MOLECULAR MORPHOLOGY 55 



similar to that of alpha keratin, from which it may be concluded that 

 some similarity of structure of the polypeptide units exists. However, 

 these proteins differ from keratin, at least superficially, in that they may 

 be composed of particulate units. In such cases the continuous polypep- 

 tide chain description requires considerable qualification. Some examples 

 of this class may be mentioned. 



Fibrinogen molecules have been well characterized with respect to 

 size and shape. They are long, thin spindle-shaped particles. The chemi- 

 cal change which occurs when these units combine to form fibrin fibers 

 is very slight, making it improbable that the internal architecture suffers 

 much alteration in the process. In this case the aggregation of the 

 particles is not readily reversible. 



Globular insulin molecules, which have relatively slight asymmetry, 

 may be converted into a fibrous modification without denaturation in 

 the strict sense of the term. The process is completely reversible, as 

 Waugh has shown. This is a particularly strategic case because much 

 is known about the composition and structure of the insulin molecule. 

 This should greatly aid in investigating the alterations which occur 

 when the globular molecules are recruited into fibrous arrays. Waugh 

 has pointed out that the phenomenon requires the assumption that 

 combination occurs at opposite poles of the molecule, otherwise only 

 random gel-like aggregation, rather than filament formation, would 

 result. It seems probable that similar considerations may apply to certain 

 other particle-fiber transformations. 



Finally the remarkable properties of the muscle protein, actin, may 

 be mentioned. In distilled water these particles or molecules occur freely 

 dissociated. The solution has relatively low streaming double refrac- 

 tion and viscosity. Merely increasing the ionic strength greatly increases 

 the double refraction and viscosity due presumably to fibrous aggrega- 

 tion of the particles. Jakus and Hall have studied the phenomenon with 

 the electron microscope and find that the filaments have widths of the 

 order of lOO A and lengths up to several microns. Reduction of ionic 

 strength causes the filaments to break up. The recent work of Szent- 

 Gyorgyi, Cori, Price and others suggests that this type of reversible 

 aggregation may be involved in muscle contraction. 



The Mechanism of Fibrogenesis 



The points just discussed bear importantly on the chemical and 

 biological mechanisms of fibrogenesis. 



