Cell Constitution 



51 



The history of muscle physiology reveals 

 that it was only when the fibi-ous proteins of 

 muscle were isolated (at least with a sem- 

 blance of purity) that rapid advances were 

 made in our concepts of the mechanism of 

 mviscle contraction. We are still far from a 

 full understanding of this process despite 

 the enormous work which has been devoted 

 to its investigation by the most competent 

 biochemists over the past few decades and 

 despite the fact that muscle proteins may be 

 obtained in any desired quantity. The prob- 

 lem of the cytoplasmic fibrous proteins is 

 enormously more difficult because their con- 

 centration in cytoplasm is low and there are 

 many other substances present which com- 

 plicate isolation without significant chemical 

 alteration. Constituents other than the pro- 

 teins may also occur as fibrous particles in 

 cells (nucleic acids and polysaccharides and 

 their complexes with proteins, lipids and 

 other materials) and this further complicates 

 the problem. 



Protein fractionation procedures which 

 proved so effective in isolating the proteins 

 of blood plasma are currently being applied 

 to the fractionation of tissue proteins in vari- 

 ous laboratories. However, even granting that 

 this had been successfully achieved, the 

 problem of the localization of components in 

 particular cell types would remain. In fortu- 

 nate instances the localization problem may 

 be overcome by the use of especially favor- 

 able material. Thus the use of the giant nerve 

 fiber of the squid permits the extrvision of 

 axoplasm uncontaminated by nonaxonal ma- 

 terial. Already one axonal protein in mono- 

 disperse, relatively pure form has been char- 

 acterized and the existence of several other 

 proteins demonstrated (Maxfield, '51). 

 Studies in the avithor's laboratories are di- 

 rected at the isolation and characterization 

 of the protein of the axon filaments (the 

 fibrous constituents of neurofibrils) and some 

 progress has already been made. Perhaps 

 when success has been achieved we shall 

 have a better notion of the role of these pro- 

 teins in nerve function. 



A beginning has also been made in the 

 isolation of proteins from single cells. Mirsky 

 ('36) had suggested that a "myosin-like" 

 protein is present in sea urchin eggs and that 

 the state of aggregation of this macromolecu- 

 lar component varies with physiological state. 

 Monroy ('50) has imdertaken to fractionate 

 the macromolecular constituents of the sea 

 urchin egg and to characterize them by elec- 

 trophoretic and ultracentrifvigal methods. 

 Though he has not yet succeeded in isolating 



individual components in pure form he has 

 obtained evidence for the presence of at 

 least five components and has demonstrated 

 changes which occur in several of the com- 

 ponents on fertilization. 



The adaptation of physical chemical meth- 

 ods (ultracentrifuge, electrophoresis, stream- 

 ing birefringence, viscosity, etc.) to very 

 small samples (e.g., from single cells) would 

 greatly facilitate the analysis of cytoplasmic 

 constituents. 



Intracellular Fibrogenesis. Fibroblasts are 

 thought to produce a protein which is the 

 precursor of the collagen fibers of connective 

 tissue. Thus far, there is little evidence for 

 the formation of fibrous collagen (identifiable 

 with the EM) inside the fibroblast cells. 

 Porter and Vanamee's ('49) EM studies of 

 collagen formation by fibroblasts in tissue 

 culture suggest that the collagen fibrils are 

 formed at or near the external surface of the 

 cells. It is possible that further detailed stud- 

 ies of this sort may lead to information abovit 

 the process by which the elongate collagen 

 molecules are formed in the cell. If, as the 

 writer suspects, the intracellular precursor 

 occurs as discrete, probably elongate mole- 

 cules, the task will require a chemical as 

 well as a morphological approach. 



For present purposes we are primarily con- 

 cerned with the mechanism of formation of 

 the fibrous materials characteristic of the 

 cell itself rather than with the "secretion" 

 of fibrous substances. Fibrous arrays are the 

 chief building stones of cell structure. Yet 

 almost nothing is known about the process 

 by which the constituent molecules are as- 

 sembled into fibers. Definitive evidence must 

 await chemical isolation and characteriza- 

 tion as discussed in the preceding section. 

 Meanwhile backgrovmd information obtained 

 by indirect methods and by a study of fibro- 

 genesis in vitro will prove useful in guiding 

 our thoughts and in devising direct experi- 

 ments in cells themselves. 



Intracellular fibrogenesis mav be thought 

 to occvir along two (not mutually exclusive) 

 lines: (a) that which may occur spontane- 

 ously without the known action of enzymes, 

 and (b) that known to be controlled enzy- 

 matically. The preciirsor molecules, which 

 become linked by covalent or electrostatic 

 bonds, may themselves be highly elongate or 

 more nearly spherical in shape. 



Fihrilization without Enzyme Action. In 

 this type the adlineation of protein molecules, 

 chiefly through electrostatic or hydrogen 

 bonds at opposite ends of molecules, is chiefly 

 involved. Stability of the fiber depends im- 



