Biomedical Engineering Reference
In-Depth Information
NEURAL STEM CELLS
In mammals, it had been believed that
neurogenesis did not continue in the adult
brain. However, recent studies have overturned that theory and have shown that a
supply of new neurons is continued even into adulthood in two distinct regions of
the brain: the subventricular zone of lateral ventricles (SVZ) and the dentate gyrus
of the hippocampus. Neural stem cells are dei ned as cells having the capacity to
self-renew and to dif erentiate into neurons, astrocytes and oligodendrocytes, and
are considered to exist in the SVZ, although the prospective isolation of NSCs
has not been achieved because of the lack of NSC-specii c markers. It has been
reported that the dif erentiation capacity of the NSCs in the SVZ is skewed only
towards neurons (but not towards astrocytes and oligodendrocytes) in the adult
brain, whereas there is a report showing that oligodendrocytes are also generated
from these NSCs (Menn
et al.
2006). h e NSC-niche in the SVZ is composed
of several types of cells
(Fig. 2)
: slow-proliferating/glial i brillary acidic protein
(GFAP)-positive astrocytes (considered as NSCs), rapidly proliferating/transient-
amplifying precursors (derived from the NSCs), neuronal progenitors migrating
towards the olfactory bulb (immature neurons) and ependymal cells lining the
lateral ventricles. h e ependymal cells are epithelial cells and come into close
contact with the NSCs expressing Notch and integrin β1 (CD29). h e ependymal
cells are therefore regarded as niche cells. Campos
et al.
(2006) reported that
integrin β1 af ected the Notch signaling pathway through its interaction with the
Notch intracellular domain.
In the dentate gyrus of the hippocampus, GFAP-positive neuron precursors
can be found and these cells are considered to have a limited self-renewal and
dif erentiation capacity.
Recent studies have shown that neuronal progenitors migrating towards the
olfactory bulb (immature neurons) express integrin β1 and polysiaric acid-binding
NCAM (PSA-NCAM), and that these adhesion molecules contribute to their
smooth migration in the restral migratory stream (Cazal
et al.
2000, Belvindrah
et al.
2007). h us, adhesion molecules contribute greatly to both the generation of
neurons and the migration of the immature neurons towards the olfactory bulb.
HEPATIC STEM CELLS
h e liver is a unique organ, having an extensive regeneration capacity. Oval
cells, small hepatocytes and hepatoblasts have been proposed as candidates of
hepatic stem cells (HpSCs). h e oval cells, having an oval-shaped nucleus and
scant cytoplasm, reside in the canals of Heling (the meeting point of hepatocyte
canaliculi and the bile duct) when the liver is in a quiescent state, but the cells
begin to proliferate, move to the parenchyma and give rise to hepatocytes and bile
