Ultrathin frozen sections. I. Methods and ultrastructural preservation.

A relatively simple method for obtaining ultrathin, frozen sections for electron microscopy has been developed. Tissues, cultured cells, and bacteria may be employed. They are fixed in 1.25-4% glutaraldehyde for 1-4 hr, are washed overnight in buffer at 3 degrees C, and are embedded in 20% thiolated gelatin or pure gelatin. Before sectioning they are partially dehydrated in 50% glycerol, frozen in liquid nitrogen on a modified tissue holder, and subsequently maintained at -70 degrees C with dry ice. Finally, they are sectioned very rapidly with glass knives on a slightly modified Porter-Blum MT-1 microtome in a commercial deep-freeze maintained at -35 degrees C and are floated in the trough of the knife on a 40% solution of dimethylsulfoxide (DMSO). The sections are picked up in plastic loops and transferred to distilled water at room temperature for thawing and removal of the DMSO, placed on grids coated with Formvar and carbon, air-dried, and stained with phosphotungstic acid, sodium silicotungstate, or a triple stain of osmium tetroxide, uranyl acetate, and lead. Large flat sections are obtained in which ultrastructural preservation is good. They are particularly useful for cytochemical studies.