56 



Cellular Structure and Activity 



erties have been attributed to it. One of 

 these 'is concerned with the mechanism of 

 ameboid movement and perhaps of cell move- 

 ment generally. 



As DeBruyn ('47) points out, current theo- 

 ries of ameboid movement again stress the 

 contractility of the plasmagel as fundamental 

 to the movement. It is also frequently sup- 

 posed that protein passes from cortical gel to 

 endoplasm at the "tail" of the ameba and 

 from endoplasm to cortical gel in the ad- 

 vancing pseudopod. It has further been sug- 

 gested that the cortical gel contains fibrous 

 proteins analogous to the actomyosin system 

 of muscle and that contraction may depend 

 upon the interaction of such proteins with 

 ATP. According to Goldacre and Lorch ('50), 

 injection of ATP into ameba causes contrac- 

 tion and liquefaction of the cortical gel; this 

 liquefied gel is squeezed forward to form more 

 gel on the surface of the advancing pseudo- 

 pod. Kriszat ('50) found that ATP causes the 

 ameba Chaos chaos to contract, presumably 

 because of an increase in the rigidity of the 

 "ground cytoplasm." Lettre ('52) suggested 

 that the stage of contraction of the cortical 

 gel depends on the ATP level and thus on 

 cell metabolism. It is interesting that Loewy 

 ('52) has demonstrated the presence of an 

 actomyosin-like substance in a myxomycete 

 Plasmodium. 



Goldacre ('52) speculates that the fibrous 

 protein chains of the cortical gel upon con- 

 traction fold and remain in this condition 

 in the plasmasol; when these particles reach 

 the front of the advancing pseudopod the 

 chains again unfold to form cortical gel. For 

 such speculations there is as yet little direct 

 evidence. 



From the ultra structural, and possibly from 

 the chemical, viewpoint the process causing 

 ameboid movement may be fundamentally 

 similar to muscle contraction, e.g., localized 

 changes in affinity of fibrous proteins under 

 the influence of ATP, together with changes 

 in configuration or relative positions or orien- 

 tations of protein particles. In muscle the 

 fibrous proteins are highly oriented, mak- 

 ing possible rapid reversible and anisodia- 

 metric contraction. In the ameba the fibrous 

 particles are apparently oriented with long 

 axes predominantly in planes parallel with 

 the surface but the structural organization 

 is of low order, making contraction relatively 

 slow and uncoordinated. 



Lewis ('47) has invoked the "contractile 

 tension" of the cortical gel to explain changes 

 in cell configuration and movements occur- 

 ring in embryogenesis. Equatorial constric- 



tion resulting in cell division is also attrib- 

 vited to contraction of the cortical gel 

 (Marsland, '51). Many other cases might be 

 cited in which investigators have considered 

 the cortex to be contractile. 



Several observations may be pertinent on 

 this matter. Although really very little de- 

 tailed knowledge exists about the ultrastruc- 

 ture and composition of the cortical gel, the 

 inference that it contains a lattice of very 

 thin protein filaments seems justified. Such 

 a system may well exhibit contractility. How- 

 ever, the contractile phenomena attributed 

 to this gel would seem to require specific 

 orientation of the fibrous components. For 

 this there is little evidence except the polar- 

 ization optical indication that the anisodi- 

 ametric protein components may lie pre- 

 dominantly in planes parallel to the cell 

 surface. Finally, direct physical measirre- 

 ments of force generated in the cortex at 

 times when contraction is supposed to occur 

 are either lacking or unconvincing. 



This is said not to reflect skepticism about 

 the contractility of the cortex but rather to 

 emphasize the desirability of a direct physi- 

 cal or physical chemical investigation of the 

 properties of this important region of the 

 cell. 



Other instances of intracellular contrac- 

 tility for which no mechanism has yet been 

 demonstrated might be mentioned. One 

 thinks, for example, of the movement of pig- 

 ment granules in melanophores. Under cer- 

 tain physiological conditions the granules 

 move out into the cell processes while under 

 other conditions they move into the center of 

 the cell. The movement in or out may be 

 produced by drugs which cause the contrac- 

 tion or relaxation, respectively, of smooth 

 muscle. A preliminary unpublished investi- 

 gation in this laboratory by J. B. Finean 

 failed to reveal a fibrous structure resolvable 

 under the conditions with the EM. Similarly 

 the mechanism of cyclosis in plant cells and 

 the rhythmic contraction of slime molds 

 (Seifriz, '43, Loewy, '49) remain challenging 

 subjects for investigation with modern micro- 

 methods. 



THE CELL MEMBRANE 



The limiting envelope or cell membrane, 

 though representing but a very small frac- 

 tion of the cell volume, is a highly critical 

 structure because so many aspects of cell 

 function depend vipon it. Being only rela- 

 tively few molecules in thickness, contigu- 

 ous with the cortical cytoplasm on the inner 



