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Vascular associated endothelial cell (ECs) progenitors are still poorly studied and their role in the newly forming vasculature at embryonic or postnatal stage remains elusive. In the present work, we first defined a set of genes highly expressed during embryo development and strongly downregulated in the adult mouse. In this group, we then concentrated on the progenitor cell marker Peg3/PW1. By in vivo staining of the vasculature we found that only a subset of cells coexpressed endothelial markers and PW1. These cells were quite abundant in the embryo vasculature but declined in number at postnatal and adult stages. Using a reporter mouse for PW1 expression, we have been able to isolate PW1-positive (PW1posECs) and negative endothelial cells (PW1negECs). PW1-positive cells were highly proliferative in comparison to PW1negECs and were able to form colonies when seeded at clonal dilution. Furthermore, by RNAseq analysis, PW1posECs expressed endothelial cell markers together with mesenchymal and stem cell markers. When challenged by endothelial growth factors in vitro, PW1posECs were able to proliferate more than PW1negECs and to efficiently form new vessels in vivo. Taken together these data identify a subset of vessel associated endothelial cells with characteristics of progenitor cells. Considering their high proliferative potential these cells may be of particular importance to design therapies to improve the perfusion of ischemic tissues or to promote vascular repair.

Adult somatic stem cells, also called progenitors, are essential for tissue regeneration and homeostasis. Stem cells have been identified in almost all tissues including those with little regenerative capacity. It is a central interest in the field of regenerative medicine to understand the signals that govern stem cell function, particularly in tissues that typically do not regenerate. Progenitors are present in small numbers in most tissues and reside in a specific cellular context that is referred to as the stem cell niche. In this article, we survey the stem cell niche in several tissues and examine its role in stem cell function. While the anatomical appearance of the stem cell niche differs among tissues, common features exist that serve similar roles. We review here the stem cell niche in two tissues capable of robust regeneration (skeletal muscle and skin) and two tissues that display poor regenerative capacity (CNS and heart). We propose that an increased understanding of the stem cell niche, particularly in tissues that are ‘poor regenerators,’ will have a significant impact upon regenerative medicine. An understanding of the molecular signals that are transmitted between the progenitors and neighboring niche cells will help identify new therapeutic targets to augment regenerative capacity as well as increase our fundamental understanding of these important cellular players.