KERATIN AND MOLECULAR BIOLOGY 



41 



secretion, of low intrinsic density or poor affinity for the fixatives and 

 stains of current electron microscopy, which normally coats the surfaces 

 of the cells (Weiss, 1960). 



A question of nomenclature arises here. The rather definite width of 

 the clear space between cells suggests that the surface coat itself has a 

 definite thickness and might well be included as an element of the plasma 

 membrane itself. This viewpoint is particularly cogent when specialized 

 developments of the cell membrane, such as those forming the myelin 

 sheath of nerve axons (Robertson, 1956 and 1957), are considered. How- 

 ever, for the purely morphological reason, that the double line bounding the 



B 



Fig. 20. The structure of the surface of contact between two cells A and 

 B (as seen electron microscopically in sections of tissues fixed in 

 osmium tetroxide or potassium permanganate). M x and M 2 are the two 

 plasma membranes which are each seen to consist of three sheets. 

 Between M x and M 2 is found an intercellular sheet (or cement) C. 



cytoplasm is so obvious a feature and definable in all cells, we will refer 

 to it here as the plasma membrane. In this terminology the lighter space 

 is intercellular and, since the material occupying this space is ultimately 

 responsible for sticking the cells together, it may be referred to as an 

 intercellular cement. These definitions are illustrated in Fig. 20. 



Desmosomes. In most epithelia even in the lowest of multicellular animals 

 opposed surfaces are studded with small dense areas called desmosomes 

 (Fig. 21). These are most common and conspicuous in stratified squamous 

 epithelia such as the vertebrate epidermis. They have been shown electron 

 microscopically (see Chapter 3) to consist of localized thickenings of the 

 membranes produced by the deposition of a layer of amorphous material 

 inside and outside the cell membrane. A similar layer is formed within 

 the opposing cell. In the epidermis these remain localized and appear to 

 resemble a pair of disks about 1 /n in diameter; in other situations, e.g. 

 the columnar epithelium of the intestine (Fig. 21 (a)), the desomosomes 

 may develop into bars or long bands running around the cell near the 

 free surface. There is evidence based on their behaviour when cells are 



