86 KERATIN AND KERATINIZATION 



plasma membranes and of the less-dense layer of rather constant width 

 which separates them and which must represent in life the site of the 

 postulated intercellular cement. Evidently the intrinsic density (electron- 

 scattering power) of this material is low and, further, it does not react 

 with any of the common fixatives (osmium tetroxide permanganate and 

 formaldehyde) to produce a denser reaction product. After a treatment of 

 the fixed material with phosphotungstic acid and/or lead hydroxide 

 (Birbeck, 1959), it becomes visible (Plate 5). These findings merely 

 suggest that the intercellular layer (or exudate) is present in low absolute 

 concentration, and that it consists of chemical substances of an unreactive 

 character. Among the various suggestions compatible with these rather 

 negative requirements, is that it consists predominantly of a polysaccharide, 

 probably with a protein moiety (mucopolysaccharide) responsible for the 

 specificity. We may suppose that during the early stages of embryogenesis 

 it is secreted by or shed from the cell surfaces, each cell type producing 

 its own specific layer. See also the remarks made above concerning the 

 composition of the more specialized desmosome p. 84. 



Since the epidermal cells undergo rapid differentiation when they 

 leave the germinal layer, we are able to find in the stream of cells (Fig. 42), 

 taking its origin in this layer and ending in the fully-keratinized layers, the 

 whole sequence of changes preserved at one time in the correct sequence. 

 It is this circumstance which further recommends the use of epidermal 

 tissues for the study of differentiation. The small volume in which the 

 changes occur makes the material ideal for electron microscopic study 

 which alone permits a visualization of the cell membranes themselves. 

 The results of the electron microscopic study of the developing epidermis 

 are given in the next two sections and of the developing hair follicle on 

 p. 95. 



The Dermoepidermal Junction 



The basal membrane is a structural feature which seems essential to 

 the establishment of an epithelium, for without it there seems no reason 

 why the intercellular adhesion postulated above should produce an 

 orderly, layered structure rather than a ball of interdigitating cells. Its 

 structure and formation therefore require special consideration. 



Owing to the difficulty of resolving the fine details of the structure of 

 the dermoepidermal boundary, its nature has been much in dispute. 

 More recently, electron micrographs of a sufficient variety of tissues drawn 

 from amphibian, avian and mammalian sources have clarified the issue 

 and show that essentially the same structure is present in all these classes 

 of organisms (Weiss and Ferris, 1954; Porter, 1956; Jackson, 1954; Selby, 

 1955; and Mercer, 1958). Proceeding from within a basal-layer cell and 

 moving towards the dermis (Fig. 39), we encounter firstly the plasma 



