DIFFERENTIATION AND PROTEIN SYNTHESIS 



99 



one from another, is their distance from the outer root sheath or from the 

 dermal environment (Fig. 42) which suggests again that some influence 

 from this direction controls differentiation. Differentiation is usually 

 recognized in the light microscope by a change in cell shape or by an 

 alteration in staining properties or cell texture which indicates the accum- 

 ulation of different chemical products. However, as stated earlier, electron- 

 microscopically the cell membranes (as defined above) can be clearly 



m 



w 



Fig. 44. (a) to (c) The conversion of a flattened into a tall columnar 

 epithelium as a consequence of the spread of intercellular contact. 

 (Refer to Fig. 43.) 



(d) The development of "tilting" in a cuticle, for example, owing to 

 the creeping of a cell tip due to enhanced adhesion. (Birbeck, 1957). 



distinguished, and their behaviour provides an earlier indication that a 

 change is taking place (see Fig. 41). 



Birbeck and the present writer (1956, 1957a and 1957b) have tried to 

 correlate the outset of differentiation with differential membrane adhesion 

 between the several presumptive layers of the hair. The course of events 

 revealed in a longitudinal section of a follicle is illustrated diagrammatically 

 in Figs. 42 and 43. The first cell layers to distinguish themselves from the 

 undifferentiated mass in the lower bulb are those of the cuticles. It may be 

 significant, from the point of view of the penetration of an inductive effect 

 from the dermis, that although these layers are of a variable distance from 



