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The mechanisms behind many important aspects of blood- and lymphatic vessel formation have yet not been elucidated in detail. The primary objectives of this thesis have therefore been to study the effects of hypoxia, platelet-derived growth factor (PDGF) and vascular endothelial growth factors (VEGFs) on vascular development and function.

In conditions of low oxygen pressure, hypoxia, the survival of the organism is critically dependent on the ability to compensate for the reduced oxygen levels by promoting blood vessel growth and oxygen-independent energy production. Many direct effects of hypoxia in cells are attributed to the induction of a family of hypoxia-inducible transcription factors (HIFs) which control the expression of specific target genes. We found that capillary endothelial cells (ECs) respond to hypoxia with upregulation of genes involved in growth and remodeling of blood vessels. On the other hand, vein ECs responded to hypoxia with increased expression of genes involved in lymphatic vessel growth. Using differentiating embryonic stem (ES) cells, we have shown that hypoxia upregulates expression of VEGF receptor-3 (VEGFR-3) on blood vascular ECs. Furthermore, we have provided evidence for a critical role of VEGFR-3 in hypoxia-induced blood vessel development.

Activation of PDGF receptor-β (PDGFR-β) on early vascular progenitors in differentiating ES cells or in mice induces blood vessel differentiation, while negatively influencing early hematopoiesis. PDGFR-β expression on vascular progenitors may therefore play a role in guiding differentiation of the vascular lineages.

We have investigated the usefulness of differentiating ES cells as a model to study early lymphatic development. Administration of VEGF-C and VEGF-A induced formation of lymphatic vessel-like structures that seemed connected to the blood vasculature, supporting the general view that lymphatic ECs are derived from blood vascular ECs.

In summary, this thesis has provided new insights in the contribution of different growth factors in hematopoietic, blood- and lymphendothelial development.