Page 52 - Atlas of Histology with Functional Correlations

which is miscible with both alcohol and paraffin.

After alcohol clearance and impregnation with xylene, the specimen is

placed in melted paraffin. Paraffin then infiltrates the specimen, after which it is
placed into a metal mold. The paraffin in the mold cools, solidifies, and encases
the specimen. The paraffin block is then trimmed to the size of the specimen and
mounted in an instrument called a microtome. The microtome precisely

advances the paraffin block, and the sections are cut at specific and
predetermined increments with a steel knife. For histological examination of the
specimen, the sections are normally cut at 5 to 10 μm thick. The thin paraffin
section is then collected and floated in a warm water bath to flatten and remove

any wrinkles from the sections and placed onto a glass slide that has been
covered with a thin layer of mounting medium, which adheres the specimen to
the glass slide or the slides are dried in an oven so that the specimen attaches to
the glass.

Staining of Sections

There are numerous stain-specific cell organelles, different cell types, fibers,
tissues, and organs. The thin paraffin sections that are placed on the glass slide

are colorless. To see the structural details in a given specimen, the sections must
be stained. To stain the specimen in the section, paraffin must first be dissolved
with solvents such as xylene and the sections rehydrated with a series of
decreasing alcohol concentrations. The hydrated sections can then be stained

with a variety of water-soluble stains, which selectively stain various
components of the specimen and allow visual differentiation between the
different cellular and tissue components. After completion of staining, the
specimen is again dehydrated and immersed in xylene, after which a suitable

mounting medium is put on the specimen and a thin protective glass coverslip
placed over the specimen on the slide. The coverslip allows for viewing of the
stained specimen on the glass slide with the light microscope via a light beam
that passes through the specimen attached to the glass slide.

Most of the stains used for histological slide preparations act like the acidic
or basic compounds. Structures in the specimen that stain with basic stains are
called basophilic and those that stain with acidic stains are called acidophilic.
The most common stains that are used for histological sections are hematoxylin
and eosin stains.

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