Biology Reference
In-Depth Information
1. INTRODUCTION
During amphibian metamorphosis, the digestive organs undergo rapid
and extensive remodeling to adapt from the aquatic herbivorous to terrestrial
carnivorous life. The major digestive organ, the small intestine dramatically
shortens and develops multiple intestinal folds (
Ishizuya-Oka & Shimozawa,
1987a; McAvoy & Dixon, 1977
). The adult intestinal epithelium after meta-
morphosis acquires a cell renewal system along the trough-crest axis of the in-
testinal folds, similar to that along the crypt-villus axis in the adult mammalian
intestine (
Bjerknes &Cheng, 1981; Cheng &Bjerknes, 1985; Madara &Trier,
1994
). This suggests that stem cells analogous to those in the adult mammalian
intestine develop in the amphibian intestine during metamorphosis. In fact, in
the
Xenopus laevis
intestine, undifferentiated cells expressing markers for mam-
malian intestinal stem cells become detectable at the onset of metamorphosis
(
Ishizuya-Oka & Shi, 2007
). In the adult mammalian intestine, the microenvi-
ronment around stem cells called as the niche is generally known to play im-
portant roles in the maintenance of the stem cells throughout adulthood and
are thus clinically important for tissue regeneration and/or cancer therapies.
However, it still remains unknown how the adult stem cells and their niche
are formed during normal development in any vertebrate intestine.
Amphibian metamorphosis can be easily manipulated by simply adding a
single hormone, thyroid hormone (TH) to the tadpoles (
Dodd &Dodd, 1976;
Kikuyama, Kawamura, Tanaka, & Yamamoto, 1993; Shi, 1999
). Our previ-
ous studies have shown that TH can organ-autonomously reproduce the
whole process of larval-to-adult remodeling including development of the
adult stem cells and their niche in the
X. laevis
intestine
in vitro
(
Ishizuya-
Oka, Shimizu, Sakakibara, Okano, & Ueda, 2003; Ishizuya-Oka &
Shimozawa, 1992
). Thus, in this animal model, TH response genes provide
important clues to clarify molecular mechanisms regulating the adult stem
cells. Until now, a number of TH response genes have been identified in
the
X. laevis
intestine by subtractive differential screening (
Amano &
Yoshizato, 1998; Shi & Brown, 1993
) and, more recently, cDNAmicroarrays
(
Buchholz, Heimeier, Das, Washington, & Shi, 2007; Heimeier, Das,
Buchholz, Fiorentino, & Shi, 2010
). Their functions can be now easily ana-
lyzed by using culture and transgenic (Tg) techniques. In this review, we will
first summarize findings obtained by previous morphological observations and
culture experiments on adult epithelial development in the
X. laevis
intestine
during metamorphosis and then survey recent progress in expression and
