DIFFERENTIATION AND PROTEIN SYNTHESIS 129 



Silk fibroin — aggregation of the molecule after unfolding. Another model 

 for fibrogenesis, suggested by its analog}' with spinning, supposes that 

 aggregation is preceded by an unfolding of the molecular chains composing 

 the precursor particle. This process seems probable in cases where fibre- 

 formation is produced by a mechanical process of extrusion and drawing, 

 as when artificial fibres are manufactured from viscous solutions of 

 cellulose derivatives or of dissolved keratin derivatives. Nevertheless, it 

 seems unlikely that it can occur generally in biological systems where 

 fibrils may appear and disappear reversibly with a slight change in 

 variables. Often a mechanical factor analogous to drawing is absent. 

 While drawing may orient fibrils once formed, i.e. in fibrillar organization, 

 it seems to play no role in fibrogenesis itself. 



The formation of silk fibre (fibroin) by silkworms seems at first sight an 

 example of fibre formation by drawing. A soluble precursor of the fibrous 

 form is produced in cells, which are rich in RNA and contain an extra- 

 ordinary development of particles and membranes (Mercer, 1957); it is 

 stored as a strong viscous solution in a dilated portion of the silk gland, 

 and is converted into a thread by being extruded through a fine spinerette. 

 Nevertheless, if the contents of the silk gland are diluted with water and 

 allowed to stand for some hours, masses of fine fibrils separate spontane- 

 ously from the solution (Mercer, 1951c). Fibrogenesis thus again seems 

 to be a spontaneous phenomenon of aggregation requiring no mechanical 

 assistance, but the extrusion and drawing occurring during spinning are 

 responsible for the orientation of the protofibrils. 



Organization of Fibrous Tissues 



Assuming that the basic fibril has been formed, we have now to consider 

 the means by which this is used as a unit for the construction of higher- 

 ordered structures. The geometrical form of the structures is most easily 

 discovered with the electron microscope, although it may be deducible 

 from X-ray photographs or even with the light microscope. The problem 

 is to find the factors, mechanical or otherwise, which organize it. There is 

 often a relation between the mechanical function of the fibrous system and 

 its structure, which may provide clues. 



Some examples of the possible arrangement of fine fibrils are shown 

 in Fig. 53. Tangled " brush heaps " arise in the absence of orienting 

 influences, such as in a fibrin clot. Fibres in which all the elementary 

 filaments are parallel have been subjected to an orienting influence either 

 during or after the formation of the filaments. The most obvious influence 

 is the shear due to flow, which probably initiates the orientation of silk 

 thread. Drawing after extrusion may improve the orientation. 



Some of the more interesting structures are to be found among the 

 fibrous membranes. Collagen in skin, and certain cuticles are often found 



