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Lymphatic endothelial cells (LECs) migrate across body to form a branched network, which is crucial for fluid drainage and immune cell trafficking of the tissues. However, the molecular mechanisms behind the spatiotemporal regulation and fine-tuning of LEC migration remain largely unknown. The rapid development and the available transgenic tools in zebrafish allow to study these processes in unprecedented resolution and provide new insights into cellular and molecular dynamics regulating lymphangiogenesis. This study aims to dissect the lymphatic development of zebrafish, with a focus of LEC migration and its regulation in the trunk. 

Proper cell migration requires signalling and guidance cue from the environment and tissue-tissue interactions. In paper I, we identified arterial mural cells as a novel source of growth factor and chemokine essential for the migration and robust formation of the lymphatic network. This is important as understanding the sources of guiding molecules can help optimizing the formation and repair of lymphatic vessels in pathological conditions.

VEGFC-VEGFR3 is an essential signalling required throughout lymphangiogenesis. However, it still remains unclear how pro-lymphangiogenic cues are interpreted by LECs to induce differential behaviours. In paper II, we investigated the secondary sprouting which is dependent on VEGFC-VEGFR3 signalling. We characterized the cell migrating behaviours and analysed the molecular signatures of the sprouts. Furthermore, we identified Ca²⁺ activities are required for proper sprouting and potentially serves as modulator for VEGFC-VEGFR3 signalling. 

In paper III, we investigated LEC proliferation driven by VEGFC-VEGFR3 signalling at different development stages. We identified three key timepoints of LEC expansion as well as the novel molecular factors regulating the proliferation. Together we demonstrated the how cell cycle machinery is driven by VEGFC-VEGFR3 signalling. 

In paper IV, we identified enhancers surrounding prox1a which drives the expression in different subsets of the developing lymphatic vasculature, suggesting the tissue specific regulation of prox1a. In addition, we identified enhancer element driving valve expression and it is required for valve development, highlighting the important roles of enhancers in development of lymphatic vasculature. 

Taken together, this work provides novel insight of the molecular dynamics regulating lymphangiogenesis. Our in vivo analysis uncovered new cell types guiding lymphatic vessel formation as well as signal dynamics during embryonic lymphangiogenesis. We believe that the novel insights from this thesis in the future will help establishing pipelines to rebuilding the lymphatic vascular network.