PHYSIOLOGIC IMPORTANCE OF LYMPH 



1065 



the mechanism by which macromolecules and parti- 

 cles entered lymphatics from the peritoneum was the 

 postulation of openings in the endothelial walls. This 

 concept was supported by the early work of von 

 Recklinghausen (181a, 181 b) and the presence or 

 absence of "stigmata" and "stomata" have been 

 debated for the last century. Cunningham (51a), in 

 reviewing the subject in 1926, concluded, "In general, 

 then, we may summarize the work which has been 

 done on the mechanism involved in the absorption of 

 particulate matter from the peritoneal cavity in the 

 following way: The earlier work all tended to estab- 

 lish the concept of the presence of actual preformed 

 physical openings between the peritoneal cavity and 

 the diaphragmatic lymphatics. This idea was gradu- 

 ally eliminated and in its place the concept of poten- 

 tial physical openings between the walls was offered. 

 In turn this hypothesis is being replaced by one which 

 assumes that most, if not all, of the particulate 

 material that is being absorbed from the peritoneal 

 cavity passes directly through the cytoplasm of the 

 mesothelial cells." 



Lane Allen and his group have more recently 

 revived the concept of potential physical openings. 

 In experiments designed to test the upper limits of 

 absorption, Allen (ib) injected intraperitoneally a 

 variety of particles, yeast, mold, paraflin, and 

 paraffin-asphalt spheres, and monitored diaphrag- 

 matic lymph for their recovery. He recovered spheres 

 of mold of 10 /x, glass beads of 12.5 n, and paraffin- 

 asphalt spheres of up to 22.5 /x in diameter. He also 

 recovered red blood cells in lymph at a level of up 

 to 16 million per mm 3 . In later experiments, Allen & 

 Weatherford (ic) injected polystyrene spheres with a 

 range from chylomicron size up to 30 n into the 

 peritoneal cavities of mice, rats, and cats and re- 

 covered the particles from regional lymph nodes. The 

 largest recovered spheres in the mouse were 16.8 ^ 

 in diameter, in the rat and cat, 24 /j. Allen (ib) 

 presents his concept of diaphragmatic lymphatic 

 absorption as follows: "As the diaphragm moves 

 upward in expiration the lymphatic plexus expands 

 and a relative negative pressure is established in the 

 lymphatic lumen. At the same time the triple- 

 layered membrane which separates the peritoneal 

 cavity from lymphatic lumen is stretched. On either 

 side of the fenestrations of the basement membrane 

 the peritoneal mesothelium and lymphatic endo- 

 thelium open, sometimes to form openings as great as 

 22.5 /x in diameter. Through these openings suspen- 

 sions are 'sucked' into the lymphatic lumen. As the 

 diaphragm contracts the tension on the lymphatic 



wall is released, the openings close, and are no longer 

 demonstrable by usual techniques, and compression 

 of the plexus results in lymphatic flow." 



The possible mechanisms of absorption of particles 

 by the lymphatics of the diaphragm have been further 

 clarified in a recent definitive study by French et al. 

 (77a) using the light and electron microscopes. 

 They point out that the mesothelial cells of the roofs 

 differ from other cells at the peritoneal surface of the 

 diaphragm in that they are more closely set, stain 

 more darkly, and separate from each other more 

 readily, particularly at the base of the intercellular 

 junctions. The cells are supported by a lattice of 

 coarse and fine fibers. In the meshes of this lattice, 

 mesothelial and lymphatic endothelial cells are 

 separated only by the basement membrane of the 

 mesothelium which may be incomplete. The authors, 

 using rabbits, injected India ink, thorium dioxide, and 

 saccharated iron oxide intraperitoneally and found 

 that the particles entered the intercellular spaces of 

 the mesothelium and spread freely within the fibers 

 of the fiber lattice. The particles appeared to pass 

 through the mesothelium by a predominantly extra- 

 cellular pathway and probably entered the lymphatic 

 lumen through temporary channels formed by 

 separation of endothelial cells at the intercellular 

 junctions. These gaps formed by separation of meso- 

 thelial and endothelial cells also permit the passage 

 of erythrocytes. The authors found that absorbed 

 colloidal particles accumulated in the cytoplasm of 

 mesothelial and lymphatic endothelial cells in the 

 roofs, and their observations suggested that some of 

 the absorbed material may be transported intracellu- 

 lar^- through these two layers in cytoplasmic vesicles. 

 In addition to uptake of particles by the endothelial 

 cells in the roofs, cells in other sites in the diaphragm 

 can also take up colloidal particles from the lumen 

 of the lymphatic. In this respect, their results are 

 similar to those of Odor (160a), who showed that 

 particles of mercuric sulphide or Thorotrast were 

 rapidly taken up from the peritoneal cavities of rats 

 by mesothelial cells over the mesentery and dia- 

 phragm. On the other hand, Felix & Dalton (70a) 

 found that melanin particles introduced intraperi- 

 toneally were actively ingested by free macrophages 

 but not by mesothelial cells. These differences may be 

 related to the difference in particle size or in the 

 electron microscope preparations. 



The evidence accumulated from recent studies thus 

 suggests at least two possible pathways for the absorp- 

 tion of large particles (and erythrocytes) from the 

 peritoneal cavity: /) an extracellular pathway 



