Among constituents of UCB it is possible to identify the following SCs:
- Embryonic Stem Cells (ESCs),
- Mesenchymal Stem Cells (MSCs),
- Hematopoietic Stem Cells (HSCs)
- Endothelial Progenitors Cells (EPCs) [26; 27].
CB-SCs can be characterized at microscopy, where they showed a fibroblast-like shape during the first culturing 4-5 days. Flow cytometry has showed that CB-SCs are positive for CD29, CD44, CD73, CD90, CD117, Oct-4 and negative for CD10, CD34, CD45, CD105, CD166 [28; 29]. By considering that CB-SCs are more skilled to form colonies in vitro and required different differentiation factors in comparison to SCs obtained from BM, they appeared more suitable for studies in the field of regenerative medicine. Therefore, the highly preserved telomeres and superior telomere lengths (and so far more stability over time and less oncogenic potentiality) rendered CB-SCs more suitable for experimental propagation protocols [30]. Recently, in literature as been reported that umbilical cord contains an equal share and/or superior of HSCs than the blood from BM [31].
CB-derived-ESCs
Recently, optimization of isolation and further characterization protocols relative to CB-derived-ESCs has been developed [32]. CB-derived-ESCs have been pointed out as very small size and density, for this reason this population has been postulated, but never identified until some years ago, due to its losing during gradient centrifugation over Ficoll-Paque as well as during routine volume depletion of UCB units before freezing. By means of new refined protocols it is now possible to recover also this population from UCB [32]. CB-derived-ESCs are reported as CD34+, CD133+ and CXCR4+; therefore CB-ESCs presented large nuclei that contain unorganized euchromatin and express Oct-4 and Nanog as well as Stage-Specific Embryonic Antigen-4 (SSEA-4) on their cellular surfaces [33] (Figure 1). Virtually, this population is able to regenerate every tissue or organ specific cell; this population has been defined as able to provide the generation of primitive and definitive hematopoietic precursors, the generation of primitive and definitive mesenchymal precursors and the generation of primitive and definitive endothelial progenitors [3; 17]. CB-derived-ESCs are virtually totipotent progenitors [4].
CB-derived-MSCs
Among three commonly used sources of mesenchymal stem cells: BM, G-CSF mobilized PBSCs and CB stem cells, CB-derived-MSCs are now considered the most appropriate source for transplantation procedures [34; 35; 36]. CB-derived-MSCs has been further characterized and have showed ability to differentiate into to chondrocytes, osteocytes and adipocytes, such as nervous cells and visceral mesoderm [35; 37]. This population show absence of HSCs markers, such as CD14, CD34 and CD45 and it is positive for adhesion markers; CD106, CD54, SH2, SH3 and SH4 are also expressed [38]. Light and electron microscopy have showed that CB-MSCs are positive for mesenchymal markers such as SMA, vimentin, nestin, desmi, for embryonic marker SSEA4 and proliferation marker Ki-67, such as Oct-4 and Nanog [39]. CB-derived-MSCs are pluripotent progenitors [4].
CB-derived-HSCs
CB-derived-HSCs are now considered the most appropriate source for transplantation procedures relative to hematological diseases [40]. CB-derived-HSCs have the potential to differentiate into bone marrow tissues as well blood components [7]. By taking in considerations the conventional definition of definitive hematopoiesis it is possible to assert that CB-derived-HSCs are so closely related to primordial hematopoiesis progenitors [41]. CB-derived-HSCs has been further characterized as blast colony forming cell that express fetal liver kinase 1, the vascular endothelial growth factor receptor 2 (VEGFR2), the mesodermal marker Brachyury and Oct-4 [42] plus as CD41+ [3; 28; 29; 41]. The first markers acquired and expressed by the hemangioblast upon differentiation are endothelial, such as Tie2, Kit, VE-cadherin, CD31 and Meca32 [43], while the molecular signature for the commitment and differentiation toward hematopoietic lineages has been detected by assessing expression of transcription factors such as Scl and Runx1, thrombopoietin receptor and erythropoietin receptor [41].
CB-derived-EPCs
CB-derived-EPCs are the most suitable candidate in integrating into vascular structures at sites of neoangiogenesis and in regenerating vascularization processes [44]. Cell therapy based on EPCs product is limited, until today, by their rarity in PB [45]. CB contains many more EPCs, which are functional and can be expanded in culture. CB-derived-EPCs clinical use requires expansion in controlled conditions and rigorous validation in preclinical models. CB-derived-EPCs serve also as a quality marker-for-frozen relative to UCB unit, showing the presence of non-HSCs [45]. CB-derived-EPCs are reported in literature as Colony-Forming units [46]. By using magnetic cell sorting, CB-derived-EPCs has been characterized as CD133+/CD34+ or KDR+/CD34+ or VEGFR2-related and CD14- [47], by using real-time reverse transcriptase polymerase chain reaction, this population has been further characterized as expressing STAT3, c-kit, CXCR4 and Oct-4 [46].
