40 



EPITHELIAL TISSUES 



kidney the marginal processes are more prominent and divergent, forming 

 'brush borders.' Where the cell membrane becomes greatly thickened on 

 the free surface of a columnar cell, it is termed a crusta. Also, in neuro- 

 epithelial cells, the free border mediates reception, the attached pole 

 transmission, of stimuli. 



(b) Modified 'Columnar Epithelium 



(1) Ciliated Epithelium. Here the columnar cells carry upon 

 their free surface a group of delicate hairlike processes called cilia, or a 

 single flagellum (flagellate cells of lower forms), which during life are 



capable of a rapid vibratory or undu- 

 latory motion, a further expression of 

 cell polarity. The direction of this 

 ciliary motion is constant and is such 

 as to produce a definite current within 

 the fluids which bathe the surface of 

 these cells whose direction is invariably 

 from within toward the external surface 

 of the body. In the human body the 

 cilia occur almost exclusively upon the 

 free extremities of columnar cells 

 (Figs. 45, 51 and 52). In some of the 

 lower animals, as for example in the 

 mouth of the frog, cilia are found also 

 upon polyhedral and pear-shaped cells. 

 In simplest form cilia are pseudopod- 

 like extensions of the cytoplasm of the 

 cell body, and may be regarded as modifications of its exoplasm. 



The ciliated border is separated from the protoplasm of the cell body 

 by a fine chromatic line, which on higher magnification resolves into a 

 number of knob4ike segments, the basal granules (Fig. 22, Chap. I). The 

 cytoplasm and nucleus of ciliated epithelium, except for the peculiarities 

 dependent upon the formation of cilia, is similar to that of the simple 

 non-ciliated columnar cells. Their cytoplasm as in other types, may 

 contain vacuoles, pigment granules, metaplasm, and even secretory 

 granules, e.g., epididymis. 



(2) Glandular Epithelium. This type derives its name from the 

 fact of the predominance of the glandular function. This condition is 



FIG. 45. COLUMNAR CILIATED EPI- 

 THELIUM FROM THE EPIDIDYMIS 

 OF A RABBIT. 



a, epithelium; 6, connective tissue; 

 c, cilia. A leukocyte is seen between 

 the bases of the columnar cells. 

 Hematein and eosin. X 550. 



