Organ-specific mechanisms of vascular development in the mesentery

Abstract: Understanding how the vascular systems are formed has significant clinical importance. General mechanisms underlying vascular development have been extensively studied during the past decades. However, the mechanisms regulating the development and function of the blood and lymphatic vessels in specific organs are poorly understood.The aim of this thesis was to investigate lymphatic vascular development in the mesentery, which is a fold of peritoneum that attaches the intestine to the abdominal wall, and contains arteries, veins, lymphatic vessels, nerves and lymph nodes. We found that mesenteric lymphatic vessels were formed through lymphvasculogenesis - coalescence of isolated lymphatic endothelial cell (LEC) clusters, rather than by lymphangiogenesis - sprouting from the veins or pre-existing lymphatic vessels. The lymphvasculogenic process was selectively sensitive to inhibition of the vascular endothelial growth factor receptor 3 (VEGFR3)/ phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) signaling pathway. Using genetic lineage tracing, we uncovered that part of the mesenteric lymphatic vasculature was derived from cKit lineage cells likely originating from the blood-forming hemogenic endothelium of major arteries (Paper I). This is in contrast to the previously accepted dogma that all mammalian lymphatic vessels are of venous endothelial origin. By characterizing a mouse mutant lacking the non-venous-derived LEC progenitors we found that an alternative venous source of LECs could however compensate to build a functional mesenteric lymphatic vasculature (Paper IV). We further described in the developing mesentery that a transient loss of venous integrity, characterized by the formation of inter-endothelial cell gaps, was accompanied by extravasation of red blood cells, which were cleared by the developing lymphatic vessels. By studying mice with defective platelet function, we revealed a previously unappreciated role of platelets in maintaining the integrity of the remodeling embryonic blood vasculature and thus preventing excessive blood-filling of lymphatic vessels (Paper III). We also studied the mechanism of vessel maturation into functional lymphatic vessels, which involves smooth muscle cell recruitment. Analysis of mice with LEC-specific deletion of Pdgfb, encoding the platelet derived growth factor B (PDGFB), showed that LEC-autonomous PDGFB was required for the recruitment of smooth muscles cells that in turn control lymphatic vessel size and function (Paper II).