Mesenchymal Stem Cell Heterogeneity and Ageing In Vitro: A Model Approach - Computational Modeling in Tissue Engineering - page 179

small, so-called RS cells, [ 14 , 15 ]. But relatively fast most of the cells develop a
fibroblast-like morphology. With increasing culture time they spread more and
more and their morphology becomes irregular. This suggests using morphologi-
cal cell properties, as e.g. cell size, as alternative sorting targets. Actually,
characterisation of high proliferative subpopulations by cell size has been
demonstrated [ 31 ]. Unfortunately, cell size does also change according to spon-
taneous transformation events during long term culture. Such transformations can
occur frequently and can start early. They result in the emergence of a cell type
characterised by an elevated proliferation, reduced plasticity and round and small
morphology [ 2 , 77 ]. Thus, transformed cells may be hardly distinguishable from
high potential untransformed MSCs by morphological characterization only.
Interestingly, the time point of spontaneous transformation seems to be sto-
chastic in nature like that of acquiring senescence [ 77 ]. Experimental results
suggest that the transformed fate is not correlated to a defined regulatory state. So,
transformed clones have been observed to differ in their expansion rate [ 77 ] as well
as in the number of chromosomes present in the cells [ 2 ]. In the following we will
focus on non-transformed MSCs and will neglect transformation as a particular
source of population heterogeneity.
In summary, experimental findings demonstrate that MSC heterogeneity is a
multi-scale phenomenon and is subject to significant changes during in vitro
cultivation.
3 On the Origin of MSC Heterogeneity
While heterogeneity is accepted to be a characteristic property of stem cell pop-
ulations, its origin is still not well understood. Heterogeneity of non-transformed
functional stem cells has been discussed as a consequence (i) of cell adaptation to
dynamic environments and (ii) of the flexibility and reversibility of stem cell fate
decisions. Additional variance in cell fates may be associated with cell ageing.
3.1 Heterogeneity as a Consequence of MSC Environmental
Adaptation (Extrinsic)
It has been demonstrated that MSCs from different tissues, including bone-marrow
stroma, adipose, skeletal muscle, synovium and umbilical cord differ in both
molecular and functional properties [ 39 , 58 , 63 , 99 ]. Typically a large number of
genes and proteins have been found to be differentially expressed (see e.g.
[ 61 , 62 ]) and, although the MSCs were expanded over many PDs in vitro, these
differences in expression appeared to be conserved. Moreover, they also manifest
in functional differences of the MSCs including their expansion and differentiation
potential. For example experiments on human bone marrow MSCs revealed that
about one-third of the clones are able to acquire phenotypes of pre-adipocytes,