SURFACE MODH'K'ATIOXS OF EI'ITIIKIAAL CELLS 37 



PIGMENTATION IN EPITHELIAL CELLS. 



In various locations epithelial cells normally contain pigment 

 granules, and this constant characteristic has lead to speaking of 

 such epithelia as being of the pigmented type. However, this does 

 not indicate the morphology of the cells, which may be rightly 

 classified as cuboidal or other types. To follow the scheme of 

 morphology in classification, such cells should be classified on the 

 basis of their form and arrangement and then the presence of the 

 pigment should be noted for the gi\en type in certain locations. 

 An example of this tissue occurs in the outer layer of the retina, 

 where hexagonal cells adjacent to the inner surface of the choroid 

 coat are heavily pigmented. Pigmentation granules also occur in 

 the lower layers of cells in stratified squamous epithelium forming 

 the epidermis of various mammals and in the liver cells of many 

 lower forms. 



SURFACE MODIFICATIONS OF EPITHELIAL CELLS. 



The free boundaries of epitlielial cells may be modified in various 

 ways. The superficial layer of protoj^lasm in some cases is con- 

 densed into a firmer portion continuous with subjacent less dense 

 protoplasm. In sections such a surface condensation appears as a 

 bright line and may extend about the entire cell, although limited 

 in some cases to the exposed surface. Occasionally this denser layer 

 presents perpendicular striations, which are considered as fine, hair- 

 like processes of protoplasm held together by intervening less 

 differentiated protoplasm. Such an arrangement, is known as a 

 striated border. The surface of other cells has a brush border in 

 which densely packed, non-motile, hair-like, protoplasmic structures 

 project slightly beyond the surface. The elements of such a border 

 are associated with small granular swellings at their base and are 

 thought to be active in absorption. The peak of such dift'erentia- 

 tion is founfl in the ciliated boundaries of various types of epithelial 

 cells where the fine, hair-like, protoplasmic processes are much 

 longer than those of the brush border. These processes, the cilia, 

 of which there may be as many as a hundred from a single cell, often 

 possess the power of movement. Less commonly cells have single, 

 whip-like, protoplasmic processes, called flagella, which are usually 

 motile. 



According to some authors, cilia possess a protoplasmic shell 

 enclosing a hollow core, which being rhythmically filled and emptied 

 with more fluid protoplasm imparts to these structures their charac- 



