ENDOTHELIAL TISSUES. 



71 



Mesothelial cells from omentum of dog. X 300. Intercellular 

 cement-substance stained by argentic nitrate. 



ENDOTHELIUM. 



The modified mesoblastic, later connective-tissue, cells that line serous surfaces, 

 including those of the pericardial, the pleural, and the peritoneal divisions of the 

 body-cavity, together with those of the blood- and lymph-vessels and the lymphatic 

 spaces throughout the body, constitute endothelium. These spaces, in principle, are 

 intramesoblastic clefts and the elements forming their lining are derivatives of the 

 great connective-tissue layer. The endothelia, therefore, belong to the connective 

 tissues and are properly regarded as 



modified elements of that class ; as Fig. 91. 



a matter of convenience, however, 

 they may be considered at this place 

 in connection with the epithelial tis- 

 sues. 



The most striking difference in 

 situation between the endothelia and 

 the epithelia is found in the fact that 

 the former cover surfaces not com- 

 municating with the atmosphere, 

 while the epithelial tissues clothe 

 mucous membranes all of which are 

 directly or indirectly continuous 

 with the integumentary surface. A 

 further contrast between these tis- 

 sues is presented in their genetic re- 

 lations with the primary blastodermic 

 layers, since the epithelia, with the 

 exception of those lining certain 

 parts of the genito-urinary tracts which are derived from the mesoblast, are the trans- 

 formations and outgrowths from the ectoblast and the entoblast, while the endo- 

 thelia are direct modifications of the mesoblastic cells. 



The young mesoblastic cells bordering the early body-cavity become differenti- 

 ated into a delicate lining, the ?nesothelium, and later give rise to the characteristic 



plate-like elements which constitute the 

 lining of the permanent serous sacs. The 

 name mesothelium is sometimes retained to 

 designate the permanent investment of the 

 great serous cavities, as distinguished from 

 the endothelium which clothes the vascular 

 and other serous spaces. 



Seen in typical preparations, as ob- 

 tained from the peritoneum after treatment 

 with argentic nitrate and subsequent stain- 

 ing with haematoxylin, the endothelial cells 

 on surface view appear as irregularly polyg- 

 onal areas mapped out by deeply tinted 

 lines. The latter represent the silver- 

 stained albuminous intercellular cement- 

 substance which unites the flattened cells 

 in a manner similar to that observed in 

 simple squamous epithelium ; this superficial likeness is so marked that it has led to 

 much confusion as to the proper classification of endothelium under the connective 

 tissues. The lines of apposition are sinuous and less regular than between epithelial 

 elements, in many cases appearing distinctly dentated. The exact form of the cells 

 and the character of their contours, however, are not constant, since they probably 

 depend largely upon the degree of tension to which the tissue has been subjected. 



Not infrequently the intercellular substance, at points where several endothelial 

 cells are in apposition, shows irregular, deeply colored areas after silver staining ; 



Fig. 92. 



Endothelial cells lining artery of dog, after silver 

 staining. X 500. 



