1062 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



permeability characteristics of lymph vessels. Light 

 microscope studies suggested that lymph and blood 

 capillaries are morphologically similar, a suggestion 

 which has been confirmed by recent studies, particu- 

 larly those of Casley-Smith & Florey (39a). These 

 authors showed that there were no species differences 

 in lymphatics of mice, guinea pigs, and rats, and 

 that the lymphatics of the ear and the deep lymphatics 

 of the diaphragm and colon were similar. In general, 

 the structure of the lymphatic capillaries and lacunes 

 appeared to be similar to that of blood capillaries. 

 All the lymphatic endothelial cells contained many 

 vesicles and caveolae intracellulares. No fenestrations 

 in the endothelium were seen, but some intercellular 

 junctions were patent, especially in diaphragmatic 

 lacunes. The basement membrane was less regular 

 than that of blood capillaries or of mesothelium 

 and in many places, especially in diaphragmatic 

 lacunes, it appeared to be absent. These results are 

 similar to those of Palay & Karlin (164) and French 

 et al. (77a). The absence of a definable basement 

 membrane would not, as Casley-Smith and Florey 

 point out, fully differentiate lymphatic from vascular 

 endothelium, since the endothelium lining large 

 blood vessels may have at best a very tenuous base- 

 ment membrane. The significance of the absence of 

 fenestrations in the lymphatic endothelium also 

 remains questionable in the absence of definite 

 information as to the importance of their presence in 

 determining permeability characteristics. The signifi- 

 cance of their other findings will be discussed further 

 below. 



The permeability pattern in lymphatics presents an 

 interesting and challenging problem. It is obvious 

 from the preceding discussion that proteins, chylo- 

 microns, and lymphocytes are normal constituents of 

 lymph as routinely collected from healthy animals. 

 Experimentally, bacteria, viruses, red blood cells, 

 graphite particles, etc. have been shown to penetrate 

 the lymphatic system with no apparent difficulty. 

 Lane Allen (ia) showed that every type of cell which 

 occurs normally in tissue fluid and blood will pene- 

 trate lymphatic endothelium. He felt he had identified 

 every cell of the hematopoietic series, except giant 

 cells, in diaphragmatic lymph after intraperitoneal 

 injection of bone-marrow suspensions. Likewise, he 

 found that the entire series of lymphoid cells will 

 enter through lymphatic endothelium. The large 

 amount of literature published before 1956 describing 

 these experiments has been thoroughly reviewed by 

 Yoffey & Courtice (234). The more recent publica- 

 tions will be discussed later. 



In spite of the apparent ease with which substances 

 can penetrate into the lymphatic vessels, the avail- 

 able evidence suggests that once these substances are 

 in the lymphatic system, they are retained and 

 eventually find their way into the blood stream via 

 the larger ducts. Thus MacCallum (132a) retroin- 

 jected the lymphatics of the diaphragm and failed to 

 force suspended particles back into the peritoneal 

 cavity, except when he used pressures sufficient to 

 rupture the lymphatics. Hudack & McMaster 

 (100a) injected dyes into the ears of mice and studied 

 the escape of these substances from the lymphatics. 

 They reported that poorly diffusible dyes (pontamine 

 sky blue, Chicago blue 6B) which pass with difficulty 

 out of blood capillaries into the tissues, tend to be 

 retained by the lymphatic wall as well, whereas more 

 highly diffusible dyes (trypan red, bromphenol blue, 

 and Neptune blue) pass out with ease. Rusznyak 

 et al. (189) have more recently reported similar results 

 using fluorescent dyes (thiazine red, acridine yellow) 

 in intestinal lymphatics of cats. Hudack and Mc- 

 Master concluded that "all the evidence we have 

 obtained supports the view that permeability of the 

 lymphatic wall resembles the permeability of the 

 capillary wall in its essential features and perhaps in 

 its degree." Drinker & Field (61 a) retroinjected 

 lymphatics of the frog web with graphite acacia and 

 found no passage of the graphite particles until rupture 

 resulted from excessive pressure. Pullinger & Florey 

 (175) found that when they injected graphite particles 

 into ear lymphatics of the mouse, the fluid leaked out 

 but the graphite particles remained. Similarly, Lee 

 (124a) found that large particles of centrifuged, 

 dialyzed India ink were retained in lymph vessels, 

 whereas small particles passed through. Courtice & 

 Steinbeck (50a) attempted to evaluate lymphatic 

 permeability by injecting T-1824-labeled plasma 

 proteins intraperitoneally into rabbits and collecting 

 lymph containing the protein from the exteriorized 

 thoracic duct. They found that the injected proteins 

 were almost entirely absorbed by the diaphragmatic 

 lymphatics. In further work (50b) they demonstrated 

 that ligation of the parasternal lymph channels in 

 rabbits, rats, and guinea pigs prevented the dye- 

 labeled protein from reaching the circulation via the 

 thoracic duct, but instead, after entering diaphrag- 

 matic and mediastinal lymph channels, it proceeded 

 to leak into the mediastinum and pleural cavities. 

 They concluded from these experiments that lympha- 

 tics leaked protein, a conclusion open to question in 

 view of the obviously unnatural conditions of their 

 experiments. Ligation of the parasternal lymph chan- 



