Epoxy resin embedding is a fundamental technique in transmission electron microscopy (TEM), used to preserve and analyze biological structures and materials at the ultrastructural level. This process enables the preparation of ultrathin sections, essential for studying cellular and molecular details with unparalleled resolution. In this article, we will guide you step-by-step through the epoxy resin embedding protocol, explaining the purpose of each stage and its importance in sample preparation for TEM.
1. Fixation with Glutaraldehyde
The initial fixation stabilizes cellular structures and prevents degradation. Glutaraldehyde reacts with amino groups in proteins, forming cross-links that preserve cellular morphology.
- Immerse the sample in a 2–4% glutaraldehyde solution in 0.1 M phosphate buffer (pH 7.4).
- Incubate for 2–4 hours at 4 °C.
2. Postfixation with Osmium Tetroxide (OsO₄)
OsO₄ fixes lipids and enhances contrast by acting as an electron-dense agent, facilitating the visualization of cell membranes under TEM.
- Transfer the sample to a 1–2% OsO₄ solution.
- Incubate at room temperature (20–25 °C) for 1–2 hours.
3. Dehydration
Removes water from the sample, making it compatible with hydrophobic resins like epoxy.
- Perform a graded ethanol or acetone series (30%, 50%, 70%, 90%, 100%).
- Immerse the sample in each concentration for 10–15 minutes with gentle agitation.
- Perform two final changes with absolute ethanol for 20 minutes each.
4. Clearing
Replaces residual ethanol or acetone with an intermediate agent, such as propylene oxide, which is miscible with both organic solvents and epoxy resins.
- Immerse the sample in two consecutive baths of fresh propylene oxide for 15 minutes each, under constant agitation.
5. Infiltration with Resin
Fully impregnates the sample with epoxy resin to ensure rigidity and stability during ultrathin sectioning.
- Prepare a resin:hardener:accelerator mixture in a ratio of 10:8:0.2.
- Perform progressive infiltration:
- Immerse the sample in a 2:1 mixture (propylene oxide:resin) for 1 hour.
- Continue with a 1:1 mixture for another hour.
- Finish with a 1:2 mixture for 1 hour.
- Switch to pure resin, performing 2–3 changes for small samples (every 2–4 hours) or 8–12 hours for larger samples.
- Allow any remaining traces of propylene oxide to evaporate before proceeding to polymerization.
6. Polymerization
Hardens the resin through heat, producing a solid block suitable for ultrathin sectioning.
- Place samples in gelatin capsules or molds pre-coated with dimethyldichlorosilane to facilitate demolding.
- Incubate at 60 °C in a convection oven for 24–48 hours.
7. Obtaining Ultrathin Sections
Produces thin sections (50–90 nm) required for TEM analysis.
- Block trimming: Use a glass knife to expose the desired surface area of the block.
- Sectioning: Adjust thickness using an ultramicrotome and collect sections on Formvar-coated copper grids.
This protocol ensures optimal results by preserving structural features and enhancing contrast in biological or material samples. If you're interested in advanced techniques for electron microscopy, keep exploring the blog!
⚠️ This content is educational. Practical implementations must be carried out exclusively by qualified personnel in authorized locations and following all relevant safety regulations.