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et al., 2003; Potten et al., 2003; van Den Brink, de Santa Barbara, & Roberts,
2001 ). More definitively, the orphan leucine-rich repeat-containing G
protein-coupled receptor 5 (LGR5), a well-established stem cell marker in
the adult mammalian intestine ( Barker et al., 2007; Sato et al., 2009 ), has been
shown to be specifically expressed in the adult progenitor cells of the X. laevis
intestine, concomitantly with their appearance at stage 60 ( Sun et al., 2010 ).
Thereafter, as the intestinal folds form, the expression of LGR5 becomes lo-
calized in the trough region of the folds where the stem cells reside. These
similarities in gene expression of the stem cell markers between the amphibian
and mammalian intestines strongly suggest the usefulness of the X. laevis in-
testineasamodelforthestudyoftheadultstemcells.
3. ORIGIN OF ADULT STEM CELLS
To investigate mechanisms underlying the amphibian intestinal remo-
deling, we have previously established an organ culture system using X. laevis
tadpoles at stage 57, when the small intestine is the longest throughout pre-
and prometamorphosis. In this culture system, TH can organ-autonomously
induce adult epithelial development in vitro as in vivo ( Ishizuya-Oka &
Shimozawa, 1992 ). Adult progenitor/stem cells are identified as small islets
after 5 days, actively proliferate, and then differentiate into the adult absorptive
epithelium expressing IFABP after 7 days. This indicates that the adult stem
cells are derived from the tadpole intestine itself but not from other organs at
latest at stage 57. If so, there are two possible origins of the stem cells: (1) the
larval epithelial cells before metamorphic climax, as proposed by previous
chronological observations ( Amano, Noro, Kawabata, Kobayashi, &
Yoshizato, 1998; Marshall & Dixon, 1978b; Schreiber, Cai, & Brown,
2005 ), or (2) nonepithelial cells that migrate into the epithelium after stage
57, as bone marrow-derived cells migrate into the epithelium during mam-
malian intestinal regeneration ( Krause et al., 2001; Okamoto et al., 2002 ).
Our recent study provided the first experimental evidence supporting the for-
mer by using Tg X. laevis tadpoles that constitutively express green fluorescent
protein (GFP) for tissue recombinant culture experiments ( Ishizuya-Oka
et al., 2009 ). The larval epithelium isolated from wild-type (Wt) or GFP
Tg intestines at stage 57 was recombined with homologous or heterologous
nonepithelial tissues (non-E), and four kinds of recombinant intestines were
cultured in the medium containing TH ( Fig. 11.2 A). In all the recombinant
intestines, as in intact ones, adult progenitor/stem cells became detectable after
5 days and then differentiated into absorptive epithelial cells expressing IFABP
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