Main steps of mammalian lymphatic vascular development. (A) LECs are specified in embryonic veins, from where they sprout toward Vegf-c–producing mesodermal cells and aggregate to form lymph sacs. Further sprouting produces the lymphatic primary plexus composed of capillary-like vessels. Myeloid cells produce cytokines and regulate lymphatic vascular morphogenesis. Primary plexus is further remodeled to form collecting, precollecting, and capillary compartments. Precollecting and collecting lymphatic vessels have intraluminal valves and basement membrane coverage. Collecting lymphatic vessels are surrounded by SMCs (red). (box) Genes important for collecting lymphatic vessel development. (B) LYVE-1 is the earliest known LEC marker. The transcription factor Prox1 is essential for the establishment of LEC identity, and its expression is controlled by Sox18. (C) Signaling via Vegf-c and Vegfr-3 regulates LEC sprouting and proliferation. The role of Vegfr-2–Vegfr-3 heterodimers and participation of Nrp2 in the Vegfr-2–Vegfr-3 complex are not fully understood. (D) Separation of lymphatic and blood vasculature requires platelet aggregation (also see Table I). Interaction of podoplanin on LECs and CLEC-2 on platelets triggers the Syk-, Slp76-, and PLC-γ2–dependent signaling cascade leading to platelet aggregation. O-glycosylation by T-synthase is important for podoplanin function.