THE KERATINIZED TISSUES 61 



underlying mesodermal domain. In the first place the external situation 

 imposes a class of differentiations and secondly the mesodermal organiza- 

 tion further limits differentiation and gives rise to site-characteristic 

 developments. As these develop and call into being an appropriate dermal 

 organization to support them, consisting in part of fibrous collagenous 

 depositions and a blood supply, the situation is reversed. The epidermal 

 tissue becomes dominant and grafting now shows that, when sufficient 

 epidermal tissue is transferred, the site characteristics (skin, claws, 

 feathers, etc.) are now preserved (Cairns and Saunders, 1954). 



xxxyxxx 



000 era ip BM 



1 o a m «* 



Fig. 27. Diagrammatic representation of the interreaction between 

 epidermis and dermis in successive stages of the establishment of a 

 differentiated epidermis (r.h.s.). O, outside environment, I, internal 

 environment, BM, basal membrane. The arrows indicate the direction 

 of dominant influence and the shading differentiated cells. 



From the numerous experiments (Zwilling, 1955; Waddington, rev. 

 1956; and McLoughlin, 1959) which show that the underlying mesen- 

 chymal tissue induces and maintains the different epidermal differentiations, 

 there is a suggestion that the basal membrane itself could be the important 

 factor. It appears to differ in thickness from site to site (200-600 A) and 

 there are differences in the types of mucopolysaccharides present. It is 

 easy to picture such a continuous layer of colloids of this type, with their 

 fixed network of charged sites, functioning as ion exchange resins (Meyer, 

 1956) and exerting a selective effect on the transfer of signal molecules 

 from the blood to the epithelial cell population. The idea is, however, 

 very insufficiently explored experimentally as yet. 



If, at some risk, one attempts to summarize the findings in a field as 

 yet imperfectly explored experimentally and in rapid development, it 

 seems possible to distinguish several successive stages in the establishment 

 of the skin in which dominant control in the dermoepidermal partnership 

 swings successively from one member to the other as suggested by the 

 arrows in Fig. 27. In the earlier phase the superficial cells, responding to 

 their exposed position (see also p. 90), begin to stick together and thus 



