By the late 1970s, it was still unclear how microtubules (MTs) operated in the mitotic spindle. Several pieces of the microtubule puzzle had been worked out: microtubules slid past each other to give cilia their movement (Satir, 1968); dynein cross-bridges linked MTs together (Gibbons, 1966); and MT polymers had polarity. That polarity was reflected by both the orientation of asymmetric subunits (Amos and Klug, 1974) and the different rates at which the MT “plus” and “minus” ends added subunits (Allen and Borisy, 1974).
In the mitotic spindle, MT orientation remained a major question whose answer would help determine what role the microtubules played in lining up and separating chromosomes. Several in vitro studies revealed that MTs could be initiated from both kinetochores and centrosomes (Telzer et al., 1975; Gould and Borisy, 1977) and also that both kinetochore and centrosome MTs polymerized with their plus ends...
