66 KERATIN AND KERATINIZATION 



plus stratum corneum) depends on the coherence of the total formation 

 and such abnormally-thickened areas as corns show a thick clear layer — 

 the most thoroughly-bonded zone. Electron micrographs of desquamating 

 cells of human skin show a simultaneous separation between both cell 

 membranes and between the intercellular fibrillar contents in the body of 

 the cell. The spontaneous exfoliation could thus be due to a failure in 

 both components. 



It is evident from Table 5 that the sequence of events in the formation 

 of a soft keratin differs somewhat from that in the formation of a hard. 

 It will be convenient to consider this problem later in Chapter 6, pp. 

 210-282. 



The specialized appendages 



Localized Epidermal Thickenings 



The epidermis and the relative thickness of its various strata show 

 some characteristic local variations. In thin skins the intermediate layers 

 (Fig. 25) {stratum granulosum and stratum lucidum) may be absent, 

 the transition between the germinal and horny layer being quite abrupt. 

 This condition may have some significance in showing that a granular 

 phase (keratohyalin intermediary) may not be necessary for cornification 

 (p. 94); nevertheless such skins can often be provoked by appropriate 

 stimuli to assume a multilayered appearance and it is possible that, during 

 the actual growth phase, granules are formed. 



The thickened areas of the epidermis have a functional purpose, e.g. 

 the horny pads of the digits, and become more thick with use. These 

 thickened areas are genetically determined and display thickening in the 

 embryo before being stimulated externally. What is also inherited is the 

 tendency of the cells in these areas to respond to friction or pressure by a 

 further proliferation, thus leading to an individually-adapted response. 



Scales 



Scales are specialized epidermal thickenings with a characteristically 

 patterned appearance which are strongly developed among reptiles where 

 they form a horny exoskeleton sometimes of considerable thickness 

 (Fig. 28). They are never separate, as in fishes, being simply localized 

 thickenings of an otherwise continuous epidermal layer. The scales of fish 

 are in fact quite distinct structures of dermal origin (p. 75) and are not 

 homologous to epidermal scales. Scales are also found on the legs of birds, 

 revealing their reptilian affinities, and among a few mammals such as 

 rodents, the scaly tails of rats being familiar. The well-developed covering 

 of the scaly ant-eater is said to be a secondary development. 



Beaks and bills are horny developments of the jaw margins, often 



