Rapid-Freezing on a polished metal surface, at the temp. of liquid helium or liquid nitrogen, followed by freeze-substitution in osmium-acetone, has been shown to be a very useful technique for capturing rapid cellular events and for preserving cell ultrastructure.
*John M. Robinson and Morris J. Karnovsky, 1991. "Rapid-freezing Cytochemistry: Preservation of Tubular Lysosomes and Enzyme Activity". J. of Histochem., Cytochem. Vol. 39, No. 6, pp 787-792