Paraformaldehyde is a commonly used tissue fixative in biological and biomedical research. It is a solid, white powder composed of a polymer of formaldehyde molecules. When paraformaldehyde is dissolved in water or buffer solutions, it rapidly hydrolyzes to release formaldehyde gas, which then reacts with tissue components to form stable cross-links, preserving the cellular and tissue structures.

Here are some key points about paraformaldehyde as a tissue fixative:

1. Fixation: Paraformaldehyde is used to fix tissues by preserving their structure and preventing decay or autolysis. It acts by cross-linking proteins, nucleic acids, and other biomolecules, essentially freezing the cellular components in their current state.

2. Cross-linking: The formaldehyde gas released from paraformaldehyde reacts primarily with amino groups of proteins, forming methylene bridges between adjacent molecules. This process helps to stabilize the tissue and prevent degradation during subsequent processing steps.

3. Preservation: Paraformaldehyde fixation is commonly used to preserve tissue samples for histological, immunohistochemical, or molecular analysis. It fixes the tissue quickly, allowing researchers to retain the morphology and antigenicity of the sample.

4. Concentration and pH: Paraformaldehyde is typically used at a concentration of 2-4% in buffered solutions. The choice of buffer and pH is crucial, as it can affect the stability of the formaldehyde solution and the quality of the fixation. Commonly used buffers include phosphate-buffered saline (PBS) and HEPES buffer.

5. Safety precautions: Formaldehyde, including the gas released from paraformaldehyde, is toxic and can irritate the skin, eyes, and respiratory system. It is essential to handle paraformaldehyde and its solutions in a well-ventilated area, wearing appropriate personal protective equipment (PPE) such as gloves and a lab coat.

6. Fixation duration: The optimal fixation time depends on the tissue type and sample size. Small tissue pieces or cells may require shorter fixation times (e.g., 15-30 minutes), while larger samples or whole organs may require several hours to overnight fixation.

7. Storage: Paraformaldehyde-fixed tissues can be stored for extended periods, provided they are properly dehydrated and stored at appropriate temperatures. They are typically stored in buffered solutions (e.g., PBS) at 4°C or -20°C to maintain their integrity.