42 KERATIN AND KERATINIZATION 



stretched (reviewed in Fawcett, 1958) that the desmosomes and terminal 

 bars are effective in holding the cells together and have been referred to as 

 attachment plaques. They are elaborately developed as the intercalated 

 disks in heart muscles where their role is again at least partly mechanical 

 in transmitting tension from cell to cell. Here the contractile fibrils enter 

 the desmosomal deposits when they reach the ends of the cells (Fig. 21 (e) ). 





(d) 



Fig. 21. Examples of desmosomes. In each case the desmosome is 



shown as a pair of opposed thickenings of the cell membrane to which 



may be attached small tufts of fibrils. 



(a) The " terminal bar " type of desmosome is found in columnar or 

 cuboidal epithelia where it forms a long band running along near the free 

 edges of the cells. It could assist in preventing the separation of the 

 cells (Plate 5A). 



(b) The simple plaque desmosome found in most squamous epithelia. 

 Extracellular sheets may be deposited between the plaques in some 

 situations (see page 92). This type of structure forms the traditional 

 " intercellular bridge." 



(c) Desmosomes of type (b) may be regularly spread over the surfaces 

 of contact which may become more or less regularly corrugated to 

 produce a " tongue and groove " effect (Plates 6A and 12C). 



(d) When a small duct passes between two cells desmosomes may 

 again form in the positions shown where they could function to contain 

 the duct. 



(e) The very elaborate desmosomal development between cells in 

 heart muscle where it is recognized as an " intercalated disk." The 

 contractile muscle filaments end in the dense deposits at the cell 

 boundaries (Fawcett, 1958). 



