38 EPITHELIAL TISSUES 



neous tissue and cornea respectively, of laboratory animals : the connective 

 tissue cells became changed into large flat cells, disposed in the manner 

 of a mesothelium. These results suggest the conclusion that the meso- 

 thelial cells of pleura, pericardium and peritoneum may be regenerated 

 in the event of destruction from exposed connective tissue cells of the 

 subepithelial stroma (W. C. Clarke, Anat. Eec., 8, 2, 1914). 



The individual squamous cells are flat plates bulging at the center 

 where the oval nucleus is located, with serrated borders. In surface view 



FIG. 42. MESOTHELIUM (surface view), FROM THE MESENTERY OF A RAT. 

 Silver nitrate and hematein. X 550. 



the endothelial cell is oblong, the long axis parallel with the long axis of 

 the vessel (Fig. 41), while the mesothelial cell is polygonal in outline 

 (Fig. 42). In sections through the nucleus, these cells in side view 

 present a flat spindle-shaped appearance. 



Mesothelia exhibit small intercellular spaces, the stigmata. They 

 have been regarded as openings between the body cavities and lymph 

 spaces and vessels; but are more probably transient structures, perhaps 

 artifacts. Abdominal mesothelia of lower forms, e.g., frog, contain also 

 permanent openings, or stomata, surrounded by specialized guard cells. 



The mesenchymal epithelial cells of synovial membranes vary greatly 

 in shape according to the degree of pressure to which they are subjected. 

 They may thus be of the cuboidal or the squamous type; and they may 

 even become pressed apart so as to expose the underlying connective 

 tissue. (Compare Figs. 44 and 240.) 



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