A Note About Tissue Preparation in Histology
To prepare living tissues for microscopic examination,
a number of steps are necessary. First
of all, the tissue is fixed with a preservative (such as formalin).
The specimen is then imbedded in a block of paraffin or a plastic
material. A cutting instrument called a microtome slices the hardened block
of tissue into wafer-thin sections, which are now thin enough to transmit
light for microscopic use.
Unfortunately, living tissues are colorless or
transparent under a microscope. Without
a little help, it would be difficult or impossible to distinguish structural
details. After a section of
tissue is placed on a microscope slide, it is stained with different
chemicals to color the various elements of tissue cells.
One of the most useful of these is hematoxylin and
eosin (H+E). This preparation
gives different colors to tissues, depending upon the biochemical molecules
present in the cells. For
example, the chromatin of a cell nucleus stains dark purplish-blue with
hematoxylin, whereas eosin stains the cytoplasm and its organelles a
For this reason, most tissue preparations stained with
H+E (as with most of the slides on this Web site) have dark blue nuclei and
reddish cytoplasm. We will
often comment about the appearance or location of the nuclei, or about the
abundance or lack of cytoplasm.
The H+E stain is why most of the slides on this Web
site appear to be “pink and blue” in color.
For special purposes, we will occasionally use certain stains that
bring out unique features, such as the myelin sheath in nerves or the islet
tissue of the pancreas.
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