Introduction Endothelial progenitors obtained from umbilical cord blood (UCB) have been suggested to improve the vascularization of bone tissue engineered constructs. Combining the UCB mononuclear cell (MNC) fraction with endothelial-supportive osteoblasts could bring strong improvements to in vivo neo-vascularization of constructs. Experimental Osteoblasts (Ob) obtained from adipose-derived stem cells were cultured on membranes of Chondrus crispus kappa/iota carrageenan until confluence. UCB-MNC were isolated by differential centrifugation, seeded on top of the Ob monolayer, co-cultured for 7 and 21 days and implanted in nude mice. Results The presence of cells positive for CD31 and vWF was confirmed along the co-culture. Gene expression analysis showed over expression of CD31 and VE-cadherin in the co-cultures at day 21. Independently of the time of implantation, H&E analysis revealed the presence of an intense inflammatory infiltrate. Human CD31 positive cells integrated the blood vessels surrounding the co-cultured implants suggesting the migration of UCB-MNC to the nascent vessels. Moreover, increased blood vessel formation was found around the co-culture implants in comparison to the controls corresponding to transplants only with Ob.Conclusion These results indicate the potential use of the UCB-MNC without further selection and supported by co-culture with osteoblasts, as a source of endothelial progenitors capable of contributing to new blood vessel formation in vivo.