DIFFERENTIATION AND PROTEIN SYNTHESIS 125 



In histological systems, unlike those found in the textile mill, " spinning 

 and weaving " usually occur together in time and space, i.e. the fibrils 

 appear precisely where and when they are required to enter into the 

 structure of the growing fabric. However, it may be possible to separate 

 the two processes artificially if the precursor of the fibrous system can be 



Micromolecules 

 * 



1" 



Macromolecules 



i 



Protofibrils 

 I 



Fibrous textures 



Stabilized tissues 



Synthesis 

 Fibrogenesis 

 Organization 

 Stabilization 



Fig. 54. Formation of secondary and tertiary fibrous structure 

 (*see also Fig. 51 for this step). 



isolated, either before it has been converted into fibres or by reversing the 

 process of fibre formation. In this case it may be possible to study the 

 formation of fibrils in vitro and, by artificially orienting these in imitation of 

 the natural system, to gain some insight into the processes which must be 

 operating to orient them in vivo. 



Fibrogenesis 



Fibrogenesis (Fig. 54) is by definition the formation of an elementary 

 elongated unit from which the more complex formations are constructed. 

 An appropriate name for such a unit, which indicates that it is the first 

 fibrous unit in a hierarchy of such structures, is the protofibril. Electron 

 microscopy definitely established the existence of protofibrils in many 

 fibre tissues although their presence had earlier been inferred from X-ray 

 diffraction and polarized-light studies. Protofibrils form the structural 

 basis of some fibrous keratins; their appearance and condensation into 

 macrofibrils and more massive formations has been described in the hair and 

 feather follicle. Unfortunately, valuable though these morphological 

 observations are, in the case of keratin they cast little light on the physico- 

 chemical processes involved. Information concerning these processes 

 could certainly be obtained if it were possible to study the phenomenon 



