1. ¹Laboratory of Microbiology, University of Brescia, Brescia, Italy
2. ²Centro Scompenso Cardiaco, Civic Hospital, Brescia, Italy
3. ³Department of Pathology, San Raffaele Hospital and Scientific Institute, Milan
4. ⁴Institute for Obstetrics and Gynaecology "L Mangiagalli", Milan, Italy
5. ⁵Division of Pathology and Laboratory Medicine Veterans Affairs Puget Sound Health Care System, University of Washington, Seattle, Washington
6. ⁶Department of Pathology, University of Washington, Seattle, Washington
7. ⁷Neuropharmacology, National Neurological Institute "C. Besta", Milan, Italy

Correspondence: Dr. Eugenio Parati, Istituto Nazionale Neurologico C. Besta, Via Celoria, 11, 20133 Milan, Italy. E-mail: parati@istituto-besta.it

Received 23 February 2001.

Abstract

Vasculogenesis, the de novo formation of new blood vessels from undifferentiated precursor cells or angioblasts, has been studied with experimental in vivo and ex vivo animal models, but its mechanism is poorly understood, particularly in humans. We used the aortic ring assay to investigate the angioforming capacity of aortic explants from 11- to 12-week-old human embryos. After being embedded in collagen gels, the aorta rings produced branching capillary-like structures formed by mesenchymal spindle cells that lined a capillary-like lumen and expressed markers of endothelial differentiation (CD31, CD34, von Willebrand factor [vWF], and fms-like tyrosine kinase-1 [Flk-1]/vascular endothelial growth factor receptor 2 [VEGFR2]). The cell linings of these structures showed ultrastructural evidence of endothelial differentiation. The neovascular proliferation occurred primarily in the outer aspects of aortic rings, thus suggesting that the new vessels mainly arose from immature endothelial precursor cells localized in the outer layer of the aortic stroma, ie, a process of vasculogenesis rather than angiogenesis. The undifferentiated mesenchymal cells (CD34+/CD31-), isolated and cultured on collagen-fibronectin, differentiated into endothelial cells expressing CD31 and vWF. Furthermore, the CD34+/CD31+ cells were capable of forming a network of capillary-like structures when cultured on Matrigel. This is the first reported study showing the ex vivo formation of human microvessels by vasculogenesis. Our findings indicate that the human embryonic aorta is a rich source of CD34+/CD31- endothelial progenitor cells (angioblasts), and this information may prove valuable in studies of vascular regeneration and tissue bioengineering.