CHAPTER IV 



The Growth of Epidermal Structures 



The epidermis as a growing organ 



The growth of epidermal structures is a subject of interest and im- 

 portance in itself; however, it gains a wider importance since, for reason 

 of the ease with which superficial changes can be observed, the epidermis 

 is often the tissue chosen for the investigation of the factors governing 

 growth in general. Obviously an account of such an enormous subject 

 would be impossible here. Nevertheless, the peculiar suitability of the 

 epidermal structures for these studies and the probability of their future 

 use, makes a limited discussion of some points desirable. 



The sum total of the epidermis and its appendages constitutes an organ 

 of a quite definite morphological and functional character, as well defined 

 as that of the internal organs. Its pattern, in considerable detail, is charac- 

 teristic of the species, with usually a male and female variation under the 

 supplementary control of the sex hormones. The fact that it is a superficial 

 organ, with much easily-observed structural detail some of which, such as 

 feather or hair, is amenable to quantitative evaluation, is the reason for its 

 use by taxonomists, geneticists, experimental physiologists and others. 

 Its pathology provides valuable signs and symptoms of less-readily 

 observed disorders. The feather, hair or nail is in fact a permanent record 

 in chronological order of the synthetic events which led to its formation. 



In the adult, the cells in most organs divide infrequently; growth has 

 practically ceased, and the residual divisions are probably those required 

 to make good " wear and tear." In certain situations, however, e.g in the 

 seminal vesicles, the intestinal mucosa, the bone marrow and in the epi- 

 dermis itself (Leblond and Storey, 1956), cell loss is a normal physiological 

 process and cell division continues as part of the normal activity of the 

 tissue. In exposed situations superficial cells are simply shed or scraped 

 off and obviously their loss must be made good. The entire range of 

 epidermal derivatives is maintained by the proliferation of the cells of the 

 continuous germinal layer underlying the whole system, a population of 

 apparently-uniform and interchangeable cells (p. 57). Thus the problem 

 of the growth of the whole formation resolves itself into the question of 

 what factors control cell division in the germinal layer and what determines 

 the course of differentiation of the cells after leaving this layer. 



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