Cell Constitution 



55 



removal of salt by dialysis. An acetic acid 

 solution of collagen containing only 0.1 per 

 cent protein may be converted into a gel 

 by dialysis against water. In this case the 

 collagen filaments are very thin ( < 50 A) 

 and very long ( ~ micra) ; there are 

 many cross-bonds between particles and a 

 gel is formed although the concentration of 

 protein is very low. When the average par- 

 ticle length is mvich less, as in gelatin, the 

 concentration necessary for gelling is much 

 higher. Sol-gel transformations of such sub- 

 stances are freely reversible. 



Gels may be broken down also by the 

 action of depolymerases which convert the 

 elongate macromolecules into lower poly- 

 mers or monomers. Ribose and desoxyribose 

 nucleic acid depolymerases and hyaluroni- 

 dase are examples of such action. About the 

 enzymatic polymerization of such nucleic 

 acid and polysaccharide molecules miich less 

 is known. 



Much has been learned by a study of sol- 

 gel transformations produced in systems con- 

 taining purified components. Indeed, only in 

 such systems can we possibly hope to evalu- 

 ate the physical and chemical factors in- 

 volved. However, biological systems mani- 

 festing sol-gel transformations may be very 

 much more complex, frequently involving a 

 system of enzj^mes, kinases and antikinases. 

 Although it is useful as a first approximation, 

 and for economy of thought, to visualize 

 protoplasmic sol-gel transformations as obey- 

 ing a common set of rules, this is by no 

 means necessarily true (see Kopac, '51). Care- 

 ful distinction must be made between specu- 

 lations based on the assumption that par- 

 ticular cellular processes behave as do known 

 partial systems and the demonstration that 

 such is, in fact, the case. To really understand 

 a system such as the cortical gel-sol trans- 

 formation or the formation of the mitotic 

 mechanism there is only one way which, in 

 the end, will suffice: the isolation of the 

 individual components and the analysis of 

 the physical chemical factors involved. This 

 may be a discouraging point of view for the 

 enthusiast who would seek a simpler ap- 

 proach by a study of the system in cells them- 

 selves — a field in which much valuable work 

 still remains to be done. However, the prob- 

 lem should not be regarded as insuperable; 

 it will certainly yield when attacked with 

 persistence and patience by modern micro- 

 methods of fractionation and physical chem- 

 ical analysis. 



Contractility is a property of most gelled 



systems of biological materials. The contrac- 

 tion may be isodiametric, as in the syneresis 

 of many gels in which the constituent par- 

 ticles have little or no preferred orientation. 

 Anisodiametric contraction presumes prefer- "n 

 ential orientation. The precise mechanism 

 of svich contractility has not yet been clearly 

 demonstrated even in the much studied case 

 of mviscle, in which the proteins may be 

 obtained in kilogram amounts. In recent 

 years emphasis has been placed on the view 

 that the protein polypeptide chains them- 

 selves contract to form a configuration more 

 highly folded or helically coiled than is char- 

 acteristic of the chains in the uncontracted 

 state. However, the alternative view, pro- 

 posed half a century ago, that the process 

 involves a change in orientation of the par- 

 ticles without change in their internal or- 

 ganization, has received strong support re- 

 cently (Huxley and Hanson, '54). V 



Space permits mention of only a few illus- 

 trative cases of intracellvilar sol-gel trans- 

 formations which are of vmquestioned physio- 

 logical and embryological significance. Dur- 

 ing mitosis the internal organization of the 

 cytoplasm undergoes striking alterations 

 leading to the formation of the spindle and 

 asters. When tested with a micromanipulator 

 the spindle is found to be a fairly stiff gel 

 capable of being moved about as a semi- 

 rigid structure (Chambers, '51). The isola- 

 tion of the gelled mitotic apparatus from 

 fragmented cells recently reported by Mazia 

 and Dan should prove very valuable in work 

 on this subject. Although the fibrous par- 

 ticles composing the spindle are oriented, as 

 shown by their positive birefringence, thev 

 must be extremely thin (probably less than 

 100 A). The robust fibers in electron micro- 

 graphs of sections shown in certain published 

 work are almost certainly aggregates due to 

 the action of the fixative. 



The region of cytoplasm lying immediately 

 below the plasma membrane and having 

 varying thickness (one to several micra de- 

 pending on the cell type), is usvially in a 

 gelled state and is known as the cortex or 

 cortical gel. From birefringence and EM 

 data one may suppose that this region con- 

 tains very fine threadlike particles, having 

 strong interaction with each other and hav- 

 ing orientation predominantly parallel with 

 the surface. Lipid molecules in the cortex are 

 oriented with paraffin chains normal to the 

 surface plane. 



Although little is known about the com- 

 position of the cortical gel, important prop- 



