Appearances of microtubules after various fixative procedures, and comparison with the appearances of tobacco mosaic virus.

Microtubules in crane-fly spermatids appeared altered when the glutaraldehyde-fixed cells were not postfixed with osmium tetroxide. The cytoplasmic microtubules were altered more than the doublet microtubules. Addition of osmium tetroxide after dehydration did not produce appearances identical with those of microtubules postfixed directly after glutaraldehyde, and thus at least some alterations occurred during dehydration, possibly due to extraction of microtubule-associated lipid. The omission of osmium tetroxide postfixation did not cause drastic alterations in the appearances of either tobacco mosaic virus (TMV), or polymerized tobacco mosaic virus protein (without RNA), suggesting that microtubule stability is different from TMV stability (with respect to the embedment procedure). The electron-dense stain associated with embedded-sectioned TMV is predominantly outside the TMV protein, as demonstrated by the known distribution of TMV protein compared with the dimensions of sectioned TMV and negatively stained TMV. The same might hold true for microtubules, as evidenced by the dimensions of negatively stained, isolated brain microtubules compared with those of embedded and sectioned brain microtubules.