THE SPATIAL-DISTRIBUTION OF SPINDLE MICROTUBULES IN ANAPHASE-3Y1 CELLS

The spatial distribution of the microtubules (MT) in the rat 3Y1 cells in mitosis was investigated by immunoelectron microscopy and computer -graphic reconstruction of serial thin sections. In anaphase the inter zone-MT increased in number gradually with advancing phase, while the kinetochore-MT in half-spindles decreased. The interzone-MT overlapped with each other at the equatorial region of the cell, and they formed a specific structure called the 'stem bodies'. The ends of the interz one-MT opposite to the stem bodies often attached to chromosomes but n ot to the poles. The stem bodies were not labeled with immunogold part icles of anti-alpha tubulin. Some of the stem bodies or MT which origi nate from stem bodies were found just beneath the plasma membrane in t he equatorial region where abundant actin filaments appear showing the formation of the contractile ring and subsequently the cleavage furro w begins. On the basis of these observations it is assumed that the in terzone-MT is involved both in the separation of chromosomes in anapha se and in the formation of the cleavage furrow in telophase.