Although certain epoxy resins have been demonstrated to be excellent embedding media for many materials, electron microscopists will continue, no doupt, to use methacrylate, for many purposes. If properly handled methacrylate is an excellent embedding medium and has the advantage of rapid penetration and consequently a much shorter delay before specimens can be examined in the electron microscope. A study of acrylic polymerization, however, indicates that the embedding procedure employed in most laboratories would not be expected to yield satisfactory results. Much of the tissue derangement, vacuolization, and distortion seen in the electron microscope result from incomplete penetration of the methacrylate, non-uniform polymerization or even chemical interaction of the methacrylate with tissue components. Non-uniform polymerization will cause uneven contraction and therefore shifting and tearing of fine structure. The final ratio of solid matter to plastic in cells or cell structures is directly related to the original water content. The density of tissue thus affects the cutting properties and preservation of structure. Local variations in hardness due to the tissue density or incomplete polymerization may lead to distortion, compression, or chatter during microtomy.
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