TRACTION FIBER - TOWARD A TENSEGRAL MODEL OF THE SPINDLE

Most current hypotheses of mitotic mechanisms are based on the ''PAC-M AN'' paradigm in which chromosome movement is generated and powered by disassembly of kinetochore microtubules (k-MTs) by the kinetochore. R ecent experiments demonstrate that this model cannot explain force gen eration for anaphase chromosome movement [Pickett-Heaps et al., 1996: Protoplasma 192:1-10]. Another such experiment is described here: a UV -microbeam cut several kinetochore fibres (k-fibres) in newt epithelia l cells at metaphase and the half-spindle immediately shortened; in se veral cells, the remaining intact spindle fibres bowed outwards as the y came under increased compression. Thus, severing of k-MTs can lead t o increased tension between chromosomes and poles. This observation ca nnot be explained by models in which force is produced by motor molecu les at the kinetochore actively disassembling k-MTs. Rather, we argue that tensile forces act along the whole k-fibre, which, therefore, can be considered as a classic ''traction fibre.'' We suggest that anapha se polewards force is generated by MTs interacting with the spindle ma trix and when k-MTs are severed, polewards force continues to act on t he remaining kMT-stub; spindle MTs act as rigid struts concurrently re sisting and being controlled by these forces. We suggest that the prin ciples of ''cellular tensegrity'' [Ingber, 1993: J. Cell Sci. 104:613- 627] derived from the behaviour and organization of the interphase cel l apply to the spindle. In an evolutionary context, this argument furt her suggests that the spindle might originally have evolved as the mec hanism by which a single tensegral unit (cytoplast) is divided into tw o cytoplasts; use of the spindle for segregating chromosomes might rep resent a secondary, more recent development of this primary function. If valid, this concept has implications for the way the spindle functi ons and for the spindle's relationship to cytokinesis. (C) 1997 Wiley- Liss, Inc.