Ultrathin frozen sections of biological tissues for electron microscopy provide certain advantages in cytochemical studies in which the penetration of cells by large molecules is necessary and in morphological studies of cellular constituents which are dissolved by the reagents employed in routine plastic embedding. The recent introduction of several types of commercially available cryo-ultramicrotomes makes it possible for many laboratories to employ this valuable tool. This paper summarizes recent improvements in the methods developed in this laboratory for preparing ultrathin frozen sections and reviews some of the inherent problems involved in their use. These procedures may serve as a baseline for other investigators who can then modify or adapt them for their specific purposes.

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