Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1166 Chapter 21: Development of Multicellular Organisms
vitelline membrane
DORSAL
VENTRAL
perivitelline space
Dpp mRNA
Sog mRNA
Twist mRNA
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.
(A)
gradient of nuclear Dorsal protein
zygotic gene transcription regulated
by Dorsal protein
secreted Dpp protein
extraembryonic tissue
dorsal
epidermis
DORSAL
VENTRAL
neurogenic
ectoderm
(B)
gradients of Dpp and Sog proteins
secreted Sog
protein
dorsoventral territories are specified
signal proteins. In particular, Dpp codes for a secreted TGFβ-family protein,
which forms a local morphogen gradient in the dorsal part of the embryo. Sog
encodes another secreted protein MBoC6 that is m22.34/22.27
produced by the neurogenic ectoderm
(which gives rise to the nervous system) and acts as an antagonist of Dpp protein.
The opposing diffusion gradients of these two signal proteins create a steep gradient
of Dpp activity: the highest Dpp activity levels, in combination with certain
other factors, cause development of the most dorsal tissue of all—an extraembryonic
membrane. Intermediate levels cause development of dorsal epidermis; and
the absence of Dpp activity allows the development of neurogenic ectoderm (Figure
21–27B).
A Hierarchy of Inductive Interactions Subdivides the Vertebrate
Embryo
The molecular genetic analysis of Drosophila development has uncovered how
a cascade of transcription regulators and signaling pathways subdivides the
embryo. The same principle of progressive pattern refinement is used during
the development of all animal embryos, including vertebrates. Remarkably, conservation
is not restricted to the general strategy of pattern formation, but also
extends to many of the molecules involved.
As mentioned previously, the earliest phases of vertebrate development are
surprisingly variable, even between closely related species, and it is even hard
to say precisely how the axes of an early fly embryo correspond to those of an
early frog or mouse embryo. Nevertheless, we shall see that amid this display of
evolutionary plasticity, some features of early development turn out to be highly