SURFACE TOPOGRAPHY OF ULTRAMICROTOME SECTIONS

During ultramicrotomy the blocks of embedding medium are compressed to a certain degree by the cutting force of the knife, and a section cut under such conditions does not necessarily correspond to a flat plane through the uncompressed block. After each stroke over the knife the block resumes its uncompressed shape, and the cut surface of the block should then faithfully reflect the plane of cutting. To determine any irregularities on this surface, methacrylate, Vestopal, and Epon blocks were studied in an interference microscope using incident, monochromatic light. With this method level differences of down to 200 Å could be measured. Blocks made of unprepolymerized methacrylate exhibited the greatest level variations, up to 5 μ or more. When prepolymerized methacrylate was used, the surface irregularities were smaller, and in Vestopal and Epon blocks the level variations were usually limited to about 0.5 μ. In addition to these level variations, the blocks containing embedded biological tissue exhibited minor localized irregularities. Differences in elastic and plastic properties within the blocks presumably represent important factors for the formation of level variations. These variations may be a source of errors in the interpretation of electron micrographs. © 1969 Academic Press, Inc.