The Role of the Endothelial Intercellular Junctions in the Functioning of the Initial Lymphatics

The fine structure of the initial lymphatics is discussed, especially that relating to the endothelial intercellular junctions and the structures (e.g., extracellular filaments) which relate to their functioning. Material enters the initial lymphatics basically via the open junctions; it is retained in them because these junctions become too small for macromolecules to traverse them during tissue compression. Various theories about what causes material to enter the lymphatics are examined. It is suggested that, except in oedema when hydrostatic pressure is important, the osmotic pressure produced by the concentration of macromolecules inside the lymphatics causes fluid to enter the vessels and that the macromolecules are carried by the fluid. The concentration is considered to be caused by the expulsion of fluid through the junctions during tissue compression.