Embryonic pulmonary vascular development during murine lung morphogenesis

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Backgrounds:Impaired pulmonary vasculature is a serious complication of cyanotic congenital heart diseases. Understanding the molecular mechanisms of the pulmonary vascular development is essential for molecular-based gene and regeneration therapies. Vascular endothelial growth factor (VEGF) and its receptors, Flk-1 and Flt-1,are known to be important factors that regulate vascular development. This study focuses on spatiotemporal expression and experimental inhibition of Flk-1 and Flt-1 during the development of pulmonary vasculature.Methods:The expressions of Flk-1 and F1t-1 in murine embryonic lungs were examined by immunohistochemistry and the functions were studied by using antisense oligonucleotides in vitro. DNA synthesis of vascular endothelial cells was studied by BrdU incorporation.Results:Based on the relationship between Vascular endothelial cells and bronchial epithelial cells, development of pulmonary vasculature is divided into 4 stages. Flk-1 is diffusely expressed in mesenchymal cells at stage I-III, and is less expressed at stage IV. Flt-1 is initially detected in mesenchymal cells surrounding bronchial epithelium at stage II, its expression peaks at stage III, and decreases at stage IV. VEGF protein is expressed both in pulmonary epithelial cells and adjacent mesenchymal cells throughout the stages. DNA synthesis of vascular endothelial cell is up-regulated at stages I and II, and down-regulated after stage III. Treatment of cultured lung buds with antisense oligonucleotides complementary to Flk-1 resulted in insufficient branching of capillaries and impaired proliferation of vascular endothelial cells. Contrary, treatment with antisense oligonucleotides complementary to Flt-1 promotes vascular branching of capillaries and increased proliferation of vascular endothelial cells.Conclusion:Expressions of Flk-1 and Flt-1 were crucial elements of the normal development of the pulmonary vasculature.