The mechanism of silver staining.
Though much research has been devoted to the elaboration of silver stains for nervous tissue, there have been few experimental investigations of the mechanism of the staining process. Liesegang (1911) claimed that during the impregnation in the silver bath, ultramicroscopic particles of metallic silver were deposited in the sections as silver 'nuclei', and it was on these 'nuclei' that further silver deposition occurred during development. Thus the final staining of the tissue was determined by the initial distribution of the silver 'nuclei'. This theory was generally accepted until Owens & Bensley (1929) and Nageotte & Guyon (1930) suggested that the staining was more dependent on the physical properties of the tissue. Zon (1936) concluded that the differences in stainability of the tissue elements were due solely to the differences in the protective properties of their colloids, which permitted varying degrees of precipitation of silver within them. Silver (1942) thought that during the development phase of silver staining negatively charged silver micelles were formed and these were precipitated by the positive charges carried by the tissues. The pH of the silver solution was important only in so far as it affected the charge on the micelle, rendering it more or less liable to flocculation, but the pH of the developing solution was of extreme importance in so far as it affected the charge on the tissues and hence the distribution of the silver. As none of the previous opinions seemed to be based on incontestable evidence, it was considered that a further investigation into the mechanism of silver staining might throw further light on the problem.
Silver impregnation methods for reticulum fibers and reticulin: A re-investigation of their origins and specifity
SummaryMaresch (1905) introduced Bielschowsky's silver impregnation technic for neurofibrils as a stain for reticulum fibers, but emphasized the nonspecifity of such procedures. This lack of specifity has been confirmed repeatedly. Yet, since the 1920's the definition of “reticulin” and studies of its distribution were based solely on silver impregnation technics. The chemical mechanism and specifity of this group of stains is obscure. Application of Gomori's and Wilder's methods to human tissues showed variations of staining patterns with the fixatives and technics employed. Besides reticulum fibers, various other tissue structures, e.g. I bands of striated muscle, fibers in nervous tissues, and model substances, e.g. polysaccharides, egg white, gliadin, were also stained. Deposition of silver compounds on reticulum fibers was limited to an easily removable substance; the remaining collagen component did not bind silver. These histochemical studies indicate that silver impregnation technics for reticulum fibers have no chemical significance and cannot be considered as histochemical technics for “reticulin” or type III collagen.
STAINING NERVE FIBERS IN MOUNTED SECTIONS WITH ALCOHOLIC SILVER NITRATE SOLUTION
The procedures to be described have developed through experiments designed to test the effects of varying some of the many conditions involved in silver staining. The previous article 1 on staining nerve fibers in unmounted celloidin sections gave the technic from which the present method has evolved. The application of silver solutions to mounted sections with subsequent reduction of adsorbed or chemically combined silver salt is not new. Sand 2 embedded nerve tissue in paraffin after fixation in a strong solution of nitric acid in acetone and later impregnated the mounted sections. Malone 3 described a method for handling paraffin sections to produce a Cajal type of stain. Brandt and Kadanoff 4 investigated the effect of varying the time of impregnation and the concentration of the silver nitrate bath on sections from celloidin, paraffin and gelatin embedded material. They chose gelatin as best suited to the O. Schultze method. More