176 Silver Impregnation I (chap. 14) 



rapid reduction by formaldehyde, thereby inducing the formation of 

 finer-grained deposits of silver.) The next step was experimentation 

 ^vith silver on tissue sections, leading to a literature full of modifications 

 of Ramon y Cajal, Bielschowsky, and others. 



Ammoniacal silver is the familiar complex, and when reduced by 

 formaldehyde to metallic silver forms a colloidal solution containing 

 negatively charged particles. These may be precipitated out by oppo- 

 sitely charged surfaces which can be changed either to repel or to absorb 

 the siher. Thus, the negatively charged silver (formed by reduction) is 

 deposited on positively charged surfaces and allows selective impregna- 

 tion of neurofibrils, reticultim, Golgi, etc. The charges of tisstie ele- 

 ments may be effected by the fixation and dehydration coagulating the 

 proteins and leaving them positively or negatively charged. The pH of 

 the silver solution is a strong factor in determining charges and depends 

 on whether an ammonium or sodium hydroxide or sodium or lithium 

 carbonate is used (Ramon y Cajal and del Rio-Hortega methods). 



As already mentioned, protective colloids, such as gimi arable and 

 mastic, and the use of protargol slow down the reduction of the silver 

 to produce a finer grain (Liesegang method). Dilute reducing reagents, 

 combined with the above, can have the same effect (Ramon y Cajal and 

 Bielschowsky methods). Temperature is a factor in this respect, because 

 it increases the kinetic energy of the particles and permits a greater 

 number of collisions of the particles against the tissue surfaces. In some 

 methods, copper is added to the silver solution supposedly to speed up 

 impregnation by initiating the reduction to metallic silver. At the same 

 time too heavy a deposit is prevented by removal of some of the silver 

 from the sohuion. Thus various applications of the principles can be 

 used to control the impregnation of different kinds of tissue elements 

 (Silver, 1942). Chemical properties of tissties and their responses to these 

 conditions all help to determine the place and amount of deposition. 



The types of silver impregnation (all ammoniacal) can be classified in 

 the following manner: (1) ammoniacal silver nitrate, (2) ammoniacal 

 silver hydroxide, and (3) ammoniacal silver carbonate. In all of these 

 solutions the silver is present largely in the form of a complex silver 

 ammonia cation [Ag(NHH)2] + . In (1) ammonia alone is used to form the 

 precipitate, the chief prodtict in solution being silver diammino nitrate. 

 In (2), the Bielschowsky method (also modified by Ramon y Cajal), 

 sodiinn hydroxide is used to form the precipitate and ammonia to re- 

 dissolve it, the chief product being silver diammino hydroxide. The 

 difference between the Bielschowsky and the Ramon y Cajal methods 

 lies in the way the silver is applied and the reduction is performed. The 



