Most of us have seen their own blood at one or the other time. The occasion might have been a small accident or in unfortunate cases a severe blood loss caused by a major injury. We also can feel our heart beating and the resulting pressure wave, the pulse. The existence of the cardiovascular system is obvious to us. Unlike the cardiovascular system, the lymphatic system has, until recently, escaped notable attention not only by the laymen, but also by the scientific community. It is unclear why the lymphatic system originally developed in higher vertebrates. Now, its main function seems to be to collect fluid that has leaked from the blood vessels and to return it into the cardiovascular system. Much of our knowledge about the development and structure of the lymphatic system is of considerable age, and it has been said that there has not been any progress in our understanding since the fine structure of the lymphatics was described with the introduction of the electron microscope.
Vascular endothelial growth factor (VEGF) is the principal direct inducer of blood vessel growth, but it does not promote the growth of lymphatic vessels. This study demonstrates for the first time specific lymphangiogenesis as a response to the VEGF homologue VEGF-C. Overexpression of full length VEGF-C under the keratin-14 promoter in the skin of transgenic mice caused a proliferation of the lymphatic endothelium and lymphatic vessel enlargement. In the chorioallantoic membrane assay, the mature form of VEGF-C was also largely specific for lymphatic endothelial cells. A newly discovered close homologue of VEGF-C, VEGF-D was then shown to have the same receptor-binding pattern as VEGF-C.
Contrary to the interaction of VEGF with its receptors, VEGF-C interaction with VEGFR-3 has not been analyzed at the molecular level. The structural determinants of VEGFR-3 binding were characterized in relation to VEGF using a non-random family shuffling approach with VEGF and VEGF-C as parent molecules. This approach led to the identification of VEGF/VEGF-C mosaic molecules that showed novel receptor binding profiles and a panel of these molecules was used to delineate the requirements of specific receptors in the induction of angiogenesis versus lymphangiogenesis.