Comparison between Culture Conditions Improving Growth and Differentiation of Blood and Bone Marrow Cells Committed to the Endothelial Cell Lineage

Endothelial progenitor cells (EPCs) promote vasculogenesis and/or ameliorate the process of angiogenesis, thereby improving both regeneration and function of ischemic organs such as the infarcted heart [1,2]. EPCs can be easily isolated from peripheral blood or bone marrow [3,4], expanded ex vivo, and then transplanted in the damaged tissues. Superficial antigens including CD34, CD133, and VEGFR-2 are often utilized to recognize EPCs during their immature stage, whilst other markers such as VE-cadherin and CD31 characterize the lineage progression towards an increased differentiation into endothelial cells [5,6].In order to obtain EPCs, three fundamental strategies are usually followed: (1) identifying EPCs by markers which are already present in the putative immature endothelial stem cells (haemangioblast) [7], (2) selecting EPCs by their ability to form endothelial-cell like clusters [8], and (3) exploiting the plasticity of mononuclear cells (MNCs) [9,10], the latter two conditions demanding the presence of specific differentiating growth factors. The first strategy represents the only way to immediately isolate EPCs after sample collection. However, the isolation output is low and it takes a long time to reach a critical cell number useful for clinic applications. In contrast, a greater number of EPCs can be more rapidly obtained from MNCs which can generate two different subpopulations, the early and the late EPCs [11,12]. The former are usually described as MNCs which are forced to express endothelial markers within a few days; their number is elevated after isolation but these cells cannot be further expanded and they incorporate poorly into new vessels. Despite these apparent negative features, early EPCs efficiently promote in vivo neoangiogenesis especially by producing specific growth factors and chemokines. On the other hand, the number of late EPCs is extremely low after isolation and they begin to proliferate only 2–3 weeks after seeding, showing by th