CHAPTER VI 



EPITHELIUM (INCLUDING MESOTHELIUM AND 



ENDOTHELIUM) 



General Characteristics. — Epithelium is derived from all three 

 germ layers. In general it covers surfaces and lines cavities and 

 diverticula from them, for example, the duct glands. Thus located, 

 it serves for protection, takes part in the formation of neuroepithelial 

 end organs for the reception of stimuli, and is the active agent in the 

 elaboration of various secretions and excretions. Epithelium con- 

 sists almost wholly of cells.- The intercellular substance is merely 

 sufficient to attach the cells to one another and is, consequently 

 known as cement substance. A characteristic of this cement substance 

 is its reaction to silver nitrate (Figs. i6, 28, 29). In some instances 

 the protoplasm of adjacent epithelial cells is seen to be even more 



^Vfs* 



^. ^ 



^l%l'>? ^%^S'^''. 



Fig. 15. — Simple Cylindrical Epithelium from Uterus of Monkey — Iron-alum- 

 hsematoxylin Stain. (Krause.) kl, cement wedges separating the free ends of the 

 cells; ck, centrioles. 



closely associated, the intervening cement substance being bridged 

 over by delicate processes of protoplasm which pass from one cell 

 to another and are known as '^ intercellular bridges'' (see Fig. 20, p. 

 77). It seems probable that the minute spaces between the processes 

 serve as channels for the passage of food (lymph) to the cells. Ref- 

 erence to Figs. 159 and 191 shows that a precipitation of silver also 

 occurs in some tubules filled with glandular secretions. It is possible 

 that the so-called intercellular cement may be of the nature of an 

 intercellular lymph. The surface cells of epithelium arc united by 

 continuous cement substance in larger amount (Fig. 15), in which 

 there are apparently no si)aces. In tliis way escape of lymph is 

 prevented. 



