Patterning of the gut into morphologically distinct regions results from the appropriate factors being expressed in strict spatial and temporal patterns to assign cells their fates in development. Often, the boundaries of gene expression early in development correspond to delineations

The digestive system is critical for maintenance of all multicellular organisms. As the organism ingests food, it must be processed and essential nutrients removed and absorbed into the body prior to expulsion of the waste products. In higher organisms, these steps are handled by a series of specialized organs. In spite of its ultimate complexity, the vertebrate gut develops from a primitive tubular structure composed of two tissue types, an underlying endoderm and a surrounding splanchnic mesoderm. As development proceeds, the endoderm and mesoderm differentiate their organ-specific characteristics, producing the organs of the gastrointestinal tract. Although relatively little is known about the molecules that pattern the gut tube, experiments have shown the importance of epithelial-mesenchymal signaling in proper formation of the gut derivatives (Kedinger et al., 1986, 1990). Epithelial-mesenchymal signaling is characterized by a cross-talk between the two tissue types, which results in patterning of the organ. Epithelial-mesenchymal signaling in the gut is characterized by a primary signal emanating from the endoderm that specifies the surrounding mesoderm to become gut mesoderm (Kedinger et al., 1986, 1990). A secreted molecule expressed throughout the gut endoderm, Sonic Hedgehog (SHH), has been implicated as this primary mesodermal specification signal (Roberts et al., 1995; Apelqvist et al., 1997). Sonic hedgehog (Shh) expression is seen as soon as the gut primordia begin to invaginate to form the gut tube at the anterior intestinal portal and the caudal intestinal portal. This expression is maintained and extends along the entire length of the gut endoderm as the gut develops (Roberts et al., 1995). Consistent with endodermally derived Shh mediating epithelial-mesenchymal interactions, the receptor for Shh is expressed in the underlying mesoderm (Marigo et al., 1996). Following this primary specification of the visceral mesoderm, the mesoderm signals back to the endoderm providing anteroposterior cues that pattern the endoderm (Kedinger et al., 1986, 1990; Roberts et al., 1998). These secondary signals contribute to the specification of differential endodermal phenotypes along the gut tube. Hence, although the endoderm is very specialized in each organ of the gut, some of the signals that determine this specification emanate from the overlying mesoderm. Hox genes, a family of genes encoding homeodomain-containing transcription factors, have been found to play a regulatory role in the mesoderm to endoderm signaling (Kondo et al., 1996; Roberts et al., 1998). The Hox genes are expressed in an organ-specific 3671 Development 127, 3671-3681 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 DEV2565