Molecular mechanisms for thyroid hormone‐induced remodeling in the amphibian digestive tract: A model for studying organ regeneration

During amphibian metamorphosis the digestive tract is extensively remodeled under the control of epithelial–connective tissue interactions. At the cellular level, larval epithelial cells undergo apoptosis, while a small number of stem cells appear, actively proliferate, and then differentiate to form adult epithelium that is analogous to its mammalian counterpart. Therefore the amphibian digestive tract is a unique model system for the study of postembryonic organ regeneration. As amphibian intestinal remodeling can be triggered by thyroid hormone (TH), the molecular mechanisms involved can be studied from the perspective of examining the expression cascade of TH response genes. A number of these genes have been isolated from the intestine of Xenopus laevis . Recent progress in the functional analysis of this cascade has shed light on key molecules in intestinal remodeling such as matrix metalloproteinase‐11, sonic hedgehog, and bone morphogenetic protein‐4. These genes are also thought to play key roles in organogenesis and/or homeostasis in both chick and mammalian digestive tract, suggesting the existence of conserved mechanisms underlying such events in terrestrial vertebrates. In this article, we review our recent findings in this field, focusing on the development of adult epithelium in the X. laevis intestine.