Mechanism of Colchicine-Mitosis 399 



17.3: Physical Action 



All inhibition ot spindle function and the destruction of its fibril- 

 lar structure can be the consequence of physical agents acting on the 

 cells during division. On the other hand, it appears most probable 

 that many of the spindle poisons which have been described do not 

 act by combining in the chemical sense of the word with the spindle 

 proteins, but by altering some of the physical conditions necessary 

 for the proper development of mitosis. 



iy.3-1: luliibition of the spindle by physieal ao;eiits. That modifi- 

 cations of the physical environment of the cell, without any mitotic 

 poison being present, may induce c-mitosis is evidenced from the action 

 of heat, cold, and high hydrostatic pressiires. 



The reversible changes of the mitotic spindle under the influence 

 of an increased temperature were described in 1933.'- Before colchi- 

 cine, heat-shock was perhaps the most reliable method for producing 

 polyploid plants (cf. Chapter 11) .lo^ It is also one of the most efficient 

 methods of inducing polyploidy in mammals, as mentioned in Chap- 

 ter 16A. In Triton inilgaris, on the contrary, larvae kept in water at 

 3°C. show a typical metaphase arrest, with chromosomes grouped in 

 a single star. The only difference with colchicine is that the alkaloid 

 does not depress prophases, and that ball metaphases (ct. C;hapter 2) 

 are more frequent.^ The hypothesis that cold should mainly affect 

 the ccntrosomcs and centromeres and prevent the orientation of 

 spindle fibers at their contact^ is interesting and deserving of further 

 study. Cold may have played a significant part in the evolution of 

 polyploid species, especially during the periods of glaciation. 



The action of high hydrostatic pressures, about 200 atmospheres, 

 is similar to that of temperature changes in that it brings reversible 

 changes of the sj^ndle, ^vhich loses its fibrous appearance.-'^ 1 his has 

 been demonstrated both in animal cells (Urechis) and in j^lants (pol- 

 len mother cells of Tradescantia) . The exact significance of these re- 

 sults is far from being understood and need not be discussed here. 



Evidently, the proper functioning of the spindle is only possible 

 within a limited range of physico-chemical conditions. It is thus not 

 surprising that changes induced by chemicals of various and unre- 

 lated structures may also arrest mitosis by inhibiting the spindle. Re- 

 search in this field will now be discussed, and the "narcosis theory" of 

 c-mitosis explained. Most of this work, for obvious experimental rea- 

 sons, has been conducted on plant cells, mainly the Allium root tip, 

 and on eggs of invertebrates or vertebrates. A k-w observations have 

 been made on tissue cultures. 



I J. ^-2: Simple aromatic and aliphatic mitotic poisons. A very ex- 

 tensive Mudy on plant cells has been condudetl by several groups of 



