their processes using frozen or paraffin sections. However, likely it's commonest use in human histology is the impregnation of reticulin fibres, for which no really satisfactory dye staining method exists.
Since mildly alkaline silver methenamine solutions can be reduced by aldehydes to give results similar to those obtained with Schiff's reagent but in black, it becomes the method of choice for the demonstration of fungi and basement membranes, where fine, thin lines must be made clearly visible. It is used for staining Treponema pallidum, the causative organism of syphilis, in both blocks and paraffin processed sections, and also for some other bacteria.
Silver has been commonly used for marking the margins of specimens, the formalin reducing it to a black deposuit, easily seen microscopically. It has become common to use tattoo inks for this purpose, since they are available in different colours which can identify different margins microscopically.