Analysis of periodic structure in model subcellular macromolecular arrays by Fourier processing of single line video signals in scanning transmission electron microscopy.

Abstract

Fourier processing of single line video signals in scanning transmission electron microscopy (STEM) is a convenient and relatively objective method for the demonstration and characterization of the periodic structure present in thick sections of biological material where order cannot be readily determined by methods such as optical diffraction of transmission electron micrographs (TEM). Following alignment of the STEM scan vector with a prominent specimen axis, the video line signals, as visualized and focused in y-mode on the video monitor, were usually recorded on a digital signal processor for evaluation. The STEM line was frequency filtered, amplified, digitized, and subjected to fast forward Fourier transformation (FFT). Frequency space was examined by producing a power spectrum. Reverse Fourier transformations of the FFT, with appropriate frequency windowing, allowed separation of inherent specimen periodicities from background noise arising from various irregularities in the specimen and other sources. Pursuant to our studies on the neuroplasmic lattice of axons in neural tissue, we present here the results obtained with three model systems having spacings in the range of most interest with respect to subcellular macromolecular arrays (5-100 nm). Systems chosen were: 1) the lamellar structure of myelin sheath in cross section, 2) the lattice structure of tropomyosin crystals in thin sections, and 3) the cross-bridge lattice in longitudinal sections of squid mantle muscle fibers. These model tests show that information regarding periodic structure of materials can be obtained from economical single line scans rather than whole picture analysis.