Abstract

Mesenchymal stem cells (MSCs) represent a promising tool for new clinical concepts in supporting cellular therapy. Bone marrow (BM) was the first source reported to contain MSCs. However, for clinical use, BM may be detrimental due to the highly invasive donation procedure and the decline in MSC number and differentiation potential with increasing age. More recently, umbilical cord blood (UCB), attainable by a less invasive method, was introduced as an alternative source for MSCs. Another promising source is adipose tissue (AT). We compared MSCs derived from these sources regarding morphology, the success rate of isolating MSCs, colony frequency, expansion potential, multiple differentiation capacity, and immune phenotype. No significant differences concerning the morphology and immune phenotype of the MSCs derived from these sources were obvious. Differences could be observed concerning the success rate of isolating MSCs, which was 100% for BM and AT, but only 63% for UCB. The colony frequency was lowest in UCB, whereas it was highest in AT. However, UCB-MSCs could be cultured longest and showed the highest proliferation capacity, whereas BM-MSCs possessed the shortest culture period and the lowest proliferation capacity. Most strikingly, UCB-MSCs showed no adipogenic differentiation capacity, in contrast to BM- and AT-MSCs. Both UCB and AT are attractive alternatives to BM in isolating MSC: AT as it contains MSCs at the highest frequency and UCB as it seems to be expandable to higher numbers.

Mesenchymal stem cells (MSCs) found in many adult tissues are an attractive stem cell source for the regeneration of damaged tissues in clinical applications because they are characterized as undifferentiated cells, able to self-renew with a high proliferative capacity, and possess a mesodermal differentiation potential [1].

Although bone marrow (BM) has been the main source for the isolation of multipotent MSCs, the harvest of BM is a highly invasive procedure and the number, differentiation potential, and maximal life span of MSCs from BM decline with increasing age [24]. Therefore, alternative sources from which to isolate MSCs are subject to intensive investigation.

One alternative source is umbilical cord blood (UCB), which can be obtained by a less invasive method, without harm for the mother or the infant [5]. However, controversy still exists whether full-term UCB can serve as a source for isolating multipotent MSCs: although some groups did not succeed in isolating MSCs [6, 7], we and other groups succeeded in isolating MSCs from full-term UCB [812].

Adipose tissue (AT) is another alternative source that can be obtained by a less invasive method and in larger quantities than BM. It has been demonstrated that AT contains stem cells similar to BM-MSCs, which are termed processed lipoaspirate (PLA) cells [13]. These cells can be isolated from cosmetic liposuctions in large numbers and grown easily under standard tissue culture conditions [13]. The multilineage differentiation capacity of PLA cells has been confirmed [13].

As BM-MSCs are best characterized, we asked whether MSCs derived from other sources share the characteristics of BM-MSCs. The aim of our study was to compare MSCs isolated from the three sources under identical in vitro conditions with respect to their morphology, frequency of colonies, expansion characteristics, multilineage differentiation capacity, immunophenotype, and success rate of isolating the cells.

We compared MSCs from BM and two alternative sources, namely UCB and AT, concerning basic MSC characteristics. All cells isolated from these three sources exhibited typical MSC characteristics: a fibroblastoid morphology, the formation of CFU-F, a multipotential differentiation capability, and the expression of a typical set of surface proteins. Whereas MSCs derived from the three sources expressed classic MSC marker proteins, but lacked hematopoietic and endothelial markers, we observed significant differences concerning the expression of CD90, CD105, and CD106. These molecules are described to be associated with hematopoiesis and cell migration [18 20]. It needs to be further investigated whether these molecules are functionally important for stroma and homing capacities. In a first approach, we created a comprehensive protein expression profile of undifferentiated UCB-MSCs, which will be extended to BM- and AT-MSCs and then correlated to functional properties [21].

Since the relevance of the observed differences of marker expression has not been properly investigated yet, differences concerning differentiation capacity seem to be more relevant for MSC quality at present. We demonstrated a multilineage differentiation capacity for BM- and AT-MSCs. Interestingly, UCB-MSCs could not be differentiated toward the adipogenic lineage, which was not related to the CFU-F origin. Actually, there are conflicting data concerning the adipogenic differentiation capacity of UCB-MSCs [9 12, 22, 23]. Nevertheless, we assume that UCB-MSCs are less sensitive toward the adipogenic differentiation (supported by results of Chang et al. [22]) which might be related to the ontogenetic age of these cells. This is further supported by the fact that adipocytes reside in adult human BM and AT but are absent in fetal BM and by the observation of an increased adipogenesis correlated with age [24]. Further comparative genomic or proteomic approaches are needed to assess the susceptibility toward adipogenesis of MSCs.

None of our UCB-MSCs showed adipogenic differentiation capacity, but all differentiated into both the chondro- and osteogenic lineages. In contrast, a tripotential differentiation capacity was observed for most AT samples but only for a few BM samples. One sample each of BM and AT was observed to undergo only the chondrogenic pathway. In accordance with this, a hierarchical or even restricted differentiation potential of MSCs has been reported [1, 13, 25].

In our study, investigations were limited to the mesodermal differentiation capacity. Based on recent reports, however, the spectrum of differentiation of MSCs does not seem to be restricted to this lineage. MSCs derived from all three tissues have been shown to differentiate into further mesodermal lineages and into endo- and ectodermal lineages as well [10 13, 26 33]. Comparative experiments need to be performed to assess responsiveness toward cardiomyogenic, endothelial, hepatic, neuronal, and pancreatic differentiation.

A high impact on clinical exploitation might be related to the abundance and expansion capacity of MSCs. Based on our results, both BM and AT are reliable sources for isolating and expanding MSCs in autologous settings since all preparations gave rise to MSCs. UCB, in contrast, had an isolation efficacy of a maximum of 63% [8]. We attribute these differences to the fact that MSCs are circulating in the prenatal organism and are residing in tissues of the adult [9]. Despite the low frequency of UCB-MSCs, the expansion potential was highest compared with other cell sources. Considering clinical applications, the resulting cell numbers may be similar to both BM and AT, which can be obtained at higher frequencies. One argument against AT might be the limited availability in some patients. However, we believe that due to the high frequency of AT-MSCs, also small fat reservoirs might be sufficient for MSC isolation. BM has been the main source for clinical application of MSCs, such as the treatment of osteogenesis imperfecta, graft versus host disease, and acute myocardial infarction [3436]. As the number, frequency, and differentiation capacity of BM-MSCs correlate negatively with age, they could be clinically inefficient when derived from elderly patients. In that case, an allogeneic approach would be required. In case a matching donor is required, BM or AT from HLA identical siblings, haplo-identical relatives, or HLA-screened donors might be best choice. Speculating on a off-the-shelf product requiring mass production, AT might be a solid starting basis due to the abundance, relatively easy harvest, and high MSCs frequency.

Transplantation of MSCs is currently a highly experimental procedure, resembling the early beginnings of hematopoietic stem cell transplantation. In the latter, BM has been replaced gradually by peripheral blood progenitor cells and umbilical cord blood. Also, in the field of MSCs, alternative sources are intensely investigated, and one day these new sources may replace BM. Taking into account all the advantages and disadvantages of the three sources discussed above, depending on the therapeutic indication, the clinical applications may be based on differentiation capacity, but more likely on the abundance, frequency, and expansion potential of the cells.

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