Biomedical Engineering Reference
In-Depth Information
for differentiation could show preferred localization to the vasculature region
before.
Thus, in addition to Sertoli cells, peritubular myoid cells, and basement
membrane, the components common to the entire basal compartment, blood
vessels, and/or interstitial cells possibly specify the niche microenvironment
(Fig.
8.6b
). This may be a direct, distant effect mediated by soluble factors
(Fig.
8.6b
,
arrows 1
and
2
), or may be an indirect effect relayed via myoid or
Sertoli cells (Fig.
8.6b
,
arrows 3
and
4
). In the latter case, myoid and/or Sertoli
cells that are located in the proximity of the vasculature would be endowed with
special characters under the influence of the blood vessels/interstitial cells
(Fig.
8.6b
,
dotted arrows
). Additionally, the specific characteristics of the base-
ment membrane could be important (Fig.
8.6b
,
arrow 5
). Intriguingly, transplan-
tation of seminiferous tubule has suggested that the remodeling of the vasculature
may accompany the relocation of the A
undiff
to the newly formed vessels (Yoshida
et al.
2007b
), suggesting that the niche region is not fixed but is plastic and
reversible. In accordance with this, testis of golden hamster is suggested to
remodel the vasculature and accompanying niche region in response to short
photoperiod (do Nascimento et al.
2009
). This is in a contrast to that in
Drosophila
GSC niches that are specified as a result of a strict organogenesis program and do
not regenerate once damaged.
Currently, identification of the components that specify the niche microenviron-
ment and regulate the A
undiff
/stem cell behaviors has not been achieved to an extent
comparable to those for
Drosophila
. However, a number of molecules are known to
functionally contribute to stem cell control. In the next section, considering these
factors together, readers will gain an insight into the functional aspects of the stem
cell niche.
8.6
Molecular Mechanisms That Could Be Involved
in the Niche Functions
8.6.1
Soluble Factors
8.6.1.1
GDNF (Glial Cell Line-Derived Neurotrophic Factor) Signaling
GDNF is the most important currently known controlling factor of mouse spermato-
genic stem cells. GDNF is a soluble ligand and a diverged member of TGF-beta
superfamily, which transmits signals through a receptor complex consisting of c-Ret
tyrosin kinase and GFRa1 co-receptor (Paratcha and Ledda
2008
). The importance
of GDNF signaling in the control of spermatogenic stem cells was first recognized
from the mutant phenotype (Meng et al.
2000
). Mutant mice heterozygous for the
Gdnf
- or
c-Ret
-targeted allele exhibited a gradual degeneration in spermatogenesis
following essentially normal establishment of spermatogenesis. This phenotype
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