A few cell types are thin enough to be viewed directly in a microscope like algae and protozoa, but most tissues such as the liver or the brain are too thick to allow light to be transmitted through them. The tissues can be sliced into very thin sections provided that they are first processed to prevent cell damage. The processing involves a series of steps, and subsequent sectioning with a microtome. One of the major delays in preparing a tissue section is the time required to dehydrate and embed the tissue. This can be overcome by direct sectioning of a frozen tissue. Typically, a piece of tissue can be quick frozen to about -15 to -20 degrees Celsius and sectioned immediately in a device called a cryostat. The cryostat is merely a microtome mounted within a freezer.

A piece of tissue is removed from an organism, placed onto a metal stub and covered with a viscous embedding compound to keep it in a form convenient for sectioning. The tissue is then placed within the cryostat and quick frozen.

This method has the advantage of speed, maintenance of most enzyme and immunological functions, and relative ease of handling as there are far fewer steps to manipulate. It has the disadvantage that ice crystals formed during the freezing process will distort the image of the cell, and the blocks tend to freeze-dry. Thus, the blocks must be used immediately and great care must be taken to guard against induced artifacts from the freezing process.

When temperature-sensitive molecules are to be studied, or where speed is of the essence, this is the preferred method. Sectioning operation with the cryostat is similar to that of the microtome, with the exception that one handles frozen sections and must be handled at reduced temperatures.