Adult organ-specific stem cells are essential for organ homeostasis and repair 
in adult vertebrates. The intestine is one of the best-studied organs in this 
regard. The intestinal epithelium undergoes constant self-renewal throughout 
adult life across vertebrates through the proliferation and subsequent 
differentiation of the adult stem cells. This self-renewal system is established 
late during development, around birth, in mammals when endogenous thyroid 
hormone (T3) levels are high. Amphibian metamorphosis resembles mammalian 
postembryonic development around birth and is totally dependent upon the 
presence of high levels of T3. During this process, the tadpole intestine, 
predominantly a monolayer of larval epithelial cells, undergoes drastic 
transformation. The larval epithelial cells undergo apoptosis and concurrently, 
adult epithelial stem/progenitor cells develop de novo, rapidly proliferate, and 
then differentiate to establish a trough-crest axis of the epithelial fold, 
resembling the crypt-villus axis in the adult mammalian intestine. We and others 
have studied the T3-dependent remodeling of the intestine in Xenopus laevis. 
Here we will highlight some of the recent findings on the origin of the adult 
intestinal stem cells. We will discuss observations suggesting that liganded T3 
receptor (TR) regulates cell autonomous formation of adult intestinal progenitor 
cells and that T3 action in the connective tissue is important for the 
establishment of the stem cell niche. We will further review evidence suggesting 
similar T3-dependent formation of adult intestinal stem cells in other 
vertebrates.