The Behavior of Spindle Fibers and Movement of Chromosomes in Dividing Grasshopper Spermatocytes

Abstract

Meiotic spindle and chromosome movements were studied during spermatocyte divisions in Chrysochraon japonicus and Trilophidia annulata with a sensitive polarizing microscope. When the nuclear envelope disintegrated, birefringent astral fibers invaded the karyoplasm pushing some chromosomes toward the mid-region of the developing spindle. During prometaphase the birefringence (BR) of the kinetochore fibers fluctuated, and fibers at this period often exhibited lateral oscillations. By full metaphase, the kinetochore fibers BR became stable. BR of the continuous fibers was rather weak throughout meiosis. During anaphase in most cells, the kinetochore fibers shortened at a constant rate of 0.7 μm per minute at 28°C. In some cells the rate of shortening slowed down in late anaphase. The overall spindle length increased during anaphase, although considerable fluctuations were observed. The number of microtubules per kinetochore fiber estimated from measured BR and fiber thickness using the Wiener equation was consistent with the direct count of microtubules on electron micrographs.