KERATIN AND MOLECULAR BIOLOGY 43 



An important feature of the desmosome, which indeed justifies the 

 special attention given to it, is that it probably represents a point of fixed 

 (quasi-permanent) intercellular attachment. This permanence one deduces 

 from the observation that the structures in one half of the formation, 

 which must have taken some time to form, are mirrored closely by similar 

 structures in the opposing half. The two halves must thus have remained 

 opposed for some time. Elsewhere, over the shared surfaces of contact 

 where one observes interdigitating folds, it would seem that the surfaces, 

 although sticky, can slide laterally over each other, i.e. the intercellular 

 cement has the properties of a viscous liquid. One can readily visualize 

 that a range of viscosity is possible, depending on the degree of cross- 

 linking of the molecules between the surfaces. It is also possible that these 

 deposits may mark the sites where special forms of communication between 

 cells takes place, but definite evidence for this is wanting. 



That fine intracellular fibrils, usually ending on desmosomes, occur in 

 many if not all cells has been long recognized (Schneider, 1902 and Schmidt, 

 1924) and recently Leblond and colleagues (Leblond et ai, 1960 and 

 Puchtler et al., 1958) by systematically applying a new staining technique 

 (successive treatment of fixed tissue with tannic acid, phosphomolybdic 

 acid and amido black: " TPA " staining) have demonstrated them in 

 many kinds of cells with exceptional clarity. The geometrical arrangement 

 of these fibrils and their attachment to studs (desmosomes) on the cell 

 membranes suggests a mechanical role in maintaining cell shape and 

 rigidity, i.e. they are literally " tonofibrils " (see p. 94). They stain as 

 basic proteins quite distinct from the extra-cellular collagen fibrils but 

 similar to the first formed fibrils in keratinizing systems. From a com- 

 parison of their location and density in several different cells, it is clear 

 that they occur in enhanced amounts in precisely the situations where 

 support is demanded. Epidermal cells and muscle cells show the most 

 marked development of TPA positive fibrils and in these cells their identity 

 with keratin and muscle fibrils respectively is obvious. It would seem 

 possible that all these fibrillar systems are composed of homologous fibrous 

 proteins and that chemical modifications have been developed to fit them 

 for special purposes. Keratinized fibrils, for example, are modified to 

 enhance their strength and stability; muscle fibres show an enhancement 

 of the latent contractility of the polypeptide chain. 



Inter digitation of confronted membranes. In some epithelia the opposed 

 cell membranes, while remaining parallel, become greatly convoluted or 

 corrugated and a tongue and groove relation may develop, which has the 

 effect of greatly increasing the area of contact and presumably the adhesion. 

 Desmosomes usually form on such surfaces to add to the adhesion 

 (Fig. 21C and Plate 6A). 



In keratinized tissues with their special requirements of strength it is to 



