Mesenchymal Osr1+ cells regulate embryonic lymphatic vessel formation.

The lymphatic system is formed during embryonic development by the commitment of specialized lymphatic endothelial cells (LECs) and their subsequent assembly in primary lymphatic vessels. Although lymphatic cells are in continuous contact with mesenchymal cells during development and in adult tissues, the role of mesenchymal cells in lymphatic vasculature development remains poorly characterized. Here, we show that a subpopulation of mesenchymal cells expressing the transcription factor Osr1 are in close association with migrating LECs and established lymphatic vessels in mice. Lineage tracing experiments revealed that Osr1+ cells precede LEC arrival during lymphatic vasculature assembly in the back of the embryo. Using Osr1-deficient embryos and functional in vitro assays, we show that Osr1 acts in a non-cell-autonomous manner controlling proliferation and early migration of LECs to peripheral tissues. Thereby, mesenchymal Osr1+ cells control, in a bimodal manner, the production of extracellular matrix scaffold components and signal ligands crucial for lymphatic vessel formation.