Serial sections of mitotic spindles of the marine cryptophycean alga, Cryptomonas, were analysed to determine what types of microtubules they contained and which of these microtubules came close enough to each other (50 nm or less) for the commonly described crossbridging to be possible. Interpolar microtubules were rare (less than or equal to 1%) but from prometaphase through anaphase there was a substantial interpolar framework of free and polar microtubules which came close enough to one another to cross-bridge and generate anaphase spindle elongation by intermicrotubule sliding. However, such elongation would also require some concomitant polar microtubule polymerization. In contrast, only about 12% of the chromosomal microtubules came within bridging distance of interpolar framework microtubules. Thus, assuming that microtubules were accurately fixed in their in vivo positions, crossbridging between chromosomal and interpolar framework microtubules is unlikely to function in chromosome-to-pole movement. In all stages the great majority of chromosomal microtubules were found to extend all the way from the chromatin to the pole. Although the identification of intermicrotubular bridges in section is often ambiguous, the best and most frequent examples of bridges in the present material were between polar microtubules and between chromosomal microtubules but not between the 2 different categories. The spindles also contained 5-nm-diameter microfilaments associated mainly with chromosomal microtubules and occasionally with polar microtubules. A specific model for the possible involvement of these filaments in mitosis is presented.

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