Tissue interactions mediate early events in pulmonary vasculogenesis

Extensive study has provided considerable insight into the mechanisms gover ning branching morphogenesis and developmental maturation of the pulmonary epithelium, The process by which the vascular tree arises in the mesodermal mesenchyme of the developing lung, however, is not known. Because normal e pithelial branching and differentiation have been shown to be dependent on interactions with the lung mesenchyme, we hypothesized that the developing pulmonary vasculature is dependent on a reciprocal interaction with pulmona ry epithelium, In this study we have defined the temporal and spatial expre ssion of flk-1 mRNA, which encodes an endothelial cell-specific vascular en dothelial growth factor (VEGF) receptor, in fetal and neonatal rat lung. Fl k-1-positive cells were observed in the lung at every prenatal stage from f etal day 11 through birth, demonstrating that vascularization has been init iated as soon as the lung evaginates from the foregut epithelium. The spati al distribution of vascular precursors was distinct and consistent in early lung (fetal days 11-16): clusters of flk-1-positive cells were localized i n the mesenchyme closely apposed to the developing epithelium. This spatial relationship between vascular precursors and the developing epithelium sug gested that vascular development in the lung may be dependent on interactio ns between the two tissue types, To investigate this possibility, day-13 di stal lung mesenchyme was cultured in the presence and absence of lung epith elium. Lung mesenchyme cultured in the absence of epithelium degenerated si gnificantly, and few flk-1-positive cells were maintained. In contrast, lun g mesenchyme recombined with lung epithelium contained abundant flk-1-posit ive cells, and their spatial distribution mimicked that observed in vivo. T hese studies provide the first detailed information regarding the temporal and spatial pattern of pulmonary vascularization in early development and s uggest that tissue interactions play an important role in growth and mainte nance of the developing lung vasculature. (C) 2000 Wiley-Liss, Inc.