Spindle microtubules: thermodynamics of in vivo assembly and role in chromosome movement.

In this paper I have presented results of experiments in which spindle microtubules were depolymerized by hydrostatic pressure, in order to examine the Inoué dynamic equilibrium concept of spindle assembly and the possible role of microtubule depolymerization-polymerization in the movement of chromosomes. Using a newly developed optical hydrostatic pressure chamber, I investigated with polarization microscopy the quantitative effects of pressure on the polymerization of spindle microtubules and, with phase contrast microscopy, the relationship of pressure-induced spindle microtubule depolymerization to chromosome movement in living cells. From results of earlier experiments, principally those of Inoué et al. with low temperature and colchicine as microtubule-depolymerizing agents, and from results of my own research, I have concluded that: (1) spindle fiber microtubules are sensitive to depolymerization by pressure (3000-7000 psi), spindle microtubules do exist in a labile equilibrium with a pool of subunits, and the Inoué simple equilibrium model does predict changes in spindle microtubule assembly at metaphase induced by pressure; (2) the stability of microtubules depends on the number of "attached ends;" (3) the longest interpolar microtubules and the longest chromosomal fiber microtubules regulate the spindle interpolar length and the chromosome-to-pole positions; (4) chromosome velocity is independent of the number of spindle microtubules, as well as of the drag force of the chromosomes; (5) the chromosomal fiber microtubules transmit the forces between the poles and between the chromosomes and the poles; and (6) polymerization of microtubules does produce pushing forces and, if controlled microtubule depolymerization does not actually produce pulling forces, at least it governs the velocity of chromosome-to-pole movement.