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 pinkish-red.
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.