TISSUES 



97 



All epithelia are similar in that their cells generally possess walls, 

 they are relatively small and compact, are crowded closely together, and 

 are usually cemented to one another by an intercellular cement secreted 

 by the cells. The connective tissue underlying an epithelium often 

 forms a thin sheet called a basement membrane (Fig. 35 C to E) to which 

 the epithelial cells are attached. In some cases intercellular bridges of 

 protoplasm connect one cell to the next, and in the absence of a basement 

 membrane the deepest layer of cells in an epithelium may be anchored 

 by rootlike projections which penetrate the tissue below them. Neither 

 nerves nor blood vessels are ordinarily found in epithelia, though this does 

 not apply to nerve terminals in sensory epithelia. 



B 



Basement membrane 



Fig. 35. — Semidiagrammatic sketches illustrating different types of epithelia. .4, 

 simple pavement epithelium, seen in surface ^•ie\v and in section. B , section of simple short 

 columnar, or cubical, epithelium, also seen in surface view and in section. C, sectional view 

 of a simple columnar epithelium. D, section of simple ciliated epithelium. E, section of a 

 stratified pavement epithelium. All highly magnified. Figure ^A also shows a single 

 very thin pavement epithelial cell. 



When epithelial cells have a secretory function the secretion may be 

 accumulated in droplets within the cells (Fig. 36). In the case of 

 enzyme-secreting cells droplets or granules containing a zymogen are 

 accumulated, and when secretion occurs the zymogen is transformed to 

 the enzyme and passed out of the cell. These droplets or granules 

 become massed at the outer end of the cell and the cells consequently 

 become markedly granular in texture. The droplets or granules dis- 

 appear when the enzyme is formed and passed out through the cell wall, 

 to reappear during the time the gland is not secreting. Examples are 

 the cells of the salivary glands and the pancreas. In some cases, how- 

 ever, as in the case of cells which secrete mucus, the droplets flow 

 together and form a great mass toward the outer end of the cell. The 

 secretion is set free by the rupturing of the cell wall. Examples are the 

 mucus-secreting goblet cells of the intestines of vertebrates. In still 



