90 RETICULAR FIBRILS PRODUCED BY CAPILLARY ENDOTHELIUM. 



are the endothelial walls, and these contain the reticular fibrils. Figure 6 well 

 represents the condition. 



RENAL BASEMENT MEMBRANES. 



Besides the glands of internal secretion, there are other organs in which fixed 

 interstitial connective-tissue cells are known to be scarce or absent, yet which 

 possess well-developed supporting fibers, and in which the capillaries are intimately 

 applied to the secreting cells whose functions they subserve. These conditions exist 

 in the cortex of the kidney, and here again the point of departure for our investiga- 

 tion is found in the work of Dr. Mall. In his first paper of 1891 he showed that 

 after digestion of sections of the kidney with pancreatin the entire structure, from 

 capsule to pelvis, including the basement membranes, is a single mass of anastomos- 

 ing fibrils which possess all the characteristics of reticulum. This statement, amply 

 confirmed by Riihle (1897), was in disagreement with the older belief in a homo- 

 geneous basement membrane. The perplexity was cleared up in 1901 by Mall's 

 discovery that both structures exist, the structureless membrane applied closely 

 to the bases of the epithelial cells, and itself intimately invested without by the 

 cylinder of interlacing fibrils. In sections prepared with connective-tissue stains 

 it is the outer coat that gives the traditional appearance of a basement membrane; 

 in the macerated and teased preparations of older histologists probably the inner 

 coat was most obvious. Mall's whole conception has recently been confirmed and 

 extended by von Frisch (1915). 



It is now generally held, therefore, that there is an intertubular stroma through- 

 out the kidney, composed of interwoven fibrils, some of which are condensed against 

 the tubules to form the membrana propria. They are said to be produced by 

 flattened nucleated cells which are more common in the kidneys of young animals; 

 according to Disse (1902), who has given a full description of the "renal stroma," 

 in adult life the cells are found chiefly in the neighborhood of the papillae and the 

 fibrils become independent of the cells. 



Application of the Bielschowsky method with counterstaining permits us to 

 observe the exact relation of the fibrils to surrounding cells with a clearness not 

 known to former observers, and it at once becomes evident that we shall have to 

 revise the conception of the stroma of the kidney as well as of the endocrine glands 

 previously described. In the renal cortex, between the convoluted tubules and 

 about the glomeruli, the "stroma" is no more nor less than a network of reticular 

 fibrils imbedded in the cytoplasm of the capillary endothelium or deposited by the 

 latter against the tubules and Bowman's capsules. True fibroblasts are very infre- 

 quent, perhaps altogether absent. Figure 7 shows the renal cortex of the adult rat. 



In the papillae and along the medullary rays there is, on the other hand, a true 

 stroma consisting of fibroblasts which produce reticular fibrils. Figure 9 is taken 

 from a medullary ray in the renal cortex of a fetal pig 145 mm. long, and well illus- 

 trates the contrasting condition. The medullary fibrils are in general much thicker 

 than those of the cortex, but no chemical difference has as yet developed, such as 

 might be expected from the different origin of the two types. 



