16 



Cellular Structure and Activity 



INHIBITION OF MITOSIS 



Many physical and chemical agents in- 

 hibit cell division without killing the cell. 

 They have been called "mitotic poisons." 

 Besides being useful tools for dissecting mi- 

 tosis into its component processes and for 

 the biochemical characterization of these 

 components, they have great practical in- 

 terest as potential inhibitors of pathological 

 growth. Some inhibitors were discovered 

 empirically to block various phases of mitosis 

 (x-rays, colchicine, nitrogen mustards, etc.). 

 The specific ways in which they affect the 

 cell are now studied in many laboratories. 

 Other inhibitors are well known tools of the 

 biochemist, specific inhibitors of certain en- 

 zymes or antimetabolites that block meta- 

 bolic processes at definite points. They have 

 revealed some of the enzymatic reactions that 

 play a role in cellular reproduction. Reviews 

 of the more recent work on mitotic inhibitors 

 have been published by Lettre ('51, '52) and 

 Lehmann ('51). 



Two phases in the course of cell division 

 are especially sensitive to external influences. 

 One is the antephase, the time when energy 

 is produced and stored for mitosis and when 

 chromosome reproduction takes place. The 

 second is characterized by the formation of 

 oriented gels, of which the swelling and 

 contraction play a role in chromosome move- 

 ment and cytokinesis. 



Antephase inhibitors prevent cells from 

 entering prophase, but do not affect those 

 already in division. They include factors 

 that interfere with glycolysis or respiration 

 and with the formation of high-energy phos- 

 phates (Bullough, '52; Krahl, '50), and those 

 that inhibit chromosome reproduction. Of 

 special interest in this second group are the 

 inhibitors of DNA synthesis. DNA is gen- 

 erally restricted to chromosomes and its syn- 

 thesis to chromosome reproduction. Synthesis 

 of DNA may be accomplished by different 

 pathways in different cells, tissues or organ- 

 isms, and in normal cells and tumor cells; 

 hence the hope for cell specific inhibitors. 

 The better known inhibitors of chromosome 

 reproduction are x-rays (Hevesy, '48: Skin- 

 per, '51), nitrogen mustard (Bodenstein and 

 Kondritzer, '48; Friedenwald and Sia;elman, 

 '53; Goldthwait, '52), folic acid antagonists 

 such as aminopterin and amethopterin (cf. 

 Peterinff. '52) and certain purines and pyrim- 

 idines (Lettre, '51). 



Cells arrested in antephase are extremely 

 labile and easily undergo defl;eneration. In 

 rapidly proliferatine; tissues the maiority of 

 cells can be accvxmvilated in this phase and 



then destroyed (Gillette and Bodenstein, '46; 

 Friedenwald, '51). Chemical agents that in- 

 terfere with chromosome reproduction have 

 cytological effects that are similar to those 

 of ionizing radiations, and therefore have 

 been called radiomimetic (cf. Loveless and 

 Revell, '49). In addition to the antephase 

 block they produce chromosome clumping 

 and chromosome breaks. 



Another group of "chromosome poisons" 

 that inhibit antephase and also later stages 

 are acridine derivatives (trypaflavin, pro- 

 flavin), investigated especially by Bauch 

 ('47) and Lettre ('51). They seem to act 

 by forming complexes with nucleic acids, 

 interfering mainly with polymerization of 

 nucleic acid. 



The movements of chromosomes and cell 

 cleavage are dependent on nuclear and cy- 

 toplasmic gels (asters, spindle, cortical gel 

 of cleavage furrow). Beginning with meta- 

 kinesis, mitosis can be blocked by agents that 

 interfere with gelation of these organelles, 

 or with contraction of chromosomal fibers 

 and cortical gel. The relative sensitivity of 

 these structixres is often somewhat different 

 so that cleavage, for instance, can be svip- 

 pressed without halting chromosome move- 

 ments, or spindle stretching may be in- 

 hibited without affecting the contraction of 

 chromosomal fibers (Ris, '49). Complete in- 

 hibition of these organelles is produced by 

 certain anticoagulants, for instance, heparin 

 (Heilbrunn, '52a), by hydrostatic pressvire 

 (Marsland, '51 ), by high or low temperature, 

 and by hypotonic media (Lewis, '34). 



The best known specific poison of the 

 spindle is colchicine. In the presence of this 

 alkaloid the spindle does not form, or, if it is 

 present, is destroyed to2:ether with asters and 

 the cleava)t?:e furrow. Chromosome reproduc- 

 tion, spiralization and breakdown of nuclear 

 membrane and the initial parallel separa- 

 tion of chromatids are not affected and nu- 

 clei may undergo several cycles of repro- 

 duction in colchicine (Zeuthen, '51). Often 

 the chromosomes clump, or are scattered, 

 giving rise to micronuclei with variable 

 numbers of chromosomes. 



Inoue ('52) has used an improved polariz- 

 ing- microscope to study the effect of col- 

 rhicine on the structure of the spindle. He 

 found that colchicine primarily destroys the 

 orientation of the spindle micelles. The spin- 

 dle material may become scattered or remain 

 in the cell as a spherical mass TGaulden and 

 Carlson, '51), depending on the colchicine 

 concentration and the type of cell. 



Lettre ('51, '52) has studied a great num- 



