Reacciones Quimicas

What is the difference

between argentaffin and

argyrophilic reactions?

Both terms relate to the formation of colloidal

metallic silver (Latin argentum, Greek argyros)

(1), which may be range in color from yellow

through brown to black depending on

particle size and density, in specific structural

components of tissues. The metal is formed at

sites of reduction of silver ions derived from a

staining solution. These ions may be simple

Ag+, as in a solution of silver nitrate, or they

may be complex ions such as silver diammine,

[Ag(NH3)2]+ or silver methenamine,

[Ag3(C6H12N4)2]3+, which are formed

respectively by adding ammonium hydroxide

or hexamethylenetetramine to solutions of

silver nitrate. (Hexamethylenetetramine,

the reaction product of formaldehyde and

ammonia, is also used as a urinary antiseptic;

its pharmaceutical name in methenamine in

the USA and hexamine in the UK.) These

complex silver ions are more easily reduced

than Ag+. By convention, argentaffin is used

when the reducing agent is present in the

tissue. Structures are said to be argyrophilic

when bright light or a solution of a reducing

agent is applied to the object after exposure

to a silver solution.

An example of an argentaffin reaction with

silver nitrate as the reagent is the histochemical

demonstration of ascorbic acid in such sites

as the adrenal cortex and the embryonic eye.

Tiny fragments of fresh tissue are immersed

in an acidified solution of silver nitrate and

then in a solution of sodium thiosulfate

(2, 3, 4). The latter reagent removes unreduced

silver, which otherwise would be slowly

reduced by the action of light, discoloring

all

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