The Questions of Developmental Biology

In 1822, the French anatomist Etienne Geoffroy Saint-Hilaire provoked one of the most
heated and critical confrontations in biology when he proposed that the lobster was but the
vertebrate upside down. He claimed that the ventral side of the lobster (with its nerve cord) was
homologous to the dorsal side of the vertebrate (Appel 1987). It seems that he was correct on the
molecular level, if not on the anatomical. De Robertis and Sasai (1996) have proposed that there
was a common ancestor for all bilateral phyla a hypothetical creature (dubbed Urbilateria) of
some 600 million years ago that was the ancestor for both the protostome and the deuterostome
subkingdoms. The BMP4-(Dpp)/chordin(Sog) interaction is an example of "homologous
processes," suggesting a unity of developmental principles among all animals (Gilbert et al.
1996).
Competence, Bias, and Neurulation
In addition to the signals coming from the
underlying chordal plate and dorsal
mesoderm, there may also be a bias
in the cells of the dorsal part of the
embryo toward becoming neural.
Phillips and colleagues
(London et al. 1988; Savage
and Phillips 1989) have shown
that the dorsal and ventral
animal pole cells of the earlycleavage
embryo differ in their expression of the Epi1 protein.
Not only do the presumptive epidermal cells express this
protein, which is not expressed in the presumptive neural cells,
but the region of cells failing to express Epi1 increases during
gastrulation.
Moreover, in the ventral mesoderm, proteins encoded by Xvex-1 and Xvex-2 block the
expression of dorsal genes (Shapira et al. 2000). Other differences between dorsal and ventral
ectoderm also become apparent at this time, prior to the notochord's movement beneath the
ectoderm (Otte and Moon 1992). The cues for this "bias" toward neurulation may be provided by
signals from the dorsal lip traveling in a planar (horizontal) fashion through the ectoderm (Figure
10.38; Sharpe et al. 1987; Dixon and Kintner 1989; Doniach et al. 1993). The molecular basis of
this planar "ectodermal" signaling system is not known. It is possible that it is more important in
some species than in others.