DIFFERENTIATION AND PROTEIN SYNTHESIS 83 



A further feature of the germinal cells, which is again indicative of their 

 unspecialized nature, is the behaviour of their plasma membranes. These 

 are seen (electron-microscopically in sections) as greatly-convoluted dense 

 lines which may often be separated by gaps of a variable width into which 

 may be thrust small surface protrusions. The pattern suggests that the 

 adhesion of the membranes to each other is not as strong as it later becomes 



(P- 84). 



In the epidermis, but less so in the basal layers of the hard keratins, 

 the cell surfaces are also studded with desmosomes (p. 41). The portion 

 of the cell surface facing the basal membrane is covered even more 

 extensively with dense, desmosomal-like deposits (Plate 14B). Between 

 these the plasma membrane appears more free and often small invagina- 

 tions are to be seen (" blebs ") which may well be associated with the 

 entrance of liquid since all the metabolites required by the epidermis 

 appear to enter through this layer (Pillai et al, 1960; Fasske et al, 1959). 

 Desmosomes have been described earlier (pp. 41-43) but, owing to 

 their importance in epidermal tissues, some further comment is required. 

 As observed electron-microscopically in sections, well-developed examples 

 appear as a pair of very dense, often planar (straight lines in sections) 

 deposits distributed over the cell surfaces. Examples may be seen in 

 Figs. 21, 36 and Plates 12C and 14B; their structure is shown dia- 

 grammatically in Fig. 21. They may be developed to varying degrees 

 ranging from a mere increase in the density of material immediately 

 adjacent to the cell membranes to large intracellular deposits in which 

 may be embedded tufts of filaments running into the cytoplasm (Plate 6) 

 and associated intercellular (extracellular) deposits. The two halves of a 

 desmosome are usually similar in degree of development. Since they 

 occur in situations where the transmission of mechanical tension from 

 cell to cell seems a reasonable supposition, most authorities think their 

 main purpose is to supplement cell adhesion and to form attachment for 

 fibrils, i.e. the desmosomes permit of an enhanced adhesion, the intra- 

 cellular deposits providing a sort of supporting backing for the fibrils 

 transmitting tension to other surfaces (p. 95). 



The dense deposits on the membrane of the layer of cells facing the 

 dermis are in a sense " half-desmosomes " since, in the absence of a second 

 cell, one half is lacking. Nevertheless here too they seem to increase the 

 adhesion of the cell to the basal membrane and thus to the collagenous 

 meshwork beneath (Fawcett, 1958; Weiss, 1959). 



Epidermal desmosomes are visible as small dots (diameter ~l/x) in 

 the light microscope. Thus the " desmosome " of the light microscopist 

 would include several distinct elements : the two cell membranes, the dense 

 intracellular deposits backing the membranes, the material between the 

 membranes and the terminations of fibrils in the dense bodies. They are 



