Qualitative and quantitative analysis of embryonic pulmonary vessel formation

Vessel formation in the lung has been described as occurring by two mechani sms: proximal, or branch, pulmonary arteries develop via angiogenesis; and distal, smaller vessels form by vasculogenesis. Connections between the pro ximal and distal vessels establish the final vascular network. The preponde rance of vessel formation has been suspected to occur during the canalicula r stage of lung development. To test these hypotheses, reporter gene expres sion under control of the regulatory domain of fetal liver kinase-l (flk), an early endothelial cell-specific marker, was used to evaluate mouse lungs from embryonic day 10.5 (E10.5) through 2 wk postnatal age. Morphologic as sessment was: performed after histochemical staining, and quantification of vessel development by a chemiluminescent assay was compared with overall e mbryonic lung growth. LacZ expression under flk promoter control allowed: ( I) early identification of differentiating endothelial cells of the branch pulmonary arteries; (2) visualization of distal vessels forming in the lung mesenchyme (primary capillary network) with subsequent remodeling; (3) rec ognition of early continuity between proximal and distal vessels, occurring by E10.5; and (4) assessment of developing pulmonary veins and venous conf luence. Quantitative analysis revealed increased flk regulated beta-galacto sidase (beta-gal) activity of 12 ng beta-gal/lung at E12.5 to 3,215 ng beta -gal/lung at 2 wk, which corresponded to overall lung growth during this pe riod as shown by an increase in total protein content per lung from 35 mu g at E12.5 to 6,456 mu g at 2 Wk after birth, We identified endothelial cell precursors of the developing pulmonary vasculature before vessel lumen for mation. Continuity between the proximal pulmonary artery and vessels formin g in the distal mesenchyme was present even at the earliest stage evaluated , suggesting endothelial cell differentiation at the site of vessel formati on (i.e., vasculogenesis) as occurs with development of the aorta. Finally, we demonstrated that lung vessel development was not accentuated during th e canalicular stage, but occur-red at all stages and directly corresponded to overall lung growth.