92 RETICULAR FIBRILS PRODUCED BY CAPILLARY ENDOTHELIUM. 



lymph-nodes, the exact nature of these cells has been in doubt. Evans (1914, 

 1915) placed them in the category of endothelial phagocytes. Downey (1915) gives 

 a good account of the two prevailing opinions, pointing out quite correctly that the 

 same cells produce the reticulum; but his further assumption that by that fact 

 they can not be endothelial, would seem to be invalid when we know that endothe- 

 lium in other places has the power to lay down fibrils. In the spleen and lymph- 

 node we may assume that the endothelial cell and that which forms reticulum are 

 one and the same. 



Our results would also seem to bear on the discussion of pathologists as to the 

 connective tissue in endothelial neoplasms. Wooley (1903) has reported the fre- 

 quent presence of "intercellular fibrils" in these tumors, which he ascribes to rever- 

 sion of the proliferating endothelial cells to a more primitive mesoblastic character. 

 We have shown that such a function is in some localities characteristic of fully 

 differentiated endothelium. 



DESCRIPTION OF FIGURES. 



All the preparations illustrated are from sections cut at 4 microns, and stained by Ferguson's modification of 

 the Bielschowsky-Maresch method. AH except those shown in figures 1, 7. and 8 were counterstained with alcoholic 

 carmine. The plates were drawn by Ralph W. Sweet. 



PLATE 1. 



FIG. 1. Corpus luteum of cow, fetuses 210 mm. long. (Preparation by Mr. J. F. Cobb, jr.) X 1,100. 



cap a blood-capillary in longitudinal section. 



retic reticular fibrils in an endothelial cell of the capillary wall. 



FIG. 2. Corpus luteum of sow, fetuses 20 mm. long, showing reticular fibrils in the capillary endothelium. X 1,160. 



cap a blood-capillary in cross-section, containing an erythrocyte. 



nuc nucleus of an endothelial cell. 



retic reticular fibrils which can be traced to a capillary, but are not themselves actually in a 



vessel-wall. 

 FIG. 3. Adrenal gland of rat, showing reticular fibrils in the endothelial cells of a sinusoidal blood-vessel in the zona 



reticularis near the point of exit of central vein from the medulla. X 1,100. 



sin sinusoid. 



fibr fibrils connected with the endothelium but not actually in the vascular wall. 



FIG. 4. Adrenal gland of rat, showing reticular fibrils in the endothelium of a capillary in the zona fasciculata. Some 



of the erythrocytes are impregnated lightly, others are intensely blackened. X 1,100. 

 FIG. 5. Hypophysis of beef, showing reticular fibrils in the endothelial cells of a sinusoidal blood-vessel. X 1,100. 



sin sinusoid. 



fibr interlacing reticular fibrils displayed where a portion of the vascular wall is cut tangentially 



PLATE 2. 



FIG. 6. Thyroid gland of rat, showing reticular fibrils in the endothelium of a capillary between two alveoli. X 1,100. 



nuc nucleus of an endothelial cell. 



eryth erythrocytes in the capillary lumen. 



FIG. 7. Kidney of adult rat. Small area of cortex, showing that the rcticular fibrils, which here form basement- 

 membranes for the convoluted tubules, are imbedded in the endothelium of the capillary blood-vessels. 



The figure also shows a portion of a very small venule, in which endothelium is free from fibiils. X 880. 



vein venule of the smallest order. 



eryth erythrocytes in capillary. 



FIG. 8. Kidney of adult rat, showing relations of the fibrils to the epithelial elements of Bowman's capsule. X 760. 



epith epithelial element of Bowman's capsule, which is slightly wrinkled, so that its cytoplasm 



is clearly visible and is seen to be distinct from the reticular fibrils. 

 Fid. 9. Kidney of fetal pig 145 mm. long. Area of medullary ray showing portions of a collecting tubule and of a 



secreting tubule (probably a distal convoluted tubule near the ascending limb of Henle's loop). The 



field contains reticular fibrils produced both by fibroblasts and by endothelial cells. X 760. 



bl. vess. . . .blood-vessels. 



retic. . . .reticular fibrils produced by endothelium, forming the basement-membrane of a secreting 

 tubule. 



fibrobl. . . .fibroblasts producing reticulum and forming supporting sheath of a collecting tubule. 



