Paraffin embedding is used in histology to provide a medium to cut sections as thin as 3 microns. Paraffin embedded tissues are well preserved, with good morphology being present in archival blocks decades after the tissue was embedded. This technique allows for the use of a wide variety of colored dyes which can simplify the identification of most tissue components. Sections from paraffin blocks can be used for immunohistochemistry, in-situ hybridization, and PCR.
Frozen Section Histology
Frozen section histology provides rapid results using unfixed tissue in as close to its natural state as possible. The morphology of frozen sectioned tissue is adequate, though not as good as paraffin sectioned material. Frozen sections are useful for techniques such as: immunohistochemistry, immunofluorescence and enzyme histochemistry. Most routine histologic stains can be performed on frozen tissue as well.
Antigen retrieval is a microwave technique that is used on de-paraffinized sections. Application of this simple technique causes antigens that may have been cross-linked by the fixative to become available for binding to antibodies. In many cases the improvement in staining has been dramatic.
Step sections & Serial Sections
For some investigators a single section from a paraffin or frozen tissue block is simply not enough information. To obtain serial sections the technician slices into the block and collects every section and mounts them on the slide (or series of slides). The paraffin sections tend to adhere to each other forming long strips or ribbons. If you've ever looked at microscope slides in an embryology course, chances are you've seen serial sections. Step sections are a form of sampling, in that sections are collected at specified depths in the block (e.g., every 50 µm). Both techniques are fairly labor-intensive and can be expensive.
Commonly requested Histologic Stains
- H&E (hematoxylin and eosin):
- H&E is the most commonly requested histologic stain. The Gill's hematoxylin/eosin Y technique stains nuclei blue and cytoplasm, muscle and connective tissue pink to red.
- The trichrome stain uses a mixture of three different dyes. Tissue stained with a trichrome stain show muscle tissue, fibrin, and collagen dyed with three different colors (the colors are dependent on the variation of this technique that is used). Trichrome stains are used to differentiate between fibrous and normal tissue. This is a good stain for detecting liver cirrhosis or fibrotic changes due to inflammation.
- PAS (periodic acid Schiff):
- The PAS stain is used for staining polysaccharides. This stain is often used to stain glomeruli in kidney tissue and to demonstrate glycogen in the liver.
- Silver Stains:
- Silver stains can be used to stain a variety of structures. These stains can be used to selectively stain melanin, reticular fibers, fungus, spirochetes, or basement membranes.
- Verhoff's or Orcien Stain:
- These stains selectively dye the elastin fibers in tissue. The stains are useful for examining lung tissues or arteries to detect the loss of elasticity (e.g. emphysema or arterial thickening).
- Other Special Stains worth noting:
- Congo Red (stains amyloid), Alcian Blue (stains mucopolysaccherides), Mucicarmine (stains mucin), Cresyl Violet (stains nerve cells and glia), Giemsa or Wright's (stains hematopoietic cells), Oil Red O (stains lipids, can only be performed on frozen sections).
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Email: histo [at] u [dot] arizona [dot] edu