Cryoultramicrotomy is a specialized microscopy technique used in the field of pathology and biology. It involves cutting extremely thin sections (typically less than 100 nanometers thick) of biological samples that have been frozen and hardened at very low temperatures, often using liquid nitrogen or helium.

The process begins by embedding the sample in a suitable medium, such as a cryoprotectant or a low-temperature wax, to prevent ice crystal formation during freezing. The embedded sample is then mounted on a specimen holder and cooled to a temperature below its glass transition point, typically around -150°C to -196°C.

Once the sample is frozen and hardened, it is cut using an ultramicrotome, a precision instrument that uses a diamond knife to slice the sample into thin sections. These sections are then collected on a grid or other support and can be stained with various dyes or stains to enhance contrast and visualization under an electron microscope.

Cryoultramicrotomy is particularly useful for studying the ultrastructure of biological samples, such as cells, tissues, and organelles, that may be sensitive to heat or chemical fixation methods commonly used in traditional histology techniques. It allows researchers to visualize details at the molecular level, providing valuable insights into cellular processes and disease mechanisms.