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Angiogenesis, the process by which new vessels sprout from pre-existing vessels, is fundamental to development, tissue growth and repair. The main aim of this thesis was to investigate the role of microglia on angiogenesis. We adapted the rat ex vivo/in vitro aortic ring model to the mouse in a 3-D culture system. In paper I, we show that ablation of microglia in the retina leads to a poorly developed vascular network. The aortic ring model, supplied with microglia, demonstrated that microglia have a direct positive effect on angiogenic sprouting. The angiogenic effect was mediated by soluble factor/factors, and cell-cell contacts were not required. We also show that the microglia-derived angiogenic factor(s) is distinct from vascular endothelial growth factor-A. Moreover, the sprouting aortic ring induces oriented migration of microglia towards the aortic ring. In paper II, we analysed the microglia transcriptome. We found that microglia express known activators and inhibitors of angiogenesis that might have a role in retinal blood vessel development. The aortic ring system was also used as a complement to in vivo analyses to address the function of sphingosine-1- phosphate receptor 1 (S1P1) on angiogenesis (paper III). The results indicate that S1P1 is required within endothelial cells to counteract VEGFA- signalling and prevent endothelial hyper-sprouting. In paper IV, expression of green fluorescent protein in endothelial/hematopoietic cells using Tie2-Cre was used to mark transplanted bone marrow-derived cells. The study aimed to address if grafted bone marrow derived cells can differentiate into pancreatic β-cells in mice. The major part of the thesis concerns the establishment and use of the mouse aortic ring as a model for angiogenesis. Importantly, application of the system enabled us to identify a direct positive effect of microglia on angiogenesis and to test putative modifiers. This could be further pursued by microarray analyses. The presented work might therefore provide a platform for the identification of molecules that regulate angiogenesis. Key words: microglia, angiogenesis, aortic ring. ISBN: 978-91-628-8285-3