CHAPTER II 



The Keratinized Tissues 



Generalized histology of the vertebrate epidermis 



Dermis and Epidermis 



The integument of vertebrates consists of two quite distinct parts— 

 an epidermis and a dermis (Fig. 25) which together form a well-bonded 

 unit for the protection of the organism. The two parts are of very different 

 character and of different origin embryonically. The epidermis is entirely 

 cellular and its most characteristic products are retained within the cells 

 producing them; the dermis is primarily a fibrous meshwork in which 

 are distributed sparsely cells of mesenchymal (mesodermal) origin which 

 secrete a dense feltwork of fibrils of collagen and elastin. The inter- 

 fibrillar spaces contain, among other constituents, gelatinous muco- 

 polysaccharides. All these materials are found outside the cells in contrast 

 to the intracellular location of the fibrils of the epidermal tissues. 



The dermis is continuous with the sub -cutaneous connective tissue 

 and with the other connective tissue of the body and has thus no definite 

 inner boundary such as the definite boundary separating it from the 

 epidermis. This surface separating the two is called the dermoepidermal 

 junction and has long been recognized, but its detailed structure and 

 special character were only revealed by electron microscopy. The con- 

 siderable early literature on the subject should be read in the light of these 

 newer findings which will be described in detail in the next chapter. 

 Membranes of a similar type seem to separate all the superficial epithelia 

 from the mesodermal tissues (BM in Plates 7, 9 and 23B). 



Although conspicuous and predominant, collagen is not the only con- 

 stituent of the intercellular spaces of the dermis. By means of special 

 staining techniques numerous other substances can be demonstrated in 

 the light microscope and some have been extracted and partly characterized 

 chemically. It is customary to distinguish both formed, fibrillar elements 

 and an amorphous colloidal ground-substance. Collagen, except in a few 

 special situations, constitutes the bulk of the fibrous material. Less 

 conspicuous are elastin fibrils, as yet poorly-characterized but distinguished 

 from collagen by their microscopic appearance and their extensibility, and 

 other finer fibrils called reticulin. 



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