DIFFERENTIATION AND PROTEIN SYNTHESIS 93 



constrained by this barrier formed beneath them. The basal membrane, 

 itself becomes the seat of further reinforcement in the form of collagen 

 fibrils. It is therefore proper to emphasize the importance of the basal 

 membrane in the architecture of the tissue; and further, one must 

 pinpoint, as the earlier crucial step in organ construction, the secretion 

 by the epithelial cells of a substance which can (a) link specifically to 

 sites on their surface and become an intercellular cement, and (b) when 

 the cells face the mesodermal space, react with mesodermal products to 

 form a basal membrane to which the cells are anchored. 



A further important development which takes place at about the same 

 time as the formation of the basement membrane is the appearance of a 

 patterned arrangement of dense deposits over the surface of the cell facing 

 the basement membrane. These are the dermoepithelial desmosomes 

 (Fig. 36 and Plate 14B). They are not identical in structure with those 

 which form between epidermal cells (epidermal-epidermal desmosomes) 

 or in similar epithelia and which are illustrated in Fig. 21, p. 42 and 

 Plate 12C. That these structures are mechanically attached to the basal 

 membrane was shown by Weiss (1958). The deposit within the epidermal 

 cell often is double-layered (Weiss refers to them as " bobbins ") or even 

 multi-layered (Porter, 1956). 



The reason for the formation of these localized deposits is not known. 

 They arise evidently in response to some external stimulus — another cell 

 or a basal layer. It could be, adapting an idea due to Weiss (1950), that 

 the localized external stimulus promotes a selective absorption of a cyto- 

 plasmic component on the plasma membrane which then provides the 

 site for further depositions. Alternatively, the cells response may be 

 due to the localized entry of some material from without which combines 

 with and precipitates an intracellular component. This is equivalent to 

 saying that the desmosomes mark the sites of porous spots. A section 

 running parallel to and through the neighbourhood of the basal membrane 

 (Fig. 39, Plate 14B) gives the impression of a rather regular arrangement 

 recalling that of the pores in the nuclear membrane (Plate 4C). The 

 concept of " membrane pores " is current as an explanation of specific 

 membrane permeabilities (Danielli, 1942). These are not thought of as 

 small " holes " but areas of modified porosity revealing themselves only 

 by the intracellular reaction provoked by an entering molecule. 



That the ends of intracellular fibrils become attached to desmosomes 

 may be accounted for by assuming that the dense deposits present favour- 

 able sites for the initiating fibril growth. The importance of these attach- 

 ments for the ultimate mechanical function of the cells is obvious (see 

 also p. 95). 



