Ultramikrotomi

Ultramikrotomi, also known as an ultramicrotome, is a precision instrument used to cut extremely thin sections of material for transmission electron microscopy and related techniques. It enables the production of ultrathin sections typically in the tens to a few hundred nanometers thick, with thicker sections used for light microscopy.

Construction and operation: An ultramicrotome features a rigid, vibration-damped frame, a specimen block mounted on a precision stage, and a sharp blade. For ultrathin sectioning the blade is usually a diamond knife; glass knives may be used for semi-thin sections. The specimen is commonly embedded in resin (such as epoxy or Spurr’s) or prepared by cryo-fixation for frozen-hydrated samples. The device advances the block or knife in micrometer increments to generate successive sections that float on water and are collected on grids or support films.

Section thickness and specimens: Ultrathin sections are typically 50–100 nanometers thick, suitable for transmission electron microscopy. Semi-thin sections (about 0.5–1 micrometer) are used for light microscopy or for locating regions of interest before TEM. The choice of knife, resin, and cutting conditions affects section quality and artifact formation.

Sample preparation: Biological specimens are typically chemically fixed, dehydrated, and infiltrated with resin before sectioning and staining with heavy metals (e.g., uranyl acetate, lead citrate) to provide contrast in TEM. For cryo-ultramicrotomy, samples are prepared by rapid freezing and cut at low temperatures to preserve native hydration.

Cryo-ultramicrotomy: In cryogenic work, sections are cut at very low temperatures and collected on grids, enabling imaging of specimens in a near-native state with cryo-TEM.

Applications and limitations: Ultramicrotomy is essential for cellular ultrastructure studies, virology, and materials science. It requires skilled operation; artifacts such as compression, chatter, and knife marks can occur, and instrument maintenance is critical.