EPITHELIAL TISSUES 33 



EPITHELIAL TISSUES 



Epithelia are cellular membranes covering the surfaces and lining 

 the internal cavities of the body. They serve for protection, secretion, ex- 

 cretion, and the reception of stimuli. The constituent cells may be of any 

 of the above enumerated forms* 1 The spheroidal types, however, are 

 found only in embryonal membranes. The term spheroidal epithelium 

 is sometimes employed to designate masses or solid columns of spheroi- 

 dal cells, such as appear in the sex cords of the developing testis and 

 ovary, and in the early stages of glands. They are in general, outgrowths 

 or evagination from embryonic or undifferentiated epithelia. 



An epithelium may consist of a single 

 layer of cells, when it is called non-strat- 

 ified or simple epithelium. A complete 

 description, however, must include the 

 name of the preponderating type of cell, 

 e.g., simple columnar epithelium, or sim- 

 pie squamous epithelium, as the case may 

 be. Moreover, an epithelium may consist FIG. 39. COLUMNAR EPITHE- 

 of several or many ]ayers, ,vhen it becomes 

 a complex or stratified epithelium. The (Profile view.) 

 uppermost type of cells gives the name to Hematein and eosin. X 550. 

 stratified epithelium; for example, in the 



epidermis the outermost cell is of the squamous type, though the middle 

 cells are polyhedral, and the innermost columnar ; hence called stratified 

 squamous epithelium (Fig. 49). 



In the stratified epithelia the superficial cells arise through cell divi- 

 sion in the deeper layers, and if they become detached by abrasion, dis- 

 integration, or by other physiological or pathological processes, they may 

 be replaced by cell reproduction occurring in the lower layers. When 

 but a single layer of cells is present, as in the simple epithelia, loss of 

 cells over large areas will obviously become more difficult of replacement 

 by cell division. Hence it is that repair of extensively destructive patho- 

 logical conditions involving such epithelial tissues becomes exceedingly 

 difficult and often impossible, as, for example, in the alveoli of the lung. 



Each epithelial cell is to some extent a secreting cell. Sometimes 



secretion is its chief function, as is the case with goblet cells, which 



might well be called 'unicellular glands,' and which secrete abundant 



mucus. The same is true of those cells which form the parenchyma of 



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