HYDROSTATIC PRESSURE EFFECTS UPON THE SPINDLE FIGURE AND CHROMOSOME MOVEMENT. II. EXPERIMENTS ON THE MEIOTIC DIVISIONS OF TRADESCANTIA POLLEN MOTHER CELLS

¹ Department of Anatomy, the Medical School, the University of Southern California, Los Angeles, California

Hydrostatic pressures have been applied to Tradescantia pollen mother cells as a technique for studying the structure of division spindles and chromosomes and the mechanics of anaphase movement. The procedure has given pertinent information by virtue of the fact that increasing pressures progressively reduce gel rigidity. Sufficiently high pressure results in liquefaction. Yet the effects are reversible.

The spindle of the first meiotic division was but slightly affected by 4,000 lbs./in.² pressure, yet was mostly liquefied by 5,000 lbs. The spindle of the second meiotic division withstood about 2,000 lbs. more pressure. The somatic cells were even more resistant.

Condensed chromosomes were significantly softened by even 1,000 lbs./in.² pressure as indicated by an undue elongation of the kinetochore stalk. Fusion bridges became particularly obvious when 3,000 lbs. was applied. Significant shortening and rounding occurred at 4,000 lbs. Total fusion and rounding, indicating complete liquefaction of the matrix, did not occur until pressures of 15,000 lbs./in.² were applied. The fusion and rounding appeared to be a surface tension effect, and suggested the existence of a true interfacial membrane between condensed chromosome and cytoplasm. Not even these highest pressures, however, affected the uncondensed prophase chromosomes so that the effect of pressure was thought to be only upon the matrix material.

Chromosome movement was limited to those pressures which did not liquefy the spindle. The presence of fusion bridges, however, resulted in very abnormal movement.

After the release of high pressures, spindles re-formed. That these were de novo structures was indicated by their sometimes abnormal orientation, by the frequency of multipolar spindles, and by abnormalities in the course of traction fibers. Thus, the traction fibers of two homologous chromosomes might go to a single pole. Abnormalities made it seem likely that the growth of traction fibers was in a large measure independent of the growth of the body of the spindle. The direction of growth of the traction fiber was not specifically oriented until it reached the oriented bulk of the spindle.

Chromosome movement in recovery material was abnormal in that the fusion bridges persisted. Thus the chromosome matrix which had been liquefied, had become highly viscous once more. Under such circumstances, homologous chromosomes frequently went to a single pole, and the traction fiber to the other pole extended all the way across the cell. However, such traction fibers were not thinner than normal.

The outstanding conclusion is that a gel structure in the spindle is essential for anaphase movement. The traction fiber apparently serves as nothing more than a semi-elastic connection between the chromosome and the main mass of the spindle which, in turn, is in motion. It is suggested that motion and force is imparted by means of sol-gel-sol transformations, with gel being added to the central bulk of the spindle while a proportional solation goes on at the poles.