54 



HISTOLOGY 



PROCESSES OF SECRETION IN EPITHELIAL CELLS. 



Many epithelial cells elaborate and discharge substances which do 

 not become parts of the tissue. Such cells are called gland cells, and their 

 products are either utilized by the body (secretions) or eliminated as waste 

 products (excretions). The process of elaboration and discharge of the 

 secretion or excretion may often be recognized by changes in the form 

 and contents of the cell. A gland cell which is full of secretion, or dis- 

 charging it, is called "active," and one in which the secretion is not 

 apparent, though it may be in process of formation, is called "resting." 

 The appearances during secretion differ in two types of gland cells the 

 serous, which produce watery secretions, like saliva; and the mucous, 

 which form thick secretions, like those of the nose and throat. These 

 will be considered in turn. 



Serous gland cells, when empty, are small and darkly staining. As 



Granule. 



Protoplasm -- 



bt _ A- JAV^M w * --. 



Basal filaments. 



Granule. 



Protoplasm. 



New granule. 



Nucleus. 



"Large nucleolus. 



A B 



FIG. 44. Two SEROUS GLAND-CELLS FROM THE SUBMAXILLARY GLAND OK A GUINEA-PIG. X 1260. 

 In cell B the granules have passed into the unstainable state; new stainable granules are beginning to 



develop in the protoplasm. 



the formation of secretion begins, the cells, if prepared with special 

 methods, exhibit granules which stain intensely. These granules have 

 become cut off from the basal filaments or mitochondria (Fig. 44, A). 

 They enlarge, lose their staining capacity, and are transformed into drops 

 of secretion. The entire cell becomes larger and clearer than before, and 

 the alveolar structure of its protoplasm is well marked (Fig. 44, B). 

 Finally the droplets become confluent and are discharged from the free 

 surface of the cell. A portion of the mitochondria remains behind as 

 the source of further secretion. In many gland cells the cytoplasmic 

 differentiation is accompanied by changes in the nucleus. In the empty 

 cell the nucleus has distinct nucleoli and a fine chromatic reticulum, but 

 in cells full of secretion the nucleoli have enlarged or disappeared and the 

 chromatin is in the form of coarse masses. Particles pass from the nucleus 

 into the cytoplasm, and these have been said to give rise to secretory 

 granules. 



In mucous cells the process of secretion also begins with granule for- 

 mation, but the mucigen granules gather near the free surface of the cell 



