Scanning acoustic microscopy reveals distinct motility domains in living cells

Abstract

This paper reports preliminary results of the observation by acoustic microscopy of living cells in vitro. The scanning acoustic microscope uses high-frequency sound waves to produce images with submicrometer resolution. The contrast observed in acoustic micrographs of living cells depends on the acoustic properties (i.e., density, stiffness, and attenuation) and on the topographic contour of the cell. Variation in distance separating the acoustic lens and the viewed cell also has a profound effect on the image. When the substratum is located at the focal plane, thick regions of the cell show a darkening that can be related to cellular acoustic attenuation (a function of cytoplasmic viscosity). When the top of the cell is placed near the focal plane, concentric bright and dark rings appear in the image. The location of the rings can be related to cell topography, and the ring contrast can be correlated to the stiffness and density of the cell. In addition , the character of the images of single cells varies dramatically when the substratum upon which they are grown is changed to a different material. By careful selection of the substratum, the information content of the acoustic images can be increased. Our analysis of acoustic images of actively motile cells indicates that leading lamella are less dense or stiff than the quiescent trailing processes of the cells. The scanning acoustic reflection microscope is a novel way of investigating biological materials with submicrometer resolution. The information content of acoustic images differs from that of optical or electron microscopic images. Acoustical images contain information about mechanical properties of the object: density, stiffness, and acoustic attenuation. An acoustic microscope with water as the coupling medium has been used to image subcellular detail in fixed cells (1). A liquid argon-coupled acoustic microscope has been used to image human meta-phase chromosomes with a resolution of 0.38 ttm (2). In this paper we present preliminary results of the observation of living cells in vitro. We have found that living cells can be viewed for several hours with no apparent disruption. The cells continue to exhibit normal patterns of motility and remain well spread on the substratum. It appears that neither the high-frequency sound waves nor the mechanical scanning cause significant cellular damage. Living cells are particularly interesting for study because little is known about the mechanical properties of a single cell and their relations to cell behavior. The acoustic microscope used in this study was developed by Lemons and Quate (3) and was modified to operate as a reflection instrument at nearly opitcal wavelengths by Jipson and Quate (4). The basic operation of a reflection acoustic microscope can be understood with reference to Fig. 1. A radio-frequency electrical pulse is used to excite a thin-film piezoelectric transducer located at one end of a sapphire crystal. The transducer generates an acoustic plane wave which propagates through the sapphire...