92 



KERATIN AND KERATINIZATION 



(iii) The superficial cells (ectoderm) lead the way in differentiation and 

 it may be supposed that diffusible products from them influence the 

 underlying cells (Fig. 27). Rose (1952) has supposed that the appearance 

 of one form of product in a group of cells will suppress a similar appearance 

 in neighbouring cells and permit a second type of differentiation to arise. 

 Certainly, however this may be, the mesodermal cells henceforth are 

 enclosed in a bag of already-differentiated cells whose products could 

 influence them. Fibroblasts soon are recognizable by their content of 



Fig. 41. Types of cell contact seen in the hair follicle. (A) the convoluted 

 cell membranes with irregular intercellular spacing seen in the undifferen- 

 tiated matrix. (B) localized contacts spreading to give uniform contact. 



(C) (See also Fig. 43.) The intercellular cement is shown hatched. 



(D) and (E) Complete intercellular formations noted in the keratinizing 



levels and higher (from Birbeck and Mercer, 1956 and 1957). 



membranes covered with dense particles (RNP). At this stage (Fig. 38) 

 the two races of cells become separated by the appearance of the basal 

 membrane (BM). The peculiarities of this membrane are its fuzzy 

 character, its poor reaction w T ith osmium tetroxide, its staining with 

 phosphotungstic acid (protein), its situation as a further sheet covering 

 the sheet of material already covering the ectodermal cell membranes and 

 which elsewhere acts as an intercellular adhesive. These features combine 

 to suggest that it is a reaction product (precipitate) formed between proteins 

 leaving the ectodermal cell and a more rapidly-diffusing substance 

 emanating from the mesodermal cells, as suggested by Fig. 39. 



Henceforth the epithelium is established as a constantly-multiplying and 

 migrating cell population whose cell movement, in an inward direction, is 



