Spindle shape changes as an indicator of force production in crane-fly spermatocytes.

SUMMARY Changes in spindle shape during the first and second meiotic divisions in living Nephrotoma svturalis spermatocytes have been studied and recorded cinematographically with phase-contrast, Nomarski differential interference, and polarization microscopy. With the Nomarski system 2 classes of discrete spindle filaments (new terminology) have been observed: continuous filaments that extend between the poles and appear to form a cage-like framework within which the chromosomes move, and kinetochore filaments which are attached to the chromosomes at their kinetochores and converge toward the poles. Spindle deformation occurs in a manner consistent with the hypothesis that whatever tensile forces are transmitted by the kinetochore filaments are balanced by compressive forces acting on the cage-like framework of continuous filaments causing them to become splayed. Just when these forces would be expected to reach a maximum, kinetochore filaments undergo a noticeable increase in contrast. It has not been determined whether this change represents a crowding of linear elements or addition of dry mass to a constant number of linear elements.