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TH directly upregulates the fibroblast-specific expression of ST3 (
Fu, Tomita,
Wang, Buchholz, & Shi, 2006; Patterton et al., 1995
), which cleaves the 67-
kDa receptor for laminin, a major ECM component of the basal lamina
(
Amano, Fu, Marshak, Kwak, & Shi, 2005
), leading to degradation of the basal
lamina and massive apoptosis of the larval epithelium (
Mathew et al., 2009;
Shi, Fu, Hasebe, & Ishizuya-Oka, 2007
). Moreover, by using Tg tadpoles that
express ST3 after heat shock, previous study has shown that ST3 expression
alone causes modification of the basal lamina following frequent
fibroblast-epithelial cell contacts, even in the absence of TH (
Fig. 11.3
C
and D;
Fu et al., 2005
). This implies an essential role of ST3 for the cell con-
tacts, although their biological significance is unclear.
In the
X. laevis
intestine during natural metamorphosis, the fibroblasts
frequently make contact with only adult progenitor/stem cells but not with
the other larval cells, while the basal lamina becomes amorphous underneath
the entire epithelium. This suggests that a juxtacrine signaling caused by the
cell contacts is involved in adult epithelial development but not in larval ep-
ithelial apoptosis. Similarly, in the mammalian intestine, previous study
reported frequent epithelial-mesenchymal cell contacts only during intesti-
nal maturation around birth (
Mathan, Hermos, & Trier, 1972
), when the
adult stem cells develop (
Harper, Mould, Andrews, Bikoff, & Robertson,
2011; Muncan et al., 2011
). However, it still remains unknown to date what
happens at the cell-to-cell contact sites in any vertebrate intestine. Given that
neither the gap junction nor fusion of cell membranes are recognized at the
contact sites of both amphibian (
Ishizuya-Oka & Shimozawa, 1987b
) and
mammalian intestines (
Mathan et al., 1972
), it is possible that TH response
genes encoding transmembrane proteins such as Notch (
Buchholz et al.,
2007
) or transient receptor potential channels (our unpublished data) may
be ones of the juxtacrine signaling components. If the genes that are specif-
ically expressed at the contact sites will be identified in the near future, their
functional analysis will pave a way to unravel molecular mechanisms under-
lying the cell contacts between the two tissues during postembryonic intes-
tinal development.
5.2. Shh/BMP4 signaling pathway
As shown by tissue recombinant experiments using dpTR tadpoles men-
tioned earlier (
Fig. 11.4
), TH-upregulated expression of Shh is an early step
toward epithelial dedifferentiation into the adult stem cells, suggesting its
key role in this process. In fact, during natural metamorphosis, Shh is highly
