84 KERATIN AND KERATINIZATION 



therefore not in any sense homogenous and histochemical tests merely 

 inform us of the presence of certain types of material without precisely 

 indicating its location. Wislocki (1951) demonstrated phospholipids and 

 Leblond (1951) describes a positive periodic- SchifT reaction indicating poly- 

 saccharides. Protein is undoubtedly present. Possibly the polysaccharide 

 is located intercellularly as the adhesive " cement " and the phospholipid 

 in the thickened cell membranes themselves (see also p. 94). By digesting 

 skin with various enzymes before fixation, Weiss (1958) was able to 

 " dissect " these basal membrane desmosomes. After pancreatic lipase 

 the dense surfaces lost their osmophilia indicating a high lipid content. 

 Amylase also freed the epidermal cell suggesting a polysaccharide 

 constituent. 



Cell contacts during differentiation 



The relation of the plasma membranes of cells in close contact in the 

 epithelial type of tissue of ecto- and endodermal origin has been described 

 in Chapter 1 (p. 40). Typically the two dense plasma membranes run 

 closely parallel to each other and are separated by a less-dense layer of 

 material usually of the order of 200 A thick. We shall refer to such a 

 contact as adhesive. Its strength may, of course, be supplemented by the 

 specialized organs of attachment, the desmosomes. In early embryonic 

 tissues generally, in many tissues of mesenchymal origin and in the germinal 

 layers of constantly proliferating tissues (epidermis, intestinal mucosa, 

 etc.), the adhesive type of contact is less extensively found. In these 

 tissues the cell surfaces are more convoluted, wider and less regular 

 intermembraneous spacings are to be seen (Fig. 37) and the cells seem 

 able to force small protrusions (microvilli, tubular pseudopods) into the 

 intercellular space. The poor adhesion is shown by the ease with which 

 embryonic material may be dispersed into single cells (Weiss, 1958; 

 Moscona, 1952, 1956 and 1957). 



Thus it appears that during the progression from embryo to adult there 

 is, in certain organs, an enhancement of intercellular adhesion. A change 

 of this nature could have several consequences which may be factors in 

 influencing the maturation of the tissues taking place in this period. We 

 have given reasons (p. 44) for regarding differentiation as a phenomenon 

 arising from the action of one cell upon another, or in other words, of 

 intercellular communication. This communication must be effected either 

 by the transfer of samples of cell product from one cell to another, or by an 

 effect produced when cells come into contact. In either case a change in 

 the nature and activity of the cell membranes will influence communication. 

 The further possibility that intercellular adhesion can also play an active 

 role in moulding the shape of cells and guiding their movements is 

 suggested by the work of Holfreter (1947 and 1948), Weiss (1958), 



