Critical‐point drying versus freeze drying for scanning electron microscopy: a quantitative and qualitative study on isolated hepatocytes

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Abstract
Critical-point drying and freeze drying were compared both quantitatively and qualitatively as preparative procedures for scanning electron microscopy [SEM]. Isolated [rat] hepatocytes were used as model cells. Nomaski differential interference contrast microscopy was used for light microscopic measurements of the hepatocytes in the unfixed, the glutaraldehyde fixed, the glutaraldehyde + OsO4 fixed, the critical-point dried and the freeze dried states. Critical point dried hepatocytes shrank to 38% of glutaraldehyde + OsO4 fixed volume, whereas optimal freeze dried hepatocytes (frozen in water saturated with chloroform and freeze dried at 183.degree. K for 84 h) shrank to 51% of glutaraldehyde + OsO4 fixed volume. Transmission and scanning electron micrographs of the critical-point dried cells showed well-preserved ultrastructure and surface structure. Micrographs of the freeze dried cells showed ultrastructure destroyed by internal ice crystals and surface structure destroyed by external ice crystals. Double-fixed isolated hepatocytes swelled during storage in buffer and shrank during storage after critical-point drying. For low magnification SEM (up to .apprx. 3000 times) both critical-point drying and freeze drying can be used. For high magnification SEM, critical-point drying is superior to freeze drying.