STRATIFIED EPITHELIUM 



47 



the cytoplasmic portion of the columnar colls with their ciliated bor- 

 ders. 



This typo of epithelium is frequently designated simply 'stratified 

 columnar'; and in fact in certain instances under conditions of further 

 modification involving a separation of the taller cells from the Itascmcni 

 membrane, it passes, over more or less extensive areas in the respiratory 

 and male genital tracts, into actual .stratified columnar epithelium. 

 Toward the proximal end of the male urethra the epithelium is of the 

 true stratified columnar (non-ciliated) type. 





3. TRAXSITIONAL EPITHELIUM 



This variety resembles somewhat stratified squamous epithelium in 

 that it is composed of several cell layers, the deeper cells of which are 

 more nearly polyhedral but are somewhat flattened upon the free sur- 

 face, but differs in having a smaller number of cell layers in which 

 respect it is 'transitional' 

 between simple and strat- 

 ified squamous epitheli- 

 um and in the charac- 

 ter of the superficial 

 cells. Transitional epi- 

 thelium is not usuallv 



\j 



more than from three to 

 ten cells deep, four to six 

 being the rule. The 

 number of cell layers and 

 the consequent actual 

 thickness of epithelial 

 membranes is to a cer- 

 tain extent dependent 

 upon their state of ten- 

 sion during life ; thus the 

 transitional epithelium of 



the urinary bladder is much thicker when the organ is collapsed than dur- 

 ing distension. 



The deepest cells are polyhedral, and these form the greater portion 

 of the membrane. Only the more superficial layers differ therefrom. 

 Those polyhedral ceils which lie in the midregion of the epithelial layer 

 possess a peculiar flask or pear shape, with well-rounded bodies and a 



FIG. 53. TRANSITIONAL EPITHELIUM FROM A TRAN- 

 SECTION OF THE URETER OF AN INFANT. 



(7, epithelium; b, connective tissue. Hematcin and 

 eosin. X 550. 



