The adventitia of large vessels is highly vascularized

The adventitia of large vessels is highly vascularized. As such, we are not surprised that our multi-parameter analytical flow cytometric analysis revealed a relatively high prevalence of Phenyl sulfate expressing surface markers consistent with the endothelial lineage, namely CD31. We also detected CD34 expression in subsets of CD31+ endothelial cells of adventitial vasa vasorum, which we classified as EPCs. Still, it is possible that the EPCs from human aortic adventitia described here may have been derived from the circulation. The presence of EPCs in the adventitia raises the question of their potential for neovascularization and their functionality in the setting of cardiovascular disease. In the human aorta, cells expressing stem cell markers STRO-1, CD34, and c-kit have been localized to the medial layer, even more so in the adventitia, and were noted to be more prevalent in both layers in cases of aortic aneurysm and dissection than in normal healthy aortas (Shen et al., 2012). A purported role for vascular wall stem cells in vascular disease is currently an area of intense interest (Nguyen et al., 2013; Tang et al., 2012). Our current finding of a discrete progenitor cell niche associated with the adventitial vasa vasorum provides additional support that these unique cell populations may play a distinct and important role in thoracic aortic disease. Although we detected pericytes capable of smooth muscle lineage progression in both normal and aneurysmal aorta, we did not compare functional differences on the basis of disease, and additional work will help to discern potential differences in surface proteome and functionality of vascular wall progenitors in aortic disease. In conclusion, the vasa vasorum and surrounding connective tissue in human adult thoracic aortic adventitia is a niche for progenitor cell populations. Importantly, their presence and the demonstrated function of pericytes as smooth muscle progenitors and potential for multi-lineage progression adds to the evolving dichotomy related to MSCs\' angiogenic potential in vascularized tissues as either progenitors of endothelial cells or local cells that secrete angiogenic growth factors in addition to being recruited to support neovessel formation. The pericyte cultures are distinct from other prior reports investigating progenitor cells in the media and adventitia of human thoracic aorta (Invernici et al., 2007; Pasquinelli et al., 2007, 2010; Shen et al., 2012) because of their association with the vasa vasorum. Pericytes, adventitial cells, and other perivascular progenitor cells such as those in the endothelial lineage associated with the vasa vasorum may influence medial biology. The function and purpose of these progenitor cells in the vasa vasorum microenvironment may prove to be pivotal in the homeostasis and/or remodeling of the aortic wall in normal or pathologic states.
Experimental Procedures
Author Contributions
Acknowledgments Research reported in this publication was supported by the National Heart, Lung and Blood Institute of the NIH under Award Number R56HL127214 (J.A.P.) and Award Number R01HL109132 (T.G.G.), the Competitive Medical Research Fund of the UPMC Health System (J.A.P.), the University of Pittsburgh Swanson School of Engineering (B.W.E.), and the University of Pittsburgh Department of Cardiothoracic Surgery (T.G.G.). The authors acknowledge the assistance of Kristin Konopka and Julie Schreiber for IRB protocols and obtaining informed patient consent. We thank Lori Walton of the McGowan Institute for Regenerative Medicine Histology Core and Benjamin Green for assistance with histology, and McKenzie Hartman for assistance with the differentiation experiments. We are grateful to our surgical colleagues Dr. Robert Kormos and Dr. Forozan Navid of the Department of Cardiothoracic Surgery, University of Pittsburgh Medical Center, for assistance with aortic specimen acquisition. The real-time qPCR work was performed in the Genomics Research Core at the University of Pittsburgh. This project used the UPCI Cytometry Facility that is supported in part by award P30CA047904.