What is Paraffin Embedded Tissue?
Paraffin embedded tissue refers to a method used in histology to preserve biological tissues for examination under a microscope. The process involves infiltrating the tissue with paraffin wax, which provides support and allows for the cutting of very thin sections.
Why is Paraffin Used?
Paraffin is used because it solidifies at room temperature, providing a firm matrix that allows for the slicing of tissue into very thin sections, typically 4-10 micrometers thick. This is essential for producing clear and detailed histological slides.
How is Tissue Prepared for Embedding?
The preparation of tissue for paraffin embedding involves several steps:
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Fixation: The tissue is first fixed, often using a chemical like formalin, to preserve its structure and prevent degradation.
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Dehydration: The tissue is then dehydrated through a series of alcohol baths, removing water which can interfere with the paraffin.
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Clearing: Next, the tissue is cleared in a solvent like xylene that is miscible with both alcohol and paraffin.
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Infiltration: Finally, the tissue is infiltrated with melted paraffin, which penetrates the tissue and replaces the xylene.
What are the Advantages of Paraffin Embedding?
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Structural Integrity: Paraffin provides excellent support for thin sections, preserving the tissue's morphology.
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Long-term Storage: Paraffin embedded tissues can be stored for many years without significant degradation.
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Compatibility: Paraffin sections are compatible with a wide range of staining techniques, including immunohistochemistry.
Are There Any Disadvantages?
Yes, there are a few disadvantages:
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Processing Time: The entire process from fixation to embedding can be time-consuming, often taking several hours to days.
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Chemical Exposure: Some of the chemicals used, like xylene, are hazardous and require proper handling and ventilation.
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Loss of Soluble Substances: Some cellular components, such as lipids, can be lost during the processing steps.
How are Paraffin Sections Cut?
Once the tissue is embedded in paraffin, it is ready for sectioning. This is typically done using a microtome, an instrument that slices the paraffin block into very thin sections. These sections are then mounted onto glass slides for staining and examination.
What Staining Techniques are Used?
Common staining techniques for paraffin sections include:
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Hematoxylin and Eosin (H&E): The most widely used stain, providing a general view of tissue structure.
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Special Stains: Such as Masson's trichrome for connective tissue or Periodic acid-Schiff (PAS) for carbohydrates.
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Immunohistochemistry: Uses antibodies to detect specific proteins within the tissue sections.
What are the Applications of Paraffin Embedded Tissue?
Paraffin embedded tissues are used in various fields:
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Pathology: For diagnosing diseases, especially cancers.
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Research: To study tissue morphology and protein expression.
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Education: As teaching material in medical and biological sciences.
Can Paraffin Embedded Tissues Be Used for Molecular Studies?
Yes, although traditionally used for morphological studies, advances have made it possible to extract nucleic acids and proteins from paraffin embedded tissues. Techniques like
PCR and
Western blotting can be applied, although the quality of extracted molecules may be lower compared to fresh or frozen tissues.
Conclusion
Paraffin embedding is a cornerstone technique in histology, providing a reliable method to preserve and prepare tissue samples for microscopic examination. Despite some limitations, its advantages in terms of structural preservation, compatibility with various stains, and long-term storage make it indispensable in both clinical and research settings.
