12 F. AERTS ANDG. WAGEMAKER
or without supplementation of additional growth factors [77, 81]. Serum-deprived
medium, supplemented with 10 ng/ml of EGF, PDGF, IGF-1 and bFGF, did not stimulate
development of CFU-F. However, if 100∗M L-ascorbic acid-2-phosphate (ASC-2P)
and 10
−8
M dexamethasone (Dexa) were added to the medium, CFE was similar to con-
trol cultures supplemented with 20% FCS. When ASC-2P or Dexa were supplemented
without the addition of exogenous growth factors, no CFU-F growth occurred [15]. Al-
though cultures, supplemented with PDGF and EGF, in presence of ASC-2P and Dexa,
showed an equivalent ability to support CFU-F growth compared to control cultures,
the average colony size of the CFU-F was significantly increased. In contrast, first pas-
sage MSCs derived from rat BM, obtained after initial culturing under serum-replete
conditions, showed optimal growth when cultured in a serum-free medium, which con-
sisted of 5∗g/ml insulin, 0.1% LA-BSA, 10 ng/ml PDGF-BB, 1 ng/ml bFGF in a
base medium of 60% DMEM-LG with 40% MCDB-201 [78]. Additionally, mouse
and human stromal cell cultures were stimulated by a serum-free conditioned medium
(SF-CM), consisting of-MEM supplemented with 2mM L-glutamine, 100 U/ml peni-
cillin, 100∗g/ml streptomycin sulphate, 10
−8
M dexamethasone, 10
−4
M L-ascorbic
acid phosphate magnesium salt n-hydrate and 0.5% ITS+, conditioned by marrow cells.
It was found that neutralizing antibodies against PDGF, TGF-◦, bFGF and EGF specif-
ically, were able to suppress all colony formation. However, growth factor dependence
varied between the different species, and the most profound inhibition of mouse CFU-
F formation was induced by anti-PDGF, anti-bFGF and anti-EGF, whereas in human
cultures anti-PDGF and anti-TGF-◦were most effective [15].
Differences between the above mentioned studies are in part due to the use of distinct
methods of selection of the cells and the culture conditions (with or without feeder layer
or conditioned medium). Of some importance may be the use of high concentrations of
growth factors or addition of BSA that are not recombinant and may therefore contain
traces of multiple serum activities (for example, growth factor-binding proteins).
7. DIFFERENTIATION POTENTIAL AND PROLIFERATIVE HIERARCHY
7.1. Orthodox differentiation potential
The differentiation potential of stromal cells derived from a number of different
mesoderm-derived tissues has been considered for a prolonged time to be restricted
exclusively to mesenchymal lineages. The stem cell-like nature and characteristics of
MSCs are now generally accepted and these cells are now commonly regarded as a
second class of adult stem cells, in addition to HSCs, that populate the BM. However,
they also constitute a variety of other adult tissues, as described above. Inin vitrocul-
ture conditions, MSCs maintain their self-renewal capacity for extended periods and
have the ability to generate a large quantity of different mesenchymal cell types. As
such, these cells take part in the regeneration of mesenchymal tissues, in response to
injury. Throughout a variety of species MSCs were found to be able to differentiate into
several tissues including bone [82, 83], cartilage [84], stroma [14], adipose [11], tendon
[85], but also neural tissue [86, 87, 88, 89], smooth muscle [90], and cardiac muscle
[91, 92, 93, 94] and a variety of other connective tissues [14], as discussed below. The