Embryonic Patterning of Xenopus Mesoderm by Bmp-4
169
(Ferguson and Anderson 1992; Wharton et al. 1993). The discovery of
the homology between the antagonizing systems chordinlsog and Bmp4/dpp in Xenopus and Drosophila strongly argues in favor of the thesis
of Geoffroy St. Hilaire, that vertebrates and arthropodes have homologous body plans, and that during evolution an inversion of the d/v axes
must have taken place (Arendt and Niibler-Jung 1994; De Robertis and
Sasai 1996).
Similar to dpp regulation of d/v pattern formation in both Drosophila
ectoderm (Ferguson and Anderson 1992; Wharton et al. 1993) and
mesoderm (Wilson et al. 1993; Staehling-Hampton et al. 1995), it seems
that in the frog the Bmp-4khordin system regulates d/v pattern formation of all three germ layers. In ectoderm, chordin induces and Bmp-4
inhibits neural tissue formation and instead induces epidermis (Sasai et
al. 1995; Wilson and Hemmati-Brivanlou 1995); in endoderm Bmp-4
represses and chordin induces dorsal endoderm (Sasai et al. 1996).
Indeed, all known BMP antagonists are able to neuralize naive ectoderm, suggesting that the ectodermal ground state is neural (HemmatiBrivanlou and Melton 1997).
10.3 Bmp-4 Acts as a Morpbogen
The conservation of d/v patterning in arthropods and vertebrates has
raised the possibility that like dpp in Drosophila (Ferguson and Anderson 1992; Wharton et al. 1993), Bmp-4 acts as a morphogen in vertebrates (Ferguson 1996; Hogan 1996; Holley et al. 1996; Piccolo et al.
1996; Zimmerman et al. 1996). Support for the possibility that a TGF-~
type growth factor acts as a morphogen in vertebrate mesoderm patterning comes from the observation that increasing concentrations of activin
and VgJ induce a ventrodorsal sequence of mesoderm from uninduced
animal cap cells (Green and Smith 1990; Green et al. 1992; Gurdon et
al. 1994; Kessler and Melton 1994). However, while this strongly supports the notion of a morphogen functioning in d/v patterning, there is
neither evidence for a graded requirement nor evidence for a graded
distribution of an activin-like molecule at present. Furthermore, there is
recent evidence that both activin and Vg J pattern mesoderm indirectly
through a relay mechanism (Reilly and Melton 1996). This has left the
question of the natural morphogen unresolved.
169
(Ferguson and Anderson 1992; Wharton et al. 1993). The discovery of
the homology between the antagonizing systems chordinlsog and Bmp4/dpp in Xenopus and Drosophila strongly argues in favor of the thesis
of Geoffroy St. Hilaire, that vertebrates and arthropodes have homologous body plans, and that during evolution an inversion of the d/v axes
must have taken place (Arendt and Niibler-Jung 1994; De Robertis and
Sasai 1996).
Similar to dpp regulation of d/v pattern formation in both Drosophila
ectoderm (Ferguson and Anderson 1992; Wharton et al. 1993) and
mesoderm (Wilson et al. 1993; Staehling-Hampton et al. 1995), it seems
that in the frog the Bmp-4khordin system regulates d/v pattern formation of all three germ layers. In ectoderm, chordin induces and Bmp-4
inhibits neural tissue formation and instead induces epidermis (Sasai et
al. 1995; Wilson and Hemmati-Brivanlou 1995); in endoderm Bmp-4
represses and chordin induces dorsal endoderm (Sasai et al. 1996).
Indeed, all known BMP antagonists are able to neuralize naive ectoderm, suggesting that the ectodermal ground state is neural (HemmatiBrivanlou and Melton 1997).
10.3 Bmp-4 Acts as a Morpbogen
The conservation of d/v patterning in arthropods and vertebrates has
raised the possibility that like dpp in Drosophila (Ferguson and Anderson 1992; Wharton et al. 1993), Bmp-4 acts as a morphogen in vertebrates (Ferguson 1996; Hogan 1996; Holley et al. 1996; Piccolo et al.
1996; Zimmerman et al. 1996). Support for the possibility that a TGF-~
type growth factor acts as a morphogen in vertebrate mesoderm patterning comes from the observation that increasing concentrations of activin
and VgJ induce a ventrodorsal sequence of mesoderm from uninduced
animal cap cells (Green and Smith 1990; Green et al. 1992; Gurdon et
al. 1994; Kessler and Melton 1994). However, while this strongly supports the notion of a morphogen functioning in d/v patterning, there is
neither evidence for a graded requirement nor evidence for a graded
distribution of an activin-like molecule at present. Furthermore, there is
recent evidence that both activin and Vg J pattern mesoderm indirectly
through a relay mechanism (Reilly and Melton 1996). This has left the
question of the natural morphogen unresolved.