Lymphatic Vascular Morphogenesis : From Progenitors to Functional Vessels

Abstract: The lymphatic vasculature is an important part of the circulatory system and crucial for normal functioning and maintenance of tissues. Yet, our understanding of the processes underlying lymphatic development and homeostasis are surprisingly limited. Recent studies have uncovered a heterogeneous origin of lymphatic endothelium within different organs as well as different mechanisms of vessel formation. The mesentery is a fold of peritoneum that attaches the intestines to the abdominal wall and harbours nerves, blood and lymphatic vessels which supply the intestines. Here, the lymphatic vasculature forms through a process termed lymphvasculogenesis, during which non-venous derived lymphatic endothelial cell (LEC) progenitors assemble into vessels. Parallel to this process, the mesenteric blood vasculature undergoes extensive remodelling. In paper II we show that this is accompanied by a transient extravasation of red blood cells (RBCs). Engulfment of RBCs by developing lymphatic vessels indicate a novel role of lymphatics in clearance of extravasated RBCs. In paper III we further analyse early LEC progenitors in the mesentery and show that they exhibit unique characteristics including membrane blebbing that may facilitate LEC migration during lymphvasculogenic vessel formation. The primitive lymphatic plexus further develops into mature vessels with blind ended, highly specialized segments termed lymphatic capillaries. Individual capillary LECs possess a characteristic oak leaf like shape and discontinuous button like junctions. In paper IV we propose a new model of cell shape regulation in lymphatic capillaries that is based on the interplay of the cytoskeleton and a unique organization of cell-cell junctions. We further report that acquisition of oak leaf shape precedes junctional specification, and is not a mere result of button junction formation in dermal lymphatics. CreERT2 mouse lines are used across many fields of biological research, including the here presented studies, because they allow for targeted gene deletion upon inducible genetic recombination. In paper I we report that, unexpectedly, several commonly used CreERT2 mouse lines exhibit a weak baseline Cre activity leading to induction-independent recombination. This has important implications for the interpretation of results from Cre/loxP experiments, especially when performing lineage tracing.Focusing on different aspects of lymphatic vascular biology, this thesis work reveals yet undescribed mechanisms by which LECs form new vessels, contribute to tissue integrity during vascular remodelling and maintain mature lymphatic vessel integrity through a unique interplay of cell shape and junctional organization.