Page 51 - Atlas of Histology with Functional Correlations

SECTION 1 Tissue Preparation and Staining

of Sections

TISSUE PREPARATION: LIGHT

MICROSCOPY

Histology is a visual as well as a very colorful science, which is studied with the

aid of a light microscope. This chapter briefly describes the methodology used in
preparing tissues for examination with microscopes and the different stains that
were used to photograph the images. Most of the illustrations in this atlas are

photographed from slides that have been prepared by the different methods
described below.

Fixation

To preserve a section of tissue or organ for histological examination, the tissue
specimen first undergoes fixation with different chemicals. Fixation
permanently preserves the structural and molecular composition of the specimen.
For light microscopy use, small pieces of the tissue specimen are immersed in
the fixative, which hardens the specimen for sectioning and causes cross-linkage

of macromolecules within the cells. This process reduces the cellular
degeneration, preserves the integrity of cells and tissues, and increases their
affinity to take up different stains that will show the composition of the

specimen. The most commonly used fixative for light microcopy is the neutral-
buffered formaldehyde.

Postfixation

After the tissue is fixed, usually overnight, water is first removed from the fixed
specimen (dehydration) and passed through a series of ascending alcohol
(ethanol) concentrations, usually from 50% to 100% ethanol. Before embedding
the specimen in paraffin (wax) for slicing it into thin sections, it is cleared of

alcohol by passing it through several changes of clearing agents such as xylene,

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