What is the unique structural modification in lymphatic capillaries that increases their permeability?

Minivalves

Explanation:

Lymphatic capillaries are so much permeable that some researchers thought that they are open from one side but scientists found that this permeability is due to some structural modification.

The endothelial cell that forms the lymphatic capillaries wall are not packed tightly and overlap each other very loosely which makes easy openings in the structure called mini valves.

When the pressure of fluid in interstitial space is high then the mini valves become open and allow the fluid to enter the lymphatic capillary. So mini valves are the unique structural modification that increases permeability in lymphatic capillaries.