Morphological and elemental integrity of freeze‐fractured, freeze‐dried cultured cells during ion microscopic analysis

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Abstract
SUMMARY: The effects of progressive ion beam bombardment on freeze‐fractured, freeze‐dried cultured cells during ion microscopic (SIMS) analysis were studied with scanning electron microscopy (SEM) and ion microscopy. The freeze‐fracture, freeze‐dry sample preparation method was generally found to preserve cell morphology to a level far exceeding the spatial resolution of the ion microscope, with splitting at the nuclear envelope being the most commonly observed artefact. SEM monitoring of surface topography of an NRK‐49F fibroblast after various ion bombardment doses showed relatively uniform erosion of cellular material, with some apparent selective retention of small cytoplasmic granules. Prolonged bombardment produced no detectable lateral elemental translocation. 41K+/24Mg+ signal ratios from Swiss 3T3 fibroblasts and RBL rat basophilic leukaemia cells were shown to vary generally by less than 10% during the course of extended ion bombardment. GM0415 human skin fibroblasts containing engorged lysosomes characteristic of Hurler's Syndrome were used to evaluate the effects of ion bombardment during a typical analysis session, where ion images of 39K+, 23Na+, 40Ca+ and 24Mg+ are sequentially recorded. This cell line was chosen as a worst‐case system, because these cells are often thinly spread and possess extreme surface topography. Thin cell edges were shown sometimes to sputter away during analysis, giving misleadingly low ion signals from these regions in some 24Mg+ micrographs. Various nonuniform sputtering phenomena occurring in the submicrometre spatial domain had little or no measurable impact on local intensities in ion micrographs, indicating that freeze‐dried, freeze‐fractured cells are sampled in a sufficiently uniform fashion that quantitative ion microscopic evaluations of intracellular elemental levels in the general cytoplasmic or nuclear regions are feasible.