BackgroundEndothelial cells (EC) sit at the forefront of dramatic physiologic changes occurring in the pulmonary circulation during late embryonic and early postnatal life. First, as the lung moves from the hypoxic fetal environment to oxygen-rich postnatal environment, marked changes in pulmonary EC structure and function facilitate a marked increase in blood flow from the placenta to the lungs. Subsequently, pulmonary angiogenesis expands the microvasculature to drive exponential distal lung growth during early postnatal life. Yet, how these marked physiologic changes alter distinct EC subtypes to facilitate the transition of the pulmonary circulation and regulate vascular growth and remodeling remains incompletely understood.MethodsIn this report, we employed single cell RNA-transcriptomics and in situ RNA imaging to profile pulmonary EC in the developing mouse lung from just before birth through this period of rapid postnatal growth.ResultsMultiple, transcriptionally distinct macro- and microvascular EC were identified in the late embryonic and early postnatal lung, with gene expression profiles distinct from their adult EC counterparts. A novel arterial subtype, unique to the developing lung localized to the distal parenchyma and expressed genes that regulate vascular growth and patterning. Birth particularly heightened microvascular diversity, inducing dramatic shifts in the transcriptome of distinct microvascular subtypes in pathways related to proliferation, migration and antigen presentation. Two distinct waves of EC proliferation were identified, including one just prior to birth, and a second during early alveolarization, a time of exponential pulmonary angiogenesis. Chronic hyperoxia, an injury that impairs parenchymal and vascular growth, induced a common gene signature among all pulmonary EC, unique alterations to distinct microvascular EC subtypes, and disrupted EC-EC and EC-immune cell cross talk.ConclusionsTaken together, these data reveal tremendous diversity of pulmonary EC during a critical window of postnatal vascular growth, and provide a detailed molecular map that can be used to inform both normal vascular development and alterations in EC diversity upon injury. These data have important implications for lung diseases marked by dysregulated angiogenesis and pathologic pulmonary vascular remodeling.

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Early in life, the lung mesenchyme is extremely dynamic. We profiled the transcriptomes and locations of mesenchymal cells (MC) in the perinatal murine lung and observed cellular progressions of fibroblasts, myofibroblasts, airway smooth muscle (ASM), and mural cells, poised to modulate the extracellular matrix (ECM) with both subtype- and temporal-specific patterns. Within one day after birth, embryonic fibroblast precursors branched into two subtypes, alveolar and adventitial fibroblasts. ASM and myofibroblasts derived from a common precursor population that specifically expressed Crh, a hormone central to glucocorticoid release. Vascular smooth muscle cells and pericytes differentiated gradually. Fibroblast, myofibroblast, and pericyte proliferation peaked at 7 days after birth. Paracrine signaling among cell types decreased after birth and was hierarchical, with pericytes at the interface between endothelial cells and other MC. Postnatal hyperoxia delayed the developmental progression in cell type abundance and gene expression, mirroring the arrested development that characterizes the neonatal lung disease, bronchopulmonary dysplasia. Hyperoxia decreased pericyte and myofibroblast abundances and proliferation, dysregulated signaling, altered ECM modulation and, in males specifically, gave rise to a novel population of contractile fibroblasts. This study identifies the mesenchymal populations orchestrating lung development at the critical transition to air-breathing life and their distinct reactions to hyperoxia.

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