733.2 Senescence
One important consideration in terms of defi ning ageing is whether it is the indi-
vidual cells which have become aged or whether the biochemical infl uence of the
older individual has infl uenced the cell’s stem cell potential. Organismal ageing is
beyond the scope of this chapter; however, it is widely known that stem cells are
subject to altered function after extrinsic toxicity. For example, ultraviolet radiation
on skin (Fuchs and Raghavan 2002 ) or chemotherapy on hemopoiesis (Richman
et al. 1976 ) are well- known effects.
A recurrently cited theory has been suggested by Hayfl ick and Moorhead. In their
investigations, they described senescence as being a state of irreversible cell division
(Hayfl ick and Moorhead 1961 ). In vivo, stem cells on the whole clearly have no fi nite
replicative capacity. Embryonic stem cells show no loss of proliferation potential
(Rosenberger 1995 ); however, MSCs in vitro have shown replicative limit. Bruder
et al. ( 1997 ) isolated MSCs from iliac crests of nine individuals. Cells were cultured
and serially passaged, and it was found that from being spindle-like shaped cells, they
became broad and fl attened in morphology. Furthermore, cells averagely lost replica-
tive capacity after 38 population doublings, that is, they became senescent.
Homing in on a cellular level, this area is somewhat fairly well documented.
The cellular markers p21, p53 and reactive oxygen species (ROS) are all
described as important markers of cellular stress. P53, commonly regarded as a
tumour- suppressor, is found to be upregulated in aged stem cells (Stolzing et al.
2008 ). Levels of p21 and beta-galactosidase levels were found to be signifi cantly
less in ADSC compared to BMSCs in one study (Chen et al. 2011 ). Zhou et al.
( 2008 ) investigated expression of these senescence-related factors, isolating
MSCs from bone marrow of donors aged 17–90 years. They found that samples
from older donors were signifi cantly more positive for senescence-related beta-
galactosidase, as well as having increased expression of p53, and its related BAX
and p21 genes.
Where DNA damage occurs, it is known that there would be increased expres-
sion of p16INK4A and p53 (Kim and Sharpless 2006 ). Upregulation of these divert
the cells’ fate to exit the cell cycle and induce senescence and/or apoptosis. This
would also occur with regard to MSCs, suggesting the mechanisms by which these
cells have shown replicative stress. The INK4a/ARF gene encodes two proteins
p16INK4a and p14ARF. p16INK4a is known to inhibit the cyclin-dependent
kinases, CDK4 and CDK6, which promote proliferation (Kim and Sharpless 2006 ).
It also increases with age as well as is being involved in regulating age-dependant
senescence (Zindy et al. 1997 ). The p14ARF protein, however, regulates cell cycle
pathways involving p53 towards senescence/apoptosis. The role of ROS shows that
it can be used to manipulate in vitro cellular fate. Previous studies have shown that
it can limit proliferation (Meagher et al. 1988 ) and, furthermore, cause DNA dam-
age inducing cellular senescence (Ko et al. 2011 ).
3 Ageing and Senescence in Mesenchymal Stem Cells