Common solid liquid
Formalin
- Definition and Composition
Formalin is an aqueous solution of formaldehyde (FA), typically containing 37% to 40% formaldehyde. It is a colorless liquid with a strong, pungent odor and is widely used in pathology and histology experiments.
- Advantages
Low Price: Formalin’s low cost makes it suitable for large-scale use, especially when processing large numbers of tissue samples.
Strong Penetration: Formalin quickly penetrates tissue, making it suitable for fixing large tissue blocks. For example, it effectively fixes whole organs or large tissue samples in pathology sections.
Long Preservation: Tissues fixed with formalin can be preserved for long periods of time, preserving the fine structure of cells and tissues. This is very practical for laboratories that need to preserve samples for long periods of time.
- Disadvantages
High Volatility: The formaldehyde in formalin is highly volatile and produces a pungent odor, which can irritate the respiratory tract and eyes of laboratory personnel. Long-term exposure may also cause health problems. Therefore, when using it, it must be handled in a well-ventilated environment and with appropriate protective equipment such as a mask, gloves, and goggles.
Impact on Antigen Detection: Formaldehyde may cross-link with proteins during the fixation process, which may mask or destroy the epitopes of certain antigens, thereby affecting the detection results of experiments such as immunohistochemistry. In such cases, de-formaldehyde treatment may be necessary to restore the activity of the antigen.
Paraformaldehyde
- Definition and Composition
Paraformaldehyde (PFA) is a polymer of formaldehyde, typically existing in solid form. It is a relatively stable formaldehyde derivative and is often used in experiments requiring long-term fixation and storage.
- Advantages
Low Volatility: Paraformaldehyde is much less volatile than formalin, making it safer to use and suitable for long-term laboratory use. This makes it the fixative of choice in many laboratories, especially when long-term fixation is required or when processing large numbers of samples.
Stable Fixation: Paraformaldehyde stabilizes tissue structure after fixation and minimizes antigenicity in cells and tissues. This makes it particularly suitable for experiments requiring antigen detection, such as immunohistochemistry, as it better preserves antigen activity.
- Disadvantages
Poor Solubility: Paraformaldehyde requires heating to dissolve, making the process somewhat complex. Typically, solid paraformaldehyde is dissolved in an appropriate amount of water, heated to around 60°C, and stirred until completely dissolved. After dissolution, it is cooled to room temperature, and a buffer is added to adjust the PH.
Relatively High Price: Paraformaldehyde is slightly more expensive than formalin, which may increase experimental costs. However, given its stability and safety, this additional cost is generally acceptable.
Other Common Fixatives
Alcohol-Based Fixatives (Ethanol, Methanol)
- Advantages
Fast Fixation: Alcohol-based fixatives (such as ethanol and methanol) offer rapid fixation, enabling cells and tissues to be fixed in a short period of time. This makes them particularly suitable for samples that require rapid processing, such as cell smears. For example, in cytology, ethanol fixation can be completed in minutes, facilitating subsequent staining and observation.
Strong Dehydration: Alcohol-based fixatives have a strong dehydrating effect, rapidly removing water from cells and tissues, making the cell structure more compact. This is highly beneficial for subsequent dehydration, clearing, and embedding steps.
- Disadvantages
May Shrink Cells: Due to the strong dehydrating effect of alcohol, cells may shrink, resulting in an unnatural morphology. This can be inconvenient in certain experiments requiring observation of cell morphology. For example, when observing fine cellular structures, shrinkage can obscure important details.
Destruction of Certain Antigens: Alcohol-based fixatives may deactivate certain antigens, affecting the detection results of experiments such as immunohistochemistry. Therefore, when choosing a fixative, the decision to use an alcohol-based fixative should be based on the specific needs of the experiment.
Heavy Metal Fixatives (such as Osmate and Acetic Acid)
- Advantages
Good Fixation of Cell Membranes and Lipid Structures: Heavy metal fixatives (such as osmate) effectively fix cell membranes and lipid structures, enhancing membrane contrast. This makes them particularly suitable for electron microscopy, allowing for clear visualization of cell membrane details. For example, in transmission electron microscopy, fixation with osmate can significantly improve the resolution of cell membranes.
Enhanced Staining: Certain heavy metal fixatives (such as acetic acid) can enhance staining, making the structure of cell nuclei and chromosomes more distinct. This is particularly useful in cytogenetic experiments, allowing researchers to better observe chromosome morphology and number.
- Disadvantages
Toxicity: Heavy metal fixatives are somewhat toxic and require strict precautions when used. For example, osmate is a strong oxidizing agent, highly irritating and toxic. Handling with protective gloves, goggles, and a mask is essential, and the work must be performed in a well-ventilated environment.
Complexity: The use of heavy metal fixatives requires stringent conditions, requiring precise control of fixation time and concentration. For example, using osmium phosphate fixation for too long may over-fix the cell structure, affecting subsequent observations.
Experimental Instructions and Suggestions
Fixative Selection
Choosing the right fixative is crucial to a successful experiment. Different experimental objectives and sample types require different fixatives. Specific recommendations are as follows:
- Select a fixative based on the experimental purpose
Pathology sections: Formalin is the classic choice because it effectively preserves the fine structure of cells and tissues, making it suitable for long-term specimen storage.
Immunohistochemistry: Paraformaldehyde is a better choice because it minimizes damage to cellular antigens and better preserves antigen activity.
Electron microscopy: Osmium phosphate is a commonly used fixative because it enhances the contrast of cell membranes and lipid structures, making it suitable for electron microscopy.
Cell smears: Ethanol or methanol fixatives offer rapid fixation and are suitable for rapid processing of cell smears.
- Consider tissue type and size
Tissue type: For lipid-rich tissues (such as brain tissue), osmium phosphate fixatives are more effective. Tissue Size: Formalin is suitable for large tissue volumes due to its strong penetrating properties; small tissue pieces or cell smears can be fixed with paraformaldehyde or alcohol-based fixatives.
Fixative Concentration and Fixation Time
Concentration: Formalin is typically used at a 10% concentration, paraformaldehyde at a 4% concentration, and ethanol at a 70% to 95% concentration.
Time: Fixation time for small tissue pieces or cell smears is typically 10 to 30 minutes; large tissue pieces may require 12 to 24 hours.
Summary
Formalin, paraformaldehyde, and other common fixatives each have their own characteristics and application scenarios. Formalin is suitable for pathological sections and large tissue volumes, while paraformaldehyde is suitable for immunohistochemistry and experiments requiring antigen retention. Alcohol-based fixatives are suitable for rapid fixation of cell smears, while heavy metal salt-based fixatives are suitable for electron microscopy.
