Pulmonary vasculature in mice develops through two main mechanisms, namely angiogenesis and vasculogenesis. At embryonic day (E) 9.5, vascular endothelial marker Tie2-driven LacZ expression in whole-mount transgenic lungs showed continuity between the primitive lung vasculature and the aortic sac . Scanning electron microscopic study of vascular casts and graphic reconstruction in the 32- and 34-somite (E10) embryos demonstrated that the primordium of the pulmonary artery (PA) arose from the proximal portion of the sixth pharyngeal arch artery and ran straight in the caudal direction . At E10.5, though the afferent vessels were not yet defined as vascular tubes, they resembled two plexiform networks that coalesce alongside the trachea. From these observations, “distal angiogenesis” was proposed as a model for the pulmonary vascular morphogenesis where PAs arise from the pharyngeal arch artery and elongate into the lung buds . Another study provided evidence to support vasculogenesis as the mechanism of both proximal and distal vessel formation during the development of murine lungs . Detailed analysis using Mercox vascular casts revealed that vasculogenesis occurred peripherally in the lungs to form isolated blood islands and that angiogenesis centrally forms the axial and lateral arteries and veins. The fusion and coalescence of these central and peripheral systems lead to the development of the pulmonary circuit . The earliest connection between the peripheral and central spaces was identified between E13 and E14.
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