The ultrastructure of spindle microtubules after freeze-etching and negative staining in situ

The ultrastructure of spindle microtubules in crane-fly spermatocytes was investigated with negative staining and freeze-etching techniques. Negative staining of microtubules in situ was accomplished with tannic acid, which infiltrates the intersticial spaces of the microtubule wall and, in combination with osmium, stains those regions, leaving the subunits of the microtubule relatively unstained. Microtubules are constructed of 13 protofilaments and a comparison of microtubules at several different stages of the first meiotic division shows that they do not undergo any changes of internal structure and therefore do not contract. These results and results obtained by freeze-etching monolayers of cells are consistent with earlier findings that the basic unit of microtubules has a cross-sectional diameter of about 40 and is about 80 long.