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 parts of the tissue. This slower reduction,

which forms the basis of several traditional

methods for showing intercellular clefts and

spaces (5), is an argyrophil, not an argentaffin

reaction, because the light serves as an

external reducer, acting probably on silver

chloride precipitated in extracellular fluid. The

same holds for von Kossa’s stain for calcified

material. In this, silver ions from aqueous

AgNO3 displace calcium from deposits of

calcium carbonate or phosphate, forming

insoluble silver carbonate or phosphate,

which is quickly decomposed by the action of

light to form black colloidal silver.

The only argentaffin reaction of importance in

histopathology is the reduction of an alkaline

solution containing silver diammine in the

Masson-Fontana stain, which detects small

quantities of melanin and, in suitably fixed

tissues, the amines derived from tyrosine

and tryptophan: dopamine, noradrenaline,

adrenaline and serotonin (6). Most types of

melanoma and some tumors of intestinal

endocrine cells give positive Masson-Fontana

argentaffin reactions (7, 8).

Nearly all the cell-types of the diffuse endocrine

system (DES) are argyrophilic (9, 10), as are

the axons of neurons and reticulin, a term that

applies to thin collagen fibers and basement

membranes. The technical details of argyrophil

silver methods vary with the structures to

be demonstrated, but in all cases there are

at least two steps. In the first, which is often

called impregnation, the tissue (whole pieces

or sections) is exposed to a silver-containing

solution. Silver ions bind non-specifically to

the tissue and at certain significant sites some

of the ions are reduced to silver

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