94 KERATIN AND KERATINIZATION 



The differentiated layers and the variety of cell products 



The Epidermis 



The epidermis is an avascular, stratified, squamous epithelium of 

 variable thickness depending on the number and thickness of the cell 

 layers it contains (Fig. 25, p. 55). It necessarily contains a germinal layer 

 and may, as in the mouse, consist of only one other layer of horny cells 

 (Setala et al., 1960); in other animals and at other sites, and in some 

 pathological conditions, it may reach an extreme thickness of some 

 millimetres. Although the thin mouse skin does not reveal the usual 

 stratification, this does not mean that the intermediate stages of differentia- 

 tion are absent or that keratinization follows a different course. Cells 

 representative of the intermediate stages are in fact present, but insuffici- 

 ently common to constitute distinct layers (Gliicksmann, 1945). After 

 various treatments causing hyperplasia, and particularly after the applica- 

 tion of carcinogenic hydrocarbons (benzpyrene, methylcholanthene, etc.), 

 the thickened skin appears typically stratified. 



When the mammalian epidermis reaches a moderate thickness (~ ten 

 cells deep) four layers can be distinguished: (a) the germinal or basal 

 layer (stratum germinativum); (b) the stratum granulosum; (c) stratum 

 lucidum; and (d) the stratum corneum (Fig. 25). The development of 

 these stages in differentiation seems inherent in the epidermis and depends 

 on the attainment of a sufficient thickness. 



Even the basement layer cells contain loose bundles of dense filaments 

 (< 100 A diameter) (Plate 7) showing that synthesis has commenced 

 at this level (Plate 9). These filaments often sprout in bundles from 

 the sense plate-like desmosomes (D in Plate 4B) on the plasma membranes. 

 In the layer of cells immediately above the basal layer of the epidermis 

 there is an increase in the number of filaments ending on desmosomes, 

 making them more conspicuous and giving to the cell the appearance of 

 being covered with small short prickles. The layer is for this reason 

 referred to as " the prickle layer". 



Tonofibrils. The fine birefringent fibrils visible in the light microscope 

 are often referred to as tonofibrils, particularly in the older literature. 

 The use of this term arises out of the view of the classical light histologists 

 who believed that the fibrils had a particular organization relating them to 

 the mechanical function of the tissue. The word " tonus " (Greek) 

 implies a " brace or support " and the word " tonofibril " expresses the 

 idea that the fibrils run from one face of the cell to another and are thus 

 capable of transmitting tension directly. Some authors, observing the 

 so-called " intercellular bridges," held further that the tonofibrillar system 

 ran continuously from one cell to another. This latter view is now un- 

 tenable since it is clear that, in several distinct cell types, fibrils certainly 



