Shape of microtubules in solutions

Although several authors have presented dark-field micrographs of axonemes or of outer doublet microtubules from sperm tails, singlet microtubules have not been observed individually by light microscopy. This study demonstrates the technical possibility of observing individual microtubules by dark-field microscopy. While polymerized brain microtubules were always observed to be straight, the outer doublet microtubules from sperm tails assumed a coiled form, as demonstrated by Summers & Gibbons 1] and Zobel 2]. The coiled conformation of the outer doublets was found to be a left-handed helix, with a nearly uniform diameter.