Endothelial Progenitor Cells—Potential New Avenues to Improve Neoangiogenesis and Reendothelialization - International Review of Cell and Molecular Biology - page 46

differentiate into ECs under the influence of cultivation factors, (3) other
circulating progenitor cells (CPCs), termed “side population cells,”and
(4) circulating mature ECs that are sheared off from vessel walls.
Several definitions of EPC have been proposed in the literature.
The major problem of defining EPC derives from the lack of specific expres-
sion molecules or functions of putative EPC ( Timmermans et al., 2009 ).
The origin of EPC is hypothesized to be the hemangioblast, a primitive pre-
cursor cell of mesodermal origin that has the capacity of differentiating into
either hematopoietic cells or vascular ECs. Hemangioblasts have been found
in the aorta-gonad-mesonephros region of a developing embryo and are
specifically linked to the endothelial lineage of precursor cells ( Zovein
et al., 2008 ). Precursors with phenotypic and functional characteristics of
embryonic hemangioblasts are also present in human adult BM ( Kocher
et al., 2001 ).
The exact development of EPC from the hemangioblast or a precursor to
the hemangioblast has not been proved yet. Further, the exact maturation
cycle of EPC has not been described nor has there been a definition of their
different mechanisms of action or their different proliferation characteristics.
Well known and accepted is that EPC are progenitors to ECs or endothelial-
like cells with stem cell-like abilities, such as proliferation and maturation,
which sets them apart from circulating endothelial cells (CECs). EPC have
a higher rate of proliferation in cell culture while having the ability to
migrate toward gradients of cytokines ( Grieb et al., 2012; Urbich and
Dimmeler, 2004 ). Further, CECs differ from EPCs by their membrane
expression pattern. Therefore, CECs are defined as nondividing mature
ECs circulating in the bloodstream ( Ingram et al., 2005a ).
2.1. Isolation of EPCs
Although EPCs were isolated for the first time in 1997, the isolation process
of EPCs still remains nonstandardized. Due to the different origins of EPC or
EPC-like cell types, isolation protocols differ. As mentioned earlier, there
are four different origins of EPC and therefore also different possibilities
for isolation (see also Fig. 2.1 ). The different isolation procedures are
described as follows.
2.1.1 Hematopoietic stem cells
The original isolation procedure as reported by Asahara et al., based on the
isolation of mononuclear cells (MNCs) from venous blood by density gra-
dient centrifugation, is still used ( Asahara et al., 1997; Kwon et al., 2011 ).