between the acid dyes (so called "neutral" dyes). In this way, the staining principles of
the original components were preserved; and, in addition, new staining properties
dependent upon the union of the component dyes were developed. These were,
therefore, termed polychromic dyes.
e. One modification of these polychromic stains is Wright's stain. This is the
stain most used in Army laboratories today. Wright's stain is a methyl alcohol solution
of an acid dye and a basic dye. The acid dye is known as eosin, which is red in color.
The basic dye, methylene blue, is blue in color. The white cells are mostly identified by
their preference for these dyes. In some cases the cells are even named for the dye
that they prefer. For example, cells that prefer a mixture of the acid and basic dye are
called neutrophils. In the staining process, a buffer solution is used to control the acid-
base balance of the stain. This is a most important function. If the buffer solution is too
acid it makes the acid dye too bright and the basic dye too faint. On the other hand, if
the buffer solution is too basic it makes the basic dye too bright and the acid dye too
faint. In either case, the result is a poorly stained slide. The acid-base balance of a
solution is measured by its pH value. A buffer solution should have a pH value between
6.4 and 6.8. This allows the best color contrast between acid and basic dyes.
f. When optimal staining conditions exist, Wright's stain is very satisfactory and
easily differentiates cells. The eosin component stains cell cytoplasm, and the
methylene blue component stains nuclear material, granules, and inclusions. Both
stains oxidize rapidly because they are in alkaline solution. Giemsa, a purified
polychrome stain, is added to compensate for this defect by maintaining the azurophilic
staining property of the mixture.
a. Principle. A small drop of blood is placed near one end of a clean glass
slide. Using a second slide as the spreader, the blood is streaked into a thin film and
allowed to dry. It is then fixed and stained with modified Wright's stain.
Venipuncture or finger puncture material.
Clean glass slides.
Wright's stain buffer.
(a) Solution A. Dissolve 9.47 grams disodium phosphate (Na2HPO4)
(dibasic), anhydrous, in a 1-liter volumetric flask containing about 750 ml of distilled
water. Dilute to the mark with distilled water.