110 



Cellular Structure and Activity 



duced experimentally with respiratory poi- 

 sons. In the irregularly shaped cell the cor- 

 tical proteins are in a state of chronic con- 

 traction that needs a high level of ATP. 

 With less ATP irregular amoeboid move- 

 ments result and the cell takes on a spherical 

 shape. 



Cytoplasmic streaming is often pronounced 

 during mitosis. Especially during anaphase 

 and cytokinesis vortical currents are visible 

 in many cells (cf. Belaf, '29). Often these 

 currents carry along pigment granules, yolk 

 platelets, mitochondria and other inchisions 

 that accumulate in the equatorial plane or 

 along the spindle surface (e.g., Nussbaum, 

 '02). Considering these changes in the phys- 

 ical state of the protoplasm and the mixing 

 up through cytoplasmic currents we can 

 understand that mitosis is generally antag- 

 onistic to cytoplasmic differentiation and 

 specific functioning of the cell. Not only 

 is there interruption in nviclear functions, 

 but also more or less severe changes in cyto- 

 plasmic organization. Cytoplasmic organelles 

 such as cilia, brushborder, and ergastoplasm 

 often disappear and specific function is in- 

 terrupted (cf. Berrill and Huskins, '36; Peter, 

 '40). Some cells divide, however, without 

 visible simplification in cytoplasmic organi- 

 zation (Dawson, '40), and many highly dif- 

 ferentiated cells are able to divide mitot- 

 ically. Differentiation is usually accompanied 

 by a decrease in the rate of mitosis, yet 

 the factors responsible for cessation of cellu- 

 lar proliferation are independent of differen- 

 tiation as such. 



An interesting change in the cytoplasm is 

 the marked decrease in PNA in late prophase 

 and metaphase (Brachet, '42; Montalenti et 

 al., '50; Battaglia and Omodeo, '49). This 

 may be related to an interruption in nu- 

 clear function during division in view of 

 the report of Brachet ('50b) that RNA in the 

 cytoplasm decreased in enucleated halves of 

 amoebae. According to Mazia and Hirshfield 

 ('50), incorporation of P^- into the cyto- 

 plasmic RNA is under nuclear control and 

 it would be interesting to study how this is 

 affected during mitosis. 



MITOSIS AND METABOLISM 



In the past the study of the metabolic 

 characteristics of the dividing cell has lagged 

 behind the analysis of the mechanisms of 

 mitosis. Only in recent years have some sig- 

 nificant advances been made in the under- 

 standing of the metabolic processes associated 

 with cell reproduction. The various aspects 



of the recent work have been summarized 

 by Brachet ('50a), Krahl ('50), Zeuthen ('51) 

 and Bullough ('52). Much of the older work 

 on the metabolism of mitosis is quite mean- 

 ingless because mitosis was treated as a uni- 

 tary process. Mitosis is a chain of individual 

 processes that are quite different in char- 

 acter, and metabolic studies of cell division 

 will make sense only when they are related 

 to the various components of cellular repro- 

 duction. We shall want to know, for in- 

 stance, what changes in metabolism are as- 

 sociated with the initiation of mitosis; are 

 there metabolic pathways specific for divid- 

 ing cells? What processes furnish the energy 

 for chromosome synthesis, for spindle for- 

 mation and chromosome movements, for 

 cytokinesis and synthesis of cytoplasmic com- 

 ponents? How do the metabolic processes 

 during the various phases of mitosis differ 

 in different types of cells? 



Eggs during cleavage have been a favorite 

 material for metabolic studies, especially 

 those of echinoderms (Krahl, '50). In these 

 eggs division occurs only in the presence of 

 oxygen. Inhibition of respiration to less than 

 30 per cent of normal blocks mitosis. A 

 cyanide-sensitive system containing an iron- 

 porphyrin catalyst (probably cytochrome c) 

 is involved. Mitosis is also blocked by inter- 

 fering with generation and transfer of en- 

 ergy-rich phosphates. The substrate oxidized 

 during cleavage has not been conclusively 

 characterized. In some eggs, however, oxygen 

 is not necessary for cleavage. For instance 

 in the frog, the toad, Fundulus and Ilyanassa 

 (references in Bullough, '52; Brachet, '50a), 

 cleavage may continue in the absence of 

 respiration. 



Several investigators have measured the 

 rate of respiration during cell division. The 

 most careful and extensive determinations 

 have been made by Zeuthen ('51), both on 

 single eggs and on large numbers of eggs 

 dividing synchronously. In the eggs of the 

 frog, Urechis and several echinoderms a 

 definite increase in respiration was demon- 

 strated with each mitosis. Zeuthen, further- 

 more, showed that the rise in respiration 

 occurred during interphase and not during 

 the actual division in Psammechinus eggs 

 (Fig. 23) and in the ciliate Tetrahymena 

 gelei. This agrees with observations of 

 Forster and Orstrom and Bataillon (refer- 

 ences in Brachet, '50a) that respiration is 

 necessary only in the first part of mitosis, 

 before metaphase. In the presence of col- 

 chicine, chromosome movements and cyto- 

 kinesis are suppressed, yet the rhythmic 



