In the amphibian intestine during metamorphosis stem cells appear and generate the adult absorptive epithelium analogous to the mammalian one under the control of thyroid hormone (TH). progenitor cells expressing sonic hedgehog became detectable on day 5 in both the recombinant intestine of Tg Ep and Tg non-Ep (Tg/Tg) and that of Tg Ep and Wt non-Ep (Tg/Wt). However in Tg/Wt intestine they did not express other stem cell markers such as Musashi-1 and never generated the adult epithelium expressing a marker for absorptive epithelial cells. Our results indicate that while it is usually unclear why some larval epithelial cells dedifferentiate into adult progenitor/stem cells TR-mediated gene expression in the surrounding tissues other than the epithelium is required for them to develop into adult stem cells suggesting the importance of TH-inducible epithelial-connective tissue interactions TSPAN31 in establishment of the stem cell niche in the amphibian intestine. small intestine a well-analyzed organ at the cellular level a single layer of primary (larval) epithelium is usually surrounded by the immature connective Clofarabine tissue and thin muscles before metamorphosis. Clofarabine At the start of metamorphic climax (stage 60; [6]) when the level of thyroid hormone (TH) in the plasma becomes high [7] most of the larval epithelial cells begin to undergo apoptosis whereas a small number of undifferentiated cells become detectable as small islets between the larval epithelium and the developing connective tissue [8-11]. These undifferentiated cells are stained strongly red with pyronin Y and notably express sonic hedgehog (Shh) Musashi-1 (Msi-1) and Akt [12-14] all of which are candidate markers for mammalian intestinal stem cells [15-18]. They gradually replace the degenerating larval epithelium through active proliferation and as morphogenesis of multiple intestinal Clofarabine folds proceeds differentiate into the secondary (adult) epithelium that consists of major absorptive cells expressing intestinal fatty acid-binding protein (IFABP) goblet cells and enteroendocrine cells [19-21]. At the completion of metamorphosis the adult epithelium acquires a cell renewal system along the trough-crest axis of intestinal folds [8 10 11 analogous to the mammalian crypt-villus axis [22 23 These chronological observations indicate that multipotent stem cells analogous to those in the mammalian adult intestine appear at the start of metamorphic climax in the intestine. Taken together with the fact that this intestinal remodeling can be easily and experimentally induced by TH both and [24] and that a number of TH response genes have been identified in the intestine [25 26 this animal model offers an excellent opportunity to study molecular mechanisms regulating organ-specific adult stem cells common to various vertebrates. Using transgenic (Tg) tadpoles constitutively expressing GFP in tissue recombination experiments we have recently shown that this adult stem cells originate exclusively from the larval epithelium [27]. Since all of the larval epithelial cells at this stage are essentially differentiated as larval-type by light and electron microscopy [8 28 and are unfavorable for stem cell markers such as Shh and Msi-1 [13 14 this result implies that some of the larval epithelial cells dedifferentiate into the adult stem cells during amphibian metamorphosis similar to “epithelial transit cells” which are Clofarabine known to dedifferentiate into the stem cells during regeneration of the adult mammalian intestine [3 29 Despite Clofarabine the biological and clinical importance of intestinal epithelial dedifferentiation its molecular mechanisms have not yet been clarified. In the intestine the developmental transition from the larval to adult epithelium including the period when dedifferentiation occurs is usually brought on by TH and requires the presence of the connective tissue [30]. Thus the question arises whether TH must act directly on the larval epithelium the connective tissue or both in order for adult epithelial development to occur. In the present study to address this question and investigate the molecular mechanisms underlying developmental of intestinal stem cells we made use of Tg tadpoles which express a dominant positive thyroid hormone receptor (dpTR) under the control of a heat shock-inducible promoter [31]. In these Tg tadpoles dpTR specifically binds to TH response elements within promoter regions of TH target genes.