56 KERATIN AND KERATINIZATION 



become progressively more specialized as they near the surface and may 

 show an internal polarization of structure. They are bounded below by 

 the basement membranes separating them from the connective tissues 

 which are vasculated. Being themselves avascular, nutrient materials 

 must reach them by passing from the vessels of the subjacent connective 

 tissue into the interstitial fluid and thence by diffusion across the basement 

 membrane. There may often be a constant loss of cells from the exposed 

 surface (squamous epithelia) which must be made good by the proliferative 

 activity of a germinal layer. 



The keratinizing epithelium is normally squamous, its hardened 

 surface cells being shed either continuously or at intervals as a whole 

 in the form of a moult. In its simplest form it may appear to consist of a 

 single layer of living germinal cells and a thin cuticle of keratinized cells 

 as in the mouse. Usually more layers can be defined and the character of 

 the skin is influenced by the number of cells in each layer. An idealized 

 generalized, keratinizing epidermis might be said to consist of at least the 

 following layers: (a) a germinal layer of cells whose proliferation main- 

 tains the entire cell population; (b) a differentiating layer in which the 

 protein is synthesized and keratinized; and (c) the dead and hardened 

 layer (see Fig. 25). If the tissue is to be of constant average thickness, the 

 cells formed over a given time must equal those lost by exfoliation in the 

 same time. The thickness varies with the total number of cells in each 

 of the layers and structures with a variety of properties may be produced by 

 variations in the proportion of differentiating cells and hardened cells, 

 i.e. in the relative thicknesses of the softer still hydrated layers and the 

 tough, cornified and relatively drier cells. The ease of exfoliation deter- 

 mines the thickness of the homy layer and obviously a thick, hard layer 

 will result if the exfoliation is slowed down. Further by supposing 

 localized differences in rate of cell formation and loss, it is possible to 

 understand, in principle, the production of the various horny appendages. 



It is sometimes said that keratinization is a degenerative phenomenon, 

 a consequence of poor nutrition, of desiccation or other deleterious 

 factors. That this is not true is shown very clearly by observations on cells 

 cultivated in vitro where conditions are under closer experimental control 

 (Fischer, 1924; Miszurski, 1937; Hardy, 1949; and Strangeways, 1931). 

 Skin cultivated in vitro readily undergoes keratinization with the pro- 

 duction of histologically-normall}' keratinized cells. The same sequence of 

 histochemical events as in vivo occurs and the product is also birefringent 

 (Litvac, 1939; Miszurski, 1937; and Hardy, 1949). Feathers have been 

 cultivated to a limited degree. Strangeways and Hardy (Strangeways, 

 1931; Hardy, 1949) also succeeded in growing hairs and noted even in 

 these abnormal conditions that histogenesis, up to a certain point, and 

 fibrillar orientation proceeded normally. 



