KERATIN AND MOLECULAR BIOLOGY 37 



phosphotungstic acid or lead salts. Useful discussions of these problems 

 have been given by Palade (1952), Sjostrand (1956), Bahr (1954) and 

 Baker (1945, 1955). 



The reliability of electron microscopic findings is assessed by comparing 

 them when possible, with those of the light microscopy or X-ray 

 diffraction (see Fig. 1), by the internal evidence of the micrographs, 

 which may suggest deleterious changes and, in the final analysis, by their 

 contribution to the understanding of the problem of the function of the 

 tissue or organism. 



We shall treat first, and in greater detail, the cell surface and its 

 specializations, since these are of greater importance for our subsequent 

 discussions ; the structures found in the cytoplasm follow next and finally 

 the nucleus. 



The Cell Surface, its Specializations and Intercellular 

 Adhesion 



The keratinized epidermal tissues are cellular in the sense that the 

 amount of intercellular material is very small; the cells surfaces are 

 effectively in contact and the whole formation owes its coherence to 

 intercellular adhesion. This is in contrast to the mesodermal tissues where 

 the extracellular material greatly predominates (Fig. 12), and coherence 

 is due in the vertebrate to the meshwork of collagen and other fibres laid 

 down by the cells. The cell membrane, the nature of intercellular adhesion 

 and its modifications in cellular tissues, adapted to withstand mechanical 

 and chemical shock, must therefore be given special consideration in 

 relation to the total phenomenon of keratinization. 



The cell membrane. The simplest unicellular organisms possess a 

 boundary, which separates the intracellular domain from the environ- 

 ment, with special properties of permeability and mechanical strength 

 (Fig. 14). This cell membrane or plasma membrane may well have been 

 the first and most primitive organelle of the cell, since its existence without 

 such a definite boundary is difficult to admit. Its earliest function was 

 probably in essentials what it is today: by virtue of its selective perme- 

 ability it retains, in the neighbourhood of the cellular apparatus, a higher 

 concentration of certain molecules than exists in the surrounding medium. 

 Accordingly the permeability properties of the plasma membrane have 

 been much studied and this work has been summarized by Davson and 

 Danielli (1952) and Danielli (1942). The membrane proves to have pre- 

 dominantly a lipid character, i.e. it is most permeable to substances 

 soluble in non-aqueous solvents, as would be required of a membrane 

 whose principal function is to act as a barrier to substances dissolved in 

 water. This finding is supported by actual chemical analysis of membranes, 

 such as that of the erythrocyte, which can be obtained largely free from 



