Ultramicrotomy

Ultramicrotomy is a technique used to cut ultra-thin sections of material for transmission electron microscopy (TEM) and related methods. The goal is to produce sections thin enough to transmit electrons while preserving fine ultrastructural details. Typical ultrathin sections are 50–100 nanometers thick; semi-thin sections used for light microscopy are generally 0.2–1 micrometer.

Specimens are usually fixed with chemical fixatives, dehydrated, and embedded in a resin such as epoxy or acrylic to provide rigidity. After polymerization, the resin block is trimmed and mounted in an ultramicrotome. A sharp knife cuts sections from the block; diamond knives are preferred for ultrathin work, while glass knives may be used for thicker sections. The cut sections float on a water surface and are collected on support grids for TEM.

Two main modes exist: room-temperature ultramicrotomy on resin-embedded specimens and cryoultramicrotomy for frozen-hydrated samples. Cryo methods preserve native structure but require specialized equipment and careful handling; vitreous ice sections are often used for cryo-TEM.

After sectioning, sections may be stained with heavy metals such as uranyl acetate and lead citrate to increase contrast, or processed for immunolabeling. In cryo-EM, sections are usually prepared with minimal staining or with specific labeling techniques as appropriate.

Ultramicrotomy enables high-resolution imaging of cellular organelles, membranes, and complexes, and is also used in materials science to examine polymers and nanostructures. Common challenges include knife-induced artifacts such as chatter, compression, or sectioning faults, which can limit resolution and interpretation.