Gene regulation by thyroid hormone during amphibian metamorphosis: Implications on the role of cell-cell and cell-extracellular matrix interactions

Amphibian metamorphosis is the developmental process initiated by thyroid hormone which transforms a tadpole into a frog. This transformation requires extensive remodeling of almost every tissue in the animal, One of the more well-studied tadpole tissues that undergoes remodeling is the small intestine, This tissue requires a shortening in length as well as internal anatomical restructuring to function in the adult frog, Briefly, the tadpole epithelial cells undergo programmed cell death (or apoptosis) and are replaced by a layer of newly formed adult epithelium. About 20 thyroid hormone-regulated genes participating in this intestinal remodeling have been identified, These genes can be divided into several groups based on the proposed functions of their products, One of these groups contains several secreted and/or signaling molecules, Most prominent among these are the Xenopus homologs of the hedgehog and stromelysin-3 genes, Based on the expression profiles and cellular localization, hedgehog appears to be involved in adult epithelial morphogenesis, Stromelysin-3 may participate in basal lamina modification which is potentially involved in the apoptosis of the larval epithelium and development of the adult epithelium. Here we mill review in detail the potential roles for these secreted factors as well as the proposed molecular mechanisms responsible for their physiological functions, Furthermore, me will examine the effect of these proteins on the extracellular environment and how this impacts upon cellular processes involved in intestinal remodeling.