The cryostat is an instrument used to freeze human tissue samples and slice them into microscopic sections. The cryostat freezes the tissue, which allows cutting it into microscopic sections. Medical labs use cryostats to store frozen tissue samples. The microtome, a tool with an extremely sharp cutting edge mounted within a cryostat, slices through the tissue to produce pieces that are fine enough for observation by the microscope.
Freezing microtomes and cryostats are the two instruments used for preparing and maintaining frozen tissues. The key differences between a freezing microtome and a cryostat are that a freezing microtome is an instrument used to prepare thin slices of frozen tissues for microscopic studies, whereas a cryostat is an instrument that keeps a sample or apparatus placed within it at a cryogenic temperature. Similar to a standard microtome, a cryostat functions for making thin (1-10mm thick) sections of tissue, but whereas a standard microtome performs this task at room temperature, the cryostat allows an operator to slice the tissue at lower temperatures (20-30°C).
Meanwhile, cryostats utilize temperatures within the range from -150°C to absolute zero for freezing tissue samples. To chill the samples, dry cryostats or cryogen-free cryostats use mechanical chillers. Low temperatures can be maintained inside the cryostat using a variety of cooling methods, the most common being the use of cryogenic liquid baths, such as liquid helium.
A liquid-cooled cryostat uses liquid cryogens, such as nitrogen or helium, to reduce the temperature of the sample. In vacuum-insulated sample environments using liquid cryogens, the cryostat uses either helium, nitrogen, or mechanical cooling devices to cool the temperature of the samples. Instead of needing liquid cryogens, cryogen-free or dry-cryostats employ mechanical cooling to reduce a sample’s space temperature.
The lower freezing capability at -35°C means that a cryostats sample processing capability is able to freeze, slice, and section the sample in just 5 minutes, without the need for staining. The only way to get fine sections, required for immunohistochemistry applications, is by using the freeze-temp chamber (while sectioning a tissue sample).
Laboratory technicians generally employ a rotary microtome to slice frozen blocks of tissue into sections that are between 8 and 15 um (microns) thick. Although rotating microtomes are typically used for cutting frozen tissues, cryostats can accommodate almost any kind of microtome, provided that the cutting tool is not rust-proof. Newer technologies, such as the Compresstome, a type of vibrating microtome, utilize embedded agarose tissues rather than a cutting-temperature-optimized compound, eliminating the need for conventional cryostat freezing, and may be used to obtain better-quality sections.
The cryostat may be used to slice histology and tissue slides outside medicine (e.g., to localize enzymes), but sectioning quality is inferior to that in standard fixed-section wax-mounted histology. The cryostat is most critical in the preparation of tissue sections for Fourier transform infrared (FTIR) spectroscopy, where even small differences in temperature between blades and the specimen carrier cause enormous quality problems in sectioned specimens. In the event that there is not an anti-rolling plate provided with the cryostat, one could use a brush to keep the edges of a tissue sample being sectioned, and lay the tissue flat onto the blade cover.