Why Biological EM Sample Preparation Is a Science in Itself

Unlike materials science samples, biological specimens are inherently soft, hydrated, and unstable under the high vacuum and intense electron beams of an electron microscope. The goal of sample preparation is to preserve the sample's native ultrastructure as faithfully as possible while rendering it compatible with EM conditions. Errors at any stage can introduce artifacts that mislead interpretation. This guide walks through the essential steps for conventional TEM of biological material.

Step 1: Primary Fixation

Fixation is the most critical step — it must stop cellular processes and crosslink proteins to preserve structural relationships. The standard approach uses glutaraldehyde (typically 2–4% in 0.1 M sodium cacodylate or phosphate buffer, pH 7.2–7.4). Glutaraldehyde crosslinks proteins via reactive amine groups, providing excellent structural preservation.

Key considerations:

  • Fix as rapidly as possible after dissection — ideally within seconds for small tissue pieces.
  • Keep samples cold (4°C) during fixation to slow enzymatic degradation.
  • Tissue pieces should be no larger than 1 mm³ to allow fixative penetration.
  • Fixation time varies from 1 hour to overnight depending on tissue density.

Step 2: Secondary Fixation with Osmium Tetroxide

Osmium tetroxide (OsO₄) at 1–2% in buffer provides secondary fixation and, critically, stains lipid membranes by reacting with unsaturated fatty acids. This is what makes cellular membranes visible in TEM. OsO₄ is highly toxic and volatile — it must be handled exclusively in a well-functioning chemical fume hood with appropriate personal protective equipment.

Secondary fixation typically takes 1–2 hours at room temperature or 4°C and provides the characteristic electron density contrast of membrane structures.

Step 3: Dehydration

Water must be removed before resin infiltration. Samples are passed through a graded series of ethanol or acetone solutions:

  1. 30% ethanol — 10 minutes
  2. 50% ethanol — 10 minutes
  3. 70% ethanol — 10 minutes (can be a holding step at 4°C)
  4. 80% ethanol — 10 minutes
  5. 90% ethanol — 10 minutes
  6. 95% ethanol — 10 minutes
  7. 100% ethanol — 3 × 15 minutes (anhydrous)

Gradual dehydration prevents osmotic shock-induced structural distortion. Rushing this step is a common cause of shrinkage artifacts.

Step 4: Resin Infiltration and Embedding

Samples are infiltrated with a liquid epoxy resin (commonly Epon 812, Spurr's, or LR White, depending on application) through a series of resin:solvent mixtures before being placed in pure resin and polymerized at 60°C for 24–48 hours. The result is a hard plastic block containing the biological sample in a solid matrix.

Spurr's resin is preferred for very small or dense samples (bone, plant tissue); LR White is used when downstream immunolabeling is required, as it better preserves antigenicity.

Step 5: Ultramicrotomy

The embedded block is trimmed and sectioned using an ultramicrotome fitted with a glass or diamond knife. Sections for TEM are typically 60–100 nm thick (ultrathin sections), which appear silver-to-gold in reflected light on the water trough of the knife. Sections are collected onto copper grids (300 mesh is standard) or specialty grids for specific applications.

Ultramicrotomy is a highly skilled technique — consistent section thickness and the ability to section specific regions of interest come with practice and careful block trimming.

Step 6: Post-staining

Ultrathin sections on grids are stained to increase electron density contrast:

  • Uranyl acetate: Stains nucleic acids and proteins; applied first, typically 10–20 minutes.
  • Lead citrate: Stains membranes and glycogen; applied after uranyl acetate, 5–10 minutes. Must be prepared CO₂-free to prevent precipitate formation.

Cryo-EM: An Alternative Approach

Modern cryo-electron microscopy (cryo-EM) bypasses chemical fixation entirely by vitrifying samples in liquid ethane at –196°C. This preserves native structure without chemical crosslinking artifacts and is now the method of choice for protein structure determination. However, it requires specialized equipment and expertise beyond the scope of conventional TEM preparation.

Common Pitfalls to Avoid

  • Delayed fixation — the single biggest source of poor-quality biological EM data.
  • Inadequate buffer matching — always match osmolarity to the tissue type.
  • Contaminated OsO₄ — reduced osmium gives poor membrane contrast.
  • Incomplete dehydration — residual water causes resin infiltration failure.
  • CO₂ contamination of lead citrate — produces dark precipitate on sections.

Mastering biological EM sample preparation takes time, but systematic attention to each step produces the kind of ultrastructural data that advances understanding of cell biology at its most fundamental level.