An unexpected role of semaphorin3a-neuropilin-1 signaling in lymphatic vessel maturation and valve formation

Emerging lymphae for the fountain of life

The lymphatic system is indispensable for the collection and cycling of tissue-extravasated fluids, macromolecules and immune cells into the bloodstream. Different mechanisms, including sprouting, ballooning and budding of lymphatic endothelial cells from the cardinal vein, have been proposed for lymphatic vessel formation during mammalial embryogenesis. Hägerling et al (2013) now provide a cell-scale model of lymphoangiogenesis by applying selective plane illumination-based ultramicroscopy (Becker et al, 2008) to wholemount-immunostained mouse embryos. They describe VEGFR-3, VEGF-C and CCBE1 as key regulators of lymphatic endothelial cell budding and migration at the early emergence of lymphatics from venous endothelium.

Flow control in our vessels: vascular valves make sure there is no way back

The efficient transport of blood and lymph relies on competent intraluminal valves that ensure unidirectional fluid flow through the vessels. In the lymphatic vessels, lack of luminal valves causes reflux of lymph and can lead to lymphedema, while dysfunction of venous valves is associated with venous hypertension, varicose veins, and thrombosis that can lead to edema and ulcerations. Despite their clinical importance, the mechanisms that regulate valve formation are poorly understood and have only recently begun to be characterized. Here, we discuss new findings regarding the development of venous and lymphatic valves that indicate the involvement of common molecular mechanisms in regulating valve formation in different vascular beds.

Molecular regulation of lymphangiogenesis in development and tumor microenvironment

A rapid progress has been made in the field of lymphatic research during the last 15 years. This includes better understanding of the cellular events and molecular players involved in the lymphatic vessel formation and remodeling in development. The key players identified in developmental lymphangiogenesis, including vascular endothelial cell growth factor-C (VEGF-C) / VEGFR-3 and angiopoietins (ANGPTs)/ TIE pathways, are also crucial for pathological lymphatic vessel growth. In solid tumor, tumor cells as well as tumor-associated stromal cells, such as tumor-infiltrating leukocytes, contribute to intra- and peri-tumoral lymphangiogenesis via secreting lymphangiogenic growth factors. Tumor-associated lymphatic endothelial cells also interact actively with tumor cells and leukocytes via secreting various chemokines. It has been well established that tumor lymphangiogenesis promotes tumor cell dissemination to regional lymph nodes. Thus manipulation of lymphangiogenic microenvironment could become another valuable approach in the combat of tumor progression.