that this subtle depression of neural development is the organizer’s function in all chordates. Nonetheless, it should be noted that Holtfreter (1947), building on a discovery of Barth (1939), suggested that neural inducers provide little information except to release the inherent capacity of ectoderm cells to develop as neural tissue. This suggestion came from Barth and Holtfreter’s findings that ectoderm would develop neural tissue if merely shocked briefly by ion imbalances or pH extremes.

Even though the organizer mode of development is distinctive to chordates, the components of the process are common to a wide range of other animals. For example, one antagonist, the Chordin protein, exhibits significant homology with the SOG protein of Drosophila. The SOG protein antagonizes a TGFβ inductive signal (called Screw) in Drosophila as part of the development of regions of neural versus epidermal development (Neul and Ferguson 1998). Furthermore, in both Drosophila and frogs, there is a specific metalloproteinase that degrades the signal-antagonist complex, releasing the signal. The chordate and Drosophila inductive processes have deep similarities, though differing in details of time, place, and circumstances of use.

As a final example of differences, the dorsoventral dimension of arthropods looks quite different from that of a mouse, but recent analysis has shown that a number of similar genes are expressed in the nerve cords, hearts, body muscle, visceral mesoderm, and gut of both. It is currently accepted that these organs were present in primordial form in a common ancestor, but the arrangement of the organs in chordates is the inverse of that in arthropods. That is, the nerve cord is dorsal in chordates and ventral in arthropods, and the heart is ventral in chordates and dorsal in arthropods. The inversion of the dorsoventral axis is thought to have occurred in the chordate line after hemichordates split off (Nübler-Jung and Arendt 1996).

Recognizing the fact that Drosophila does not share all details of early development and organogenesis with vertebrates, researchers have begun a systematic collection of developmental mutants of the zebrafish, a small vertebrate with a short life cycle (see Chapter 7), suitable for the production of a large mutant collection. The organs of embryonic zebrafish, more than the organs of Drosophila, resemble those of mammalian embryos in structure and function. In light of the extensive conservation of developmental processes found thus far, it is expected that in most cases what is true for fish development, as learned from those mutants, will be true for mammalian development, down to the level of molecular details of components and processes. That is not meant to deny differences among organisms (e.g., mammals undergo placental development with extensive extra-embryonic tissues not found in a zebrafish), nor to dismiss the possibility that developmental biologists might be misled in some instances by the study of model organisms. The greater part of mammalian development can be understood, however, by the study of other organisms’ development. Ultimately, mammalian development will have to be understood in all the details of its differ-

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