Evidence for a second valve system in lymphatics: endothelial microvalves

Area of Science:

• Physiology
• Lymphatic System Biology

Background:

• The mechanism of interstitial fluid uptake into initial lymphatics is not fully understood.
• While intralymphatic valves prevent backflow, they are insufficient for unidirectional flow in initial lymphatics.

Purpose of the Study:

• To investigate the hypothesis of a secondary valve system within initial lymphatics that controls interstitial fluid entry.
• To determine if lymphatic endothelium prevents the escape of fluid and particles from initial lymphatics.

Main Methods:

• Micropipette manipulation techniques were used to study the transport of fluorescent microspheres across the endothelium of initial lymphatics in rat cremaster muscle.
• Lymphatic endothelial sections were examined during lymph pumping to observe interendothelial junction status.

Main Results:

• Microspheres readily passed from the interstitium into the initial lymphatic lumen but could not be expelled back out.
• The lymphatic endothelium blocked the reaspiration of microspheres within the lumen, even after endothelial puncture.
• Open interendothelial junctions were observed during lymph pumping, unlike in resting initial lymphatics.

Conclusions:

• Initial lymphatics have a primary endothelial valve system that, with secondary intralymphatic valves, facilitates unidirectional flow.
• This endothelial valve system is crucial for fluid uptake and preventing backflow into the interstitium during lymphatic pumping.