Analysis of birefringence and ultrastructure of spindles in primary spermatocytes of Nephrotoma suturalis during anaphase

Spindle apparatuses in crane-fly (Nephrotoma suturalis) primary spermatocytes have been analyzed with the aim of characterizing their ultrastructural organization during anaphase and of elucidating the relationship between ultrastructure and birefringence of chromosomal fibers. The results lead to several conclusions. Chromosomal fibers that are observed with the polarizing microscope are actually bundles of densely packed microtubules. During anaphase kinetochore microtubules, which extend from the kinetochores to the poles, shorten. The results are consistent with the hypothesis that microtubules are the structural basis of spindle fiber birefringence. However, changes in birefringence that occur during anaphase in living cells cannot be accounted for by changes either in the number or in the orientation of kinetochore microtubules. These findings have been interpreted as evidence for the hypothesis that the tensile forces operating between kinetochores and poles may induce strain birefringence in the microtubules of chromosomal fibers, which are maximally birefringent when tension appears to be greatest.