4 8 



HISTOLOGY 



Epithelia differ from one another, not only in origin, but also in the 

 shape of their cells, the number of layers of which they are composed, and 

 the differentiation of their cells. These features should be examined in 

 every specimen studied, and something under each heading should be 

 recorded in any complete description of an epithelium. 



SHAPES OF EPITHELIAL CELLS AND THE NUMBER OF LAYERS. 



An epithelium which consists of but one layer of cells is called a simple 

 epithelium, and its cells may be flat, cuboidal or columnar. These terms 

 refer to the appearance of the cells when cut in a plane perpendicular to 

 the free surface. If in such a section the outlines of the cells are approxi- 

 mately square, as along the upper surface in Fig. 34, the epithelium is 

 cuboidal; if they are stretched out in a thin layer so that they appear 

 linear, as along the lower surface in Fig. 34, the epithelium is flat. Endo- 

 thelium is an extremely flat epithelium, in which the cells are so thin that 

 the nuclei cause local bulgings of the cell membrane. If the epithelial 

 cells are laterally compressed, so that tall forms result as in Fig. 35, B, 

 the epithelium is columnar. Such epithelium is less accurately called 



Cuboidal epithelium. 

 " Connective tissue. 



Flat epithelium. 



FIG. 34. PORTION OF THE MEMBRANES SURROUNDING A PIG EMBRYO MEASURING 60 MM. (Allantios 



above, and amnion below.) 



cylindrical, and both cuboidal and flat epithelia are sometimes referred 

 to as pavement epithelium. Intermediate forms, which are described 

 as low columnar or low cuboidal, frequently occur. The cells of certain 

 epithelia change their shape temporarily, as in the bladder during disten- 

 tion, in the oesophagus during deglutition, and, to some extent, in the 

 arteries^ with every pulsation. During post-mortem contraction the 

 arterial endothelium is considerably thickened. Moreover during embry- 

 onic' development, epithelial cells may change from one form to another. 



On surface view the epithelial cells of all types are polygonal and usually 

 six-sided (Figs. 33 and 35, A). Geometrically a circle would come in 

 contact with six surrounding circles of equal diameter, and a cell is usually 

 in contact with six surrounding cells. The cells, however, vary in di- 

 ameter, and are often surrounded by five or seven cells and occasionally 

 by four or eight. 



An epithelium which consists of several superimposed layers is known 

 as stratified epithelium (Fig. 37). In such cases the basal cells are usually 



