Remnant of the buccopharyngeal membrane
Figure 13.1 Sagittal sections through embryos at various stages of development demonstrating the effect of cephalocaudal and lateral folding on the position of the endoderm-lined cavity. Note formation of the foregut, midgut, and hindgut. A. Pre-somite embryo. B. Embryo with 7 somites. C. Embryo with 14 somites. D. At the end of the first month.
Differentiation of various regions of the gut and its derivatives is dependent upon a reciprocal interaction between the endoderm (epithelium) of the gut tube and surrounding splanchnic mesoderm. Mesoderm dictates the type of structure that will form, for example lungs in the thoracic region and descending colon from the hindgut region, through a HOX code similar to the one that establishes the anterior (cranial) posterior (caudal) body axis. Induction of this HOX code is a result of sonic hedgehog (SHH) expressed throughout the gut endoderm. Thus, in the region of the mid- and hindgut, expression of SHH in gut endoderm establishes a nested expression of the HOX code in the meso-derm (Fig. 13.2). Once the mesoderm is specified by this code, it instructs the endoderm to form the various components of the mid- and hindgut regions, including the small intestine, cecum, colon, and cloaca (Fig. 13.2). Similar interactions are responsible for partitioning the foregut.
Figure 13.2 Diagrams of the mid- and hindgut regions. The morphogen sonic hedgehog (SHH) is secreted by gut endoderm and induces a nested expression of HOXgenes in surrounding mesoderm. HOX expression then initiates a cascade of genes that "instruct" gut endoderm to differentiate into its regional identities. Signaling between the two tissues is an example of an epithelial-mesenchymal interaction.
. large intestine
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