DIFFERENTIATION AND PROTEIN SYNTHESIS 95 



end on the desmosomes and do not pass across the cell membranes. 

 One of the best established examples of this is found in heart muscle 

 (mentioned above, p. 42) where parallel bundles of myofilaments are 

 seen to enter and fuse with elaborately- developed deposits of the 

 desmosome type which cover the cell membranes and are visible in the 

 light microscope as " intercalated disks". 



The situation is not as clear in epithelial cells. Certainly many fibrils 

 end on desmosomes (Porter, 1956; Charles and Smiddy, 1957), but it 

 remains to be shown what proportion of fibrils have both ends attached. 

 In view of the extreme thinness of the sections used in electron microscopy 

 ( < 500 A) proof of continuity is difficult to obtain. From a mechanical 

 point of view it is not necessary to assume that they all do since the whole 

 mass of fibrils is ultimately fused together during keratinization. There 

 is thus a use for both terms " fibrils and tonofibrils " and the latter may be 

 used when an author wishes to assert adherence to the view that the 

 fibrils run from one desmosome to another within the cells. In other 

 cases the less restrictive word " fibrils " will be used here. 



Keratohyalin. In the lower layers of the epidermis the synthetic 

 activities of the cell appear to proceed directly to the formation of filaments 

 (F in Plate 9) and in this respect they resemble the cells of the hair cortex 

 to be discussed later. In the stratum granulosum of the epidermis, 

 however, a new product makes its appearance in the form of rounded 

 droplets which, accumulating, give the cells a granular appearance. 

 These droplets consist of a distinct substance called keratohyalin and a 

 great deal of study has been devoted to it, the main point at issue being 

 whether it is, or is not, to be regarded as a precursor of the fibrous keratin 

 of the stratum corneum (see also p. 228). The granules disappear in the 

 next cellular layer, the clear, glassy, highly-birefringent stratum lucidum* 

 The appearance of keratohyalin granules is one of the definitive 

 characteristics of the " soft keratinization " of the epidermis (see Table 5, 

 p. 65) to be contrasted with the keratinization of a hard keratin, such as 

 hair (next section) which proceeds without the formation of a granular 

 layer. In the squamous cells ot the inner root sheath of the hair follicle a 

 similar granular layer appears (Plates 22 and 23). 



The Hair Follicle 



The hair follicle forms embryonically as a down-growth of the basal 

 layer of the epidermis. The first sign, indicating a locally-increased rate 

 of cell division, is a cluster of smaller crowded cells above the basal 



* Ranvier in 1879 introduced the term " eleiden " (from the Greek word for 

 " oil ") as a name for the clear glassy substance of the stratum lucidum. Although 

 this word occurs frequently in the older literature there does not seem to be any 

 need for it in the present description. 



