Paraffin embedded - Histology

Paraffin embedding is a routine technique in histology used to prepare tissue samples for microscopic examination. This process involves infiltrating tissues with paraffin wax, which allows thin sections to be cut and mounted on slides for detailed analysis. The technique is essential for preserving cellular structure and morphology, making it a cornerstone of diagnostic and research histology.

Paraffin embedding is employed because it provides a stable medium that supports the tissue during sectioning. The wax infiltrates the tissue, providing structural support, which is crucial for obtaining thin, consistent sections. This is particularly important for examining cellular details, identifying histopathological changes, and diagnosing diseases.

The process involves several key steps:

Fixation: The tissue is preserved using fixatives like formalin to prevent degradation.
Dehydration: The tissue is gradually dehydrated using increasing concentrations of alcohol to remove water.
Clearing: A clearing agent, such as xylene, replaces the alcohol, making the tissue transparent.
Infiltration: The tissue is infiltrated with molten paraffin wax.
Embedding: The tissue is embedded in a block of paraffin wax, which is then cooled to harden.

Paraffin embedding offers several benefits:

It preserves tissue morphology and cellular details.
It allows for long-term storage of tissue samples without significant degradation.
It facilitates the cutting of thin sections (4-5 micrometers) using a microtome.
It is compatible with various staining techniques, including H&E (hematoxylin and eosin) staining.

Despite its advantages, paraffin embedding has some limitations:

It is a time-consuming process, often taking several hours to complete.
Some tissue components may be lost during processing, especially lipids.
It may not be suitable for certain immunohistochemistry applications without additional steps such as antigen retrieval.

While paraffin embedding is widely used, there are alternatives:

Frozen sections: Rapid freezing of tissues allows for quick sectioning and immediate examination, often used in intraoperative consultations.
Resin embedding: Provides better preservation of ultrastructural details, typically used in electron microscopy.
Cryoembedding: Utilizes cryoprotectants and freezing for specific applications like enzyme histochemistry.

Once the tissue is embedded in paraffin, it is sectioned using a microtome to obtain thin slices. These sections are then floated on a water bath to remove wrinkles and mounted on glass slides. After drying, the sections undergo staining protocols to highlight different cellular components. Common stains include H&E, which provides general tissue morphology, and special stains like PAS (Periodic acid–Schiff) for carbohydrates.

Paraffin embedding is a fundamental technique in histology that ensures the preservation and detailed examination of tissue samples. Despite its limitations, it remains the gold standard for many diagnostic and research applications. Understanding the process, advantages, and alternatives allows histologists to choose the best method for their specific needs.