Studies on kinetochore function in mitosis. I. The effects of colchicine and cytochalasin on mitosis in the diatom Hantzschia amphioxys.

Cytochalasin does not affect mitosis (including half spindle separation and disassembly) while cleavage is partially or totally inhibited. While colchicine stops central spindle growth, it resists breakdown even after prolonged exposure. Prometaphase chromosome movement soon ceases, and some attached chromosomes slowly detach; these phenomena are correlated with a loss of the numerous microtubules (MTs) that emanate from the poles, with which chromosomes interact. "C-anaphase", often seen, is marked in vivo by spindle elongation and unequal polar distribution of those chromosomes still attached to the central spindle; this stage is characterized ultrastructurally by the accumulation of dense matrix material, probably the "collar" previously described, at the poles. Kinetochores often remain tightly associated with this matrix. We believe this result is significant, since it clearly demonstrates that the kinetochores are attached to a spindle component other than microtubules. We suspect that this matrix is contractile and part of the mitotic machinery for moving chromosomes. These colchicine effects are not reversible.