Biopsies are tissue samples removed during surgery or routine medical procedures. In the operating room, biopsies are fixed in vials o formalin or processing and microscopic analysis in a pathology laboratory. If results of such analyses are required before the medical procedure is completed, or example to know whether a growth is malignant before the patient is closed, a much more rapid processing method is used. The biopsy is rapidly frozen in liquid nitrogen, preserving cell structures and making the tissue hard and ready for sectioning. A microtome called a cryostat in a cabinet at subfreezing temperature is used to section the block with tissue, and the frozen sections are placed on slides or rapid staining and microscopic examination by a pathologist.
Freezing of tissues is also effective in histochemical studies of very sensitive enzymes or small molecules because freezing, unlike fixation, does not inactivate most enzymes. Finally, because clearing solvents often dissolve cell lipids in fixed tissues, frozen sections are also useful when structures containing lipids are to be studied histologically.