Describe the processes involved in tissue processing

Tissue processing is a crucial step in histology for preparing tissue samples for microscopic examination. The main goal is to infiltrate the tissue with a supporting medium that allows for thin sectioning. The general processes involved are:

1. Fixation: This preserves the tissue structure by preventing autolysis and putrefaction. Common fixatives include formalin and ethanol.

2. Grossing: The tissue is examined visually, and representative sections are selected for further processing.

3. Decalcification (if applicable): For tissues containing calcium (like bone), this step removes calcium salts to allow for easier sectioning. Acids like EDTA or nitric acid are used.

4. Dehydration: Water is removed from the tissue by passing it through a series of increasing concentrations of alcohol (e.g., 70%, 80%, 95%, absolute ethanol).

5. Clearing: The alcohol is replaced with a clearing agent, such as xylene or toluene, which is miscible with both alcohol and the embedding medium. This makes the tissue translucent.

6. Infiltration: The clearing agent is replaced with a molten embedding medium, typically paraffin wax. The tissue is immersed in the wax until fully infiltrated.

7. Embedding: The infiltrated tissue is placed in a mold, oriented correctly, and surrounded by molten paraffin wax. The wax is then allowed to solidify, forming a block that supports the tissue.

8. Sectioning: The paraffin block is trimmed and then cut into very thin sections using a microtome.

9. Staining: The thin sections are mounted on glass slides and stained to visualize cellular and tissue components. Common stains include Hematoxylin and Eosin (H&E).