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noted that TH-upregulated expression of Shh is specific for intervillus
pockets where the adult stem cells develop in the mammalian intestine
(
Kolterud et al., 2009
). This supports our proposal that Shh may function
as an early key player to establish the stem cell niche common to the amphib-
ian and mammalian intestines. More importantly, adult stem cells develop
from fetal epithelial cells during mammalian intestinal maturation (
Harper
et al., 2011; Muncan et al., 2011
) similar to those from the larval epithelial
cells during amphibian intestinal remodeling (
Ishizuya-Oka et al., 2009
).
Subsequently, the descendants of the adult stem cells in both mammalian
and amphibian intestines replace the preexisting suckling-type or the larval
epithelial cell by active proliferation and differentiation to generate the adult
epithelium possessing a cell renewal system. These similarities strongly sug-
gest the existence of evolutionarily conserved mechanisms underlying de-
velopment of the adult stem cells and their niche (
Ishizuya-Oka & Shi,
2011
). What is unique to this amphibian model is that the stem cell niche
can be experimentally induced by TH at any time both
in vitro
and
in vivo
and be studied through the use of numerous TH response genes. By taking
advantages of these facts, it is an interesting and challenging issue to clarify
how the stem cell niche is formed through cross talks between Shh and other
paracrine and/or juxtacrine signaling pathways.
TH is generally known to be essential not only for amphibian metamor-
phosis but also homeostasis of frog organs as well as adult mammalian ones
(
Yoshizato, 2007
). Although the identified TH response genes are still lim-
ited in the adult mammalian intestine (
Kress, Samarut, & Plateroti, 2009
),
there is a growing body of evidence that signaling molecules induced by
TH in the amphibian intestine are also expressed in the mammalian intestine
to maintain its epithelial cell renewal. For example, Shh is specifically
expressed in the crypt where adult stem cells reside throughout adulthood
(
Crosnier, Stamataki, & Lewis, 2006; de Santa Barbara et al., 2003;
Varnat, Zacchetti, & Ruiz i Altaba, 2010
), and its expression is upregulated
when the stem cells actively proliferate during mammalian intestinal regen-
eration (
Berman et al., 2003; Nielsen, Williams, van den Brink, Lauwers, &
Roberts, 2004
). In addition, many studies reported intestinal diseases caused
by mutations in components of Shh/BMP4 signaling pathway (
Batts, Polk,
Dubois, & Kulessa, 2006; Beachy, Karhadkar, & Berman, 2004; Berman
et al., 2003; Haramis et al., 2004; He et al., 2007, 2004; Litingtung, Lei,
Westphal, & Chiang, 1998
), although their functions on the adult stem cells
have not yet been clarified enough. Further,
b
-catenin gene, one of TH re-
sponse genes identified in the
X. laevis
intestine (
Buchholz et al., 2007
), has
