Neural Inducers Are Really Antineural Inhibitors

In Chapter 3, we discussed how the sog gene, which is expressed in the neural ectoderm of the early fly embryo, functions to block the neural suppressive activity of the dpp gene. Cells in the nonneural region of the embryo secrete the Dpp signal onto each other, thus inhibiting one another from following the default developmental path of ectodermal cells which is to become neural. In the neural region of the embryo, however, this mutual neural inhibitory signaling is blocked by Sog, which allows these cells to develop as neurons.

One of the first indications that the standard interpretation of the classic frog experiments might be backward was that vertebrate counterparts of the fly dpp and sog genes were found. Intriguingly, it was observed that these genes were expressed (i.e., transcribed as mRNA) in the same cell types as flies. Thus, BMP4, the frog version of Dpp, was expressed in the nonneural region of the frog, and the counterpart of Sog called Chordin was produced by cells in the neuralizing Spemann organizer (Fig. 5.5; see also Plate 2B, C for actual expression of bmp4 and chordin in frog embryos). The fact that these vertebrate genes were expressed in the same tissue types as their fly counterparts suggested that the mechanisms by which these genes acted might also be the same in flies and frogs, in which case the default state of the ectoderm should be neural, not epidermal. As an interesting aside, the neural ectoderm forms dorsally in vertebrates (i.e., the spinal cord runs along the dorsal

When cells are placed in a test tube and spun in a centrifuge the artificial gravity created by the circular motion pushes the cells to the bottom of the tube where they pile up on top of one another.

Dissociate ectoderm into single cells

Dissociate ectoderm + BMP4

Dissociate ectoderm + BMP4 +Chordin

Dissociate ectoderm into single cells

Dissociate ectoderm + BMP4

Dissociate ectoderm + BMP4 +Chordin oo

Reaggregate cells by centrifugation

Allow reaggregated cells to develop into skin versus neural tissue

Allow reaggregated cells to develop into skin versus neural tissue

Neural Skin Neural

Conclusion: Default ectodermal cell fate is neural

FIGURE 5.4. The ectoderm dissociation experiment.

Neural Skin Neural

Conclusion: Default ectodermal cell fate is neural

FIGURE 5.4. The ectoderm dissociation experiment.

midline), whereas in flies and other invertebrates, the nervous system forms ventrally. It is likely that during the course of vertebrate evolution the position of the neuroectoderm was flipped with respect to invertebrates, an idea I refer to as the "invertedbrate" hypothesis (Fig. 5.5). The position of the neural ectoderm notwithstanding, the main point to bear in mind is that cells in both vertebrates and invertebrates that will give rise to epidermis express BMP4 or its cousin Dpp, respectively, whereas Chordin and Sog are present in the neural ectoderm.

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