Polarity of midbody and phragmoplast microtubules.

Abstract

A newly discovered method for displaying the molecular polarity of microtubules (MT) was slightly modified and applied to the midbodies of cultured mammalian cells and the phragmoplasts of Haemanthus katherinae endosperm. The method involves the decoration of preexisting MT in lysed cells with curved ribbons of tubulin protofilaments; the direction of curvature of these C-shaped appendages as seen in cross section reflects the intrinsic polarity of the MT. In transverse sections of midbodies from HeLa and PtK cells, essentially all the MT in a given region of the structures have the same direction of hook curvature and the same polarity. The midbody MT that lie on 1 side of the spindle equator show the opposite polarity from those on the other side, indicating that the midbody is constructed from 2 families of antiparallel MT. Midbody MT are arranged with their fast-growing ends overlapping at the spindle equator, consistent with the hypothesis that the midbody is formed by the interdigitation of aster MT. The polarities of the MT from the phragmoplast of endosperm cells are the same as those found in the mammalian midbody. These results eliminate 1 model for mitosis but are consistent with others. The systematic and reproducible polarities observed favor the concept that the MT polarity is an important factor in the formation and/or the function of these 2 mitotic structures.