The use of dimethylsulfoxide for fixation of yeasts for electron microscopy.

Conventional methods of chemical fixation are often inadequate for preserving yeast ultrastructure. The thick cell wall severely limits penetration of fixatives rendering poor detail of the cell wall, membranes, and overall anatomy. Dimethylsulfoxide (DMSO) enhances penetration of chemicals and has been added to fixatives to improve cell preservation. At high concentrations (5 to 50%), however, it affects ultrastructure unpredictably. We found that adding 0.1% DMSO to fixatives greatly improved retention of yeast ultrastructure. Candida albicans, C. glabrata and Aspergillus fumigatus were fixed for 3 hr in 3% paraformaldehyde, 1% glutaraldehyde, 1 mM MgCl2, 1 mM CaCl2, 0.1% DMSO in 0.1 M sodium cacodylate buffer followed by 1% OsO4, 1% K2Cr2O7, 0.85% NaCl, 0.1% DMSO in the same buffer. Thin epoxy sections were post-stained in uranyl acetate and lead citrate. The multilayered character of the cell wall was distinct and well structured. Addition of ruthenium red or alcian blue to the fixatives further enhanced the outer fibrillar layer. The plasma membrane was contiguous and tightly adjacent to the inner mannoprotein layer of the cell wall. The cytoplasm was well preserved and the overall preservation of the yeast ultrastructure was significantly improved.