Lymphovenous hemostasis and the role of platelets in regulating lymphatic flow and lymphatic vessel maturation.

Aside from the established role for platelets in regulating hemostasis and thrombosis, recent research has revealed a discrete role for platelets in the separation of the blood and lymphatic vascular systems. Platelets are activated by interaction with lymphatic endothelial cells at the lymphovenous junction, the site in the body where the lymphatic system drains into the blood vascular system, resulting in a platelet plug that, with the lymphovenous valve, prevents blood from entering the lymphatic circulation. This process, known as "lymphovenous hemostasis," is mediated by activation of platelet CLEC-2 receptors by the transmembrane ligand podoplanin expressed by lymphatic endothelial cells. Lymphovenous hemostasis is required for normal lymph flow, and mice deficient in lymphovenous hemostasis exhibit lymphedema and sometimes chylothorax phenotypes indicative of lymphatic insufficiency. Unexpectedly, the loss of lymph flow in these mice causes defects in maturation of collecting lymphatic vessels and lymphatic valve formation, uncovering an important role for fluid flow in driving endothelial cell signaling during development of collecting lymphatics. This article summarizes the current understanding of lymphovenous hemostasis and its effect on lymphatic vessel maturation and synthesizes the outstanding questions in the field, with relationship to human disease.