In the animal pole region, where Nodal levels are low relative to Lefty, Lefty blocks Nodal from binding to its receptors. In the vegetal region, there is an excess of Nodal, resulting in Nodal pathway activation.
along the dorsoventral axis, BMp is widely present but chordin and Noggin are concentrated at the dorsal side: there, they bind to BMp and block its binding to receptors. the resulting patterns of Nodal and BMp activity are illustrated at the bottom of the figure.
will differentiate into blood (a ventral mesodermal tissue) when diverted from their original fate by exposure to intermediate concentrations of Nodal and high concentrations of BMP. Similarly, mouse or human embryonic stem cells can be coaxed to generate specific cell types by exposing them in culture to appropriate combinations of signal molecules. In this way, the insights gained through studies of animal development can be used to generate the cell types needed for regenerative medicine, as we discuss in the next chapter.
the Insect Dorsoventral axis corresponds to the Vertebrate Ventral-Dorsal axis
The signaling systems that pattern the D-V axis in Drosophila and in vertebrates are similar. In Drosophila, as we saw, Dpp and its inhibitor Sog are responsible, whereas in vertebrates, BMP and its inhibitors Chordin and Noggin do the job. Dpp is a member of the BMP family, while Sog is a homolog of Chordin. Both in flies and frogs, high levels of the inhibitors define the region that is neurogenic, and high levels of BMP/Dpp activity define the region that is not. These and other molecular parallels strongly suggest that this aspect of body patterning has been conserved in evolution from insects to vertebrates. Curiously, however, the axis is inverted: dorsal in the fly corresponds to ventral in the vertebrate (Figure 21–31). At some point in evolution, it seems that the ancestor of one of these classes of animals took to living life upside-down.
Hox Genes control the Vertebrate a-p axis
The conservation of developmental mechanisms between Drosophila and vertebrates extends beyond the D-V signaling system. Hox genes are found in almost every animal species studied, where they are often grouped in complexes similar to the insect Hox complex. In mice and humans, for example, there are four such complexes—called the HoxA, HoxB, HoxC, and HoxD complexes—each on a different chromosome. Individual genes in each complex can be recognized by their sequences as counterparts of specific members of the Drosophila set. Indeed, mammalian Hox genes can function in Drosophila as partial replacements for the corresponding Drosophila Hox genes. It appears that each of the four mammalian Hox complexes is, roughly speaking, the equivalent of one complete insect Hox complex (that is, an Antennapedia complex plus a Bithorax complex) (Figure 21–32).
The ordering of the genes within each vertebrate Hox complex is essentially the same as in the insect Hox complex, suggesting that all four vertebrate complexes originated by duplications of a single primordial complex and have preserved its basic organization. Most tellingly, when the expression patterns of the Hox genes are examined in the vertebrate embryo, it turns out that the members of each complex are expressed in a head-to-tail series along the axis of the body, just as they are in Drosophila. As in Drosophila, vertebrate Hox gene expression patterns are often aligned with vertebrate segments. This alignment is especially clear in the hindbrain (see Figure 21–32), where the segments are called rhombomeres.
The products of the vertebrate Hox genes, the Hox proteins, specify positional values that control the A-P pattern of parts in the hindbrain, neck, and trunk (as well as some other parts of the body). As in Drosophila, when a posterior Hox gene is artificially expressed in an anterior region, it can convert the anterior tissue to