The lymphatic system

Abstract The lymphatic vascular system is an integral component of the circulatory system; it forms a one-way conduit that transports tissue interstitial components back to the venous circulation through lymph nodes. Lymphatic vessels extend to most tissues and contribute to the regulation of interstitial fluid homeostasis, trafficking of immune cells, and absorption of dietary fats from the gut. Developmentally, lymphatic vessels originate from embryonic veins and specialized angioblasts. A number of molecules have been identified in the commitment of endothelial cells to the lymphatic lineage, and the sprouting, expansion and maturation of the lymphatic vascular tree. Importantly, the vascular endothelial growth factor (VEGF) family members VEGFC and VEGFD, together with their receptors VEGFR2 and VEGFR3 have been implicated as critical regulators of lymphangiogenesis. Lymphatic vessels are involved in several human diseases, including cancer, where they contribute to tumour metastasis, the leading cause of cancer-related deaths. Lymphatic vessels regulate immune responses against foreign pathogens by transporting leucocytes to lymph nodes, but are also in involved in the regulation of self-tolerance. Defects in the lymphatic vascular system are causal for the development of lymphoedema.