The Questions of Developmental Biology

As Figure 15.27 shows, the stomach develops as a dilated region close to the pharynx.
More caudally, the intestines develop, and the connection between the intestine and yolk sac is
eventually severed. At the caudal end of the intestine, a depression forms where the endoderm
meets the overlying ectoderm. Here, a thin cloacal membrane separates the two tissues. It
eventually ruptures, forming the opening that will become the anus.
Specification of the gut tissue
As the endodermal tubes form, the endodermal epithelium is able to respond differently
to different regionally specific mesodermal mesenchymes. This enables the digestive tube and
respiratory tube to develop different structures in different regions. Thus, as the mammalian
digestive tube meets new mesenchymes, it differentiates into esophagus, stomach, small intestine,
and colon (Gumpel-Pinot et al. 1978; Fukumachi and Takayama 1980; Kedinger et al. 1990).
The specificity of the mesoderm is thought to be controlled by its interactions with the
endodermal tube during earlier stages of development. As the gut tube begins to form from the
anterior and posterior ends, it induces the splanchnic mesoderm to become regionally specific.
Roberts and colleagues (1995, 1997) have implicated Sonic hedgehog in this specification. Early
in development, the expression of Shh is limited to the posterior endoderm of the hindgut and the
pharynx. As the tubes extend toward the center of the embryo, the domains of Shh expression
increase, eventually extending throughout the gut endoderm. Shh is secreted in different
concentrations at different sites, and its target appears to be the mesoderm surrounding the gut
tube. (The splanchnic mesoderm cells around the gut contain Patched protein, the receptor for the
Hedgehog protein family, in their cell membranes.) In the hindgut, the secretion of Shh by the
endoderm induces in the mesoderm a nested set of "posterior" Hox gene expression. As in the
vertebrae (see Chapter 11), the anterior borders of the expression pattern delineate the
morphological boundaries of the regions that will form the cloaca, large intestine, cecum, midcecum
(at the midgut/hindgut border), and the posterior portion of the midgut (Figure 15.28;
Roberts et al. 1995; Yokouchi et al. 1995). When Hox-containing viruses cause the misexpression
of these Hox genes in the mesoderm, the mesodermal cells alter the differentiation of the adjacent
endoderm (Roberts et al. 1998). Thus, the endodermal expression of Shh in the hindgut seems to
induce a nested expression of Hox genes in the mesoderm. These Hox genes probably specify the
mesoderm so that it can interact with the endodermal tube and specify its regions.