Spindle fiber interpolar

Fig. 1. A living primary spermatocyte of Pardalophora apiculata (a grasshopper) as viewed in Inoue s rectified polarizing microscope. A., Metaphase kinetochores of one bivalent are indicated by arrows ("k ) and polar regions by p". Birefringent chromosomal spindle fibers run from each kinetochore toward a pole the diffuse background birefringence of interpolar fibers is identified only with difficulty In the prints but Is readily measured. B., Metaphase the opposite compensator setting. C, D. Anaphase 22 and 36 minutes, respectively, after A. XI,500.

ANAPHASE. The two forms of anaphase motion can be directly related to the spindle mechanics reviewed above as follows (Ris, 1949) The movement to the poles depends upon force production or at least transmission by shortening chromosomal spindle fibers. Spindle elongation results from lenghtening of the interpolar fibers, which produces increased chromosome separation because the chromosomal fibers are anchored to the interpolar fibers. There is at present no reason to doubt the correctness of this consistent and straightforward view (reviewed by Ris, 1955 Mazia, 1961). 

Anaphase involves poleward forces and variable degrees of spindle elongation (through lengthening of interpolar spindle fibers). 

This model proposes a functional differentiation of chromosomal and interpolar spindle fibers in good accord with the cytological features of prometaphase congression and separate poleward motion and spindle elongation in anaphase, with the static mechanical properties, and with the slight evidence for differences in chromosomal and interpolar fibers from their differing responses to colchicine and chloral hydrate (p. 245). Any more extended discussion is unwarranted until the motions of granules and displaced chromosomes have been fully described in a variety of forms specific predictions are readily formulated. 

The critical features of spindle formation are the origin of distinct chromosomal and interpolar fibers and the development of a common spindle axis. Where both polar and kinetochoric organizers are present the interpretation in terms of assembly, parallel alignment, and organizing centers is obvious. But where no polar centers are involved... 

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