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after 7 days. Importantly, whenever the epithelium was derived from Tg in-
testine, both the adult progenitor/stem cells and their differentiated absorptive
cells expressed GFP, whereas neither of them expressed GFP whenever the
epithelium was derived fromWt intestine (
Fig. 11.2
B). These results indicate
that the adult stem cells originate from the larval epithelium but not from the
other non-E. However, previous light and electron microscopical studies
reported that all of the epithelial cells in the tadpole intestine before stage
60 are essentially differentiated as larval type (
Hourdry & Dauca, 1977;
Marshall & Dixon, 1978b
) and are immunohistochemically negative for
any stem cell marker examined (
Ishizuya-Oka et al., 2003, 2001
). Taken to-
gether, it can be concluded that some of the larval epithelial cells dedifferen-
tiate into the adult stem cells toward stage 60, similar to the mammalian
epithelial “transit amplifying cells,” which are partially differentiated but
can dedifferentiate into adult stem cells during intestinal regeneration
(
Potten, 1998; Potten, Booth, & Pritchard, 1997
). Supporting this conclusion,
we recently found that the expression of nuclear lamins, which are interme-
diate filament proteins lining the inner nuclear membrane (
Gruenbaum et al.,
2003
), changes in association with development of the adult stem cells in the
X. laevis
intestine (
Hasebe, Kajita, Iwabuchi, Ohsumi, & Ishizuya-Oka, 2011
).
Before stage 60, almost all of the differentiated epithelial cells express lamin A
(LA), whose expression is generally known to be specific to differentiated cells
of mammalian organs (
Rober, Weber, & Osborn, 1989
) but not lamin LIII
(LIII), an embryo-specific lamin (
Doring & Stick, 1990
). At the early meta-
morphic climax (stage 60), the expression of LA and LIII is down- and
upregulated, respectively, only in the adult progenitor/stem cells concomi-
tantly with their appearance. These expression profiles of LA and LIII are sim-
ilar to those during mammalian somatic cell reprogramming induced by
X. laevis
egg extracts (
Bru et al., 2008; Mitalipov et al., 2007; Miyamoto
et al., 2007
). Given that the larval epithelial cells dedifferentiate into the adult
stem cells by the inductive action of TH, the next question arises whether TH
acts on the larval epithelial cells directly or through interactions with the
nonepithelial cells.
4. NICHE ESSENTIAL FOR ADULT STEM CELL
DEVELOPMENT
It is noteworthy that adult stem cell development in the
X. laevis
in-
testine spatiotemporally correlates well with extensive changes in the sur-
rounding connective tissue (
Ishizuya-Oka & Shimozawa, 1987a; Marshall
