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Although the process of blood vasculature formation has been well documented, very little is known about the development of the lymphatic vasculature, despite its importance in normal and pathologic conditions. The lack of specific markers that allow us to monitor the normal and pathologic growth of the lymphatic vasculature has hampered the progress in this field. However, recent identification of genes participating in the formation of the lymphatic vasculature has begun to pave the way toward a new era in which better diagnoses and therapeutic treatment(s) of lymphatic disorders could become a reachable goal.
Although few life-threatening diseases result from malfunction of the lymphatic system, failure of lymph transport promotes lymphoedema, a disfiguring, disabling and occasionally life-threatening disorder for which present treatment is limited and ineffective. Malignant tumours can directly activate lymphangiogenesis and lymphatic metastasis; so, understanding the basic biology that is involved in lymphatic development is vital. Metastasis of most cancers mainly occurs through the lymphatic system, and the extent of lymph-node involvement is a useful prognostic indicator. An increased understanding of normal lymphatic development should allow us to address pathological lymphatic conditions that lead to inflammation, autoimmunity and cancer, and to improve the clinical treatment of primary and secondary forms of lymphoedema.
We identified Prox1 as the first specific marker of lymphatic endothelial cells, a finding that allowed us to confirm the century-old theory of F. Sabin, who proposed that the development of the lymphatic vasculature (lymphangiogenesis) is the result of venous endothelial cell budding (Wigle and Oliver, 1999). This finding led us to elucidate some of the mechanisms of early lymphangiogenesis, to demonstrate that blood vasculature is the normal default state of budding venous endothelial cells, and to show that Prox1 is required for the differentiation of the lymphatic endothelial cells and its activity is sufficient to promote lymphatic differentiation in cultured blood endothelial cells (Wigle and Oliver, 1999; Wigle et al., 2002; Oliver and Detmar, 2002; Hong et al., 2002). In the generated Prox1-mutant embryos, no lymphatic vasculature is detected and Prox1 heterozygous mice become obese.
We have recently determined that functional inactivation of a single allele of the homeobox gene Prox1 led to adult-onset obesity due to abnormal lymph leakage from mispatterned and ruptured lymphatic vessels (Harvey et al., 2005). Therefore, this is the first report of a novel cause for adult onset obesity as a consequence of lymphatic vasculature defects. Prox1 heterozygous mice are a new model for adult-onset obesity and lymphatic vascular disease.
More recently and by performing genetic lineage tracing we were able to further confirm Sabin’s original proposal indicating that the lymphatic vasculature is venous derived (Srinivasan et al., 2007).