Fig. 1

Mesenchymal stem cells (MSC) can be found in the lungs, and the vascular system is composed of large to smallest blood vessels. The first layer of a three-layered structure is the intima, which has contact with the lumen. The second layer is tunica media, where vascular smooth muscle cells exist and encircle the vascular lumen and create many layers. Finally, it is the outermost adventitia, which interfaces between the vessel wall and around tissues, which is called perivascular space. The perivascular space is not circular compared to the adventitia. It is shown that lung-resident MSCs (LR-MSCs; blue) exist mainly in the wall of the large lung vessels in the adventitia. In this niche, there are other kinds of stem cells, too: HSCs (shown by green cells) and EPCs (indicated by yellow cells). Different MSCs and their relative progenitors reside between endothelial and epithelial cells, and the interstitium space (mesenchyme progenitor cells (PC) is shown by light blue cells) [135]