Figure 21–27 How morphogen gradients guide a patterning process along the dorsoventral axis of the Drosophila embryo. (a) Initially, a gradient of Dorsal protein defines three broad territories of gene expression, marked here by the expression of three representative genes—Dpp, Sog, and Twist. (B) Slightly later, the cells expressing Dpp and Sog secrete, respectively, the signal proteins Dpp (a tGFβ family member) and Sog (an antagonist of Dpp). these two proteins then diffuse and interact with one another (and with certain other factors) to create the dorsoventral (D–V) territories shown.
conserved. The same is true of later developmental stages also, often to an astonishing degree. From our own anatomy, it is obvious that we are cousins to birds and fish. But looking at molecular mechanisms, we see that we are cousins to flies and worms too.
In the following pages, we discuss how vertebrate embryos are patterned by the interplay of signaling molecules and transcription regulators. We begin by discussing the formation and patterning of the embryonic axes in amphibians, taking the frog Xenopus as our example. We have already broached this topic earlier in the chapter. Here, we pick up the thread and draw comparisons with the fly.
As noted earlier, the origins of the embryonic axes and the three germ layers in the frog can be traced back to the blastula (see Figure 21–3A). By labeling individual blastomeres, we can track cells through all their divisions, transformations, and migrations and see what they become and where they come from. The precursors of ectoderm, mesoderm, and endoderm are arranged in order along the animal-vegetal axis of the blastula: the endoderm derives from the most vegetal blastomeres, the ectoderm from the most animal, and the mesoderm from a middle set. Within each of these territories, the cells have diverse fates according to their positions along the D-V axis of the later embryo. For ectoderm, epidermal precursors are located ventrally, and future neurons are found dorsally; for mesoderm, precursors for notochord, muscle, kidney, and blood are arranged from dorsal to ventral. All this can be represented by a fate map that shows which cell types derive from which regions of the blastula (Figure 21–28). The fate map confronts us with the central question: how are the cells in different positions driven toward their different fates? We have already explained how maternal factors deposited in the developing frog egg define its animal-vegetal axis, and how cortical rotation triggered by fertilization defines the orientation of the dorsoventral axis (see Figure 21–14). But how does the establishment of axes lead on to the subdivision of the embryo into the future body parts?
The maternal gene products lead to the formation of signaling centers on the vegetal and dorsal sides of the embryo. The dorsal signaling center in particular has a special place in the history of developmental biology. Experiments in the early twentieth century identified it as a small cluster of cells, located on the dorsal side of the amphibian embryo, with an extraordinary property: when the cells were transplanted to an opposite site, they could trigger a radical reorganization of the neighboring tissue, causing it to form a second whole-body axis (Figure 21–29). The discovery of this signaling center, called the Organizer, led the way to a pioneering analysis of the chain of inductive interactions that establish the framework of the vertebrate body.
In contrast to the Drosophila syncytial embryo, the fertilized frog egg undergoes rapid cleavage divisions that result in an embryo consisting of thousands of cells. Patterning must therefore be mediated by extracellular signal molecules