28 



CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



Stratified Epithelium. The term stratified epithelium is employed 

 to describe the type found in the skin or its derivatives in which the cells 

 forming the epithelium are arranged in a considerable number of superim- 

 posed layers. The shape and size of the cells of the different layers, as well 

 as the number of layers, vary in different situations. Thus the superficial 

 cells may be either squamous or columnar in shape and the deeper cells 

 range from polygonal to columnar in form. 



Stratified Squamous. The intermediate cells are polygonal in shape and 

 approach more to the flat variety the nearer they are to the surface, and to the 



FIG. 26. Squamous Epithelium Scales from the Inside of the Mouth. X 260. (Henle.) 



columnar as they approach the lowest layer. In many of the deeper layers 

 of epithelium in the mouth and skin, the outline of the cells is very irregular, 

 in consequence of processes passing from cell to cell across these intervals. 

 Such cells, figure 28, are termed "prickle" cells. These "prickles" are the 

 intercellular bridges which run across from cell to cell, the interstices being 

 filled by the transparent intercellular lymph. When this increases in quan- 



FIG. 27. Vertical Section of the Stratified Epithelium Covering the Front of the 

 Cornea. Highly magnified. (Schafer.) c, Lowermost columnar cells; p, polygonal cells 

 above these;/, flattened cells near the surface. The intercellular channels, bridged by 

 minute cell processes, are well seen. 



tity in inflammation the cells are pushed further apart, and the connecting 

 fibrils or "prickles" are elongated and more clearly visible. 



The columnar cells of the deepest layer are distinctly nucleated; they 

 multiply rapidly by division; and as new cells are formed beneath, they press 

 the older cells forward, to be in turn pressed forward themselves toward the 

 surface, gradually altering in shape and chemical composition until they 

 die and are cast off from the surface. 



