Freezing damages cell membranes and reduces the histological readability of biological specimens. This is a discussion of why, the consequences, and how to minimize or avoid the damage introduced when using freezing as the means to harden biological tissue in order to cut thin sections for histology.
Pure water can exist in the solid state in three forms. Two forms are crystalline: one with a hexagonal lattice and the other with a cubic lattice. The third form capitalizes on the fact that water can be frozen so rapidly that it does not have time to form a crystal lattice and remains amorphous (vitreous form). Crystal formation is responsible for the expansion of water as it freezes (a property unique of water), which creates specimen preparation artifacts. Vitreous ice does not expand upon solidification. This makes it the only desirable form of ice appropriate for biological specimens.