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

MECHANISMS OF PATTERN FORMATION
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conserved. The same is true of later developmental stages also, often to an astonishing
degree. From our own anatomy, it is obvious that we are cousins to birds
and fish. But looking at molecular mechanisms, we see that we are cousins to flies
and worms too.
In the following pages, we discuss how vertebrate embryos are patterned by
the interplay of signaling molecules and transcription regulators. We begin by discussing
the formation and patterning of the embryonic axes in amphibians, taking
the frog Xenopus as our example. We have already broached this topic earlier
in the chapter. Here, we pick up the thread and draw comparisons with the fly.
As noted earlier, the origins of the embryonic axes and the three germ layers
in the frog can be traced back to the blastula (see Figure 21–3A). By labeling individual
blastomeres, we can track cells through all their divisions, transformations,
and migrations and see what they become and where they come from. The precursors
of ectoderm, mesoderm, and endoderm are arranged in order along the
animal-vegetal axis of the blastula: the endoderm derives from the most vegetal
blastomeres, the ectoderm from the most animal, and the mesoderm from a
middle set. Within each of these territories, the cells have diverse fates according
to their positions along the D-V axis of the later embryo. For ectoderm, epidermal
precursors are located ventrally, and future neurons are found dorsally; for
mesoderm, precursors for notochord, muscle, kidney, and blood are arranged
from dorsal to ventral. All this can be represented by a fate map that shows which
cell types derive from which regions of the blastula (Figure 21–28). The fate map
confronts us with the central question: how are the cells in different positions
driven toward their different fates? We have already explained how maternal factors
deposited in the developing frog egg define its animal-vegetal axis, and how
cortical rotation triggered by fertilization defines the orientation of the dorsoventral
axis (see Figure 21–14). But how does the establishment of axes lead on to the
subdivision of the embryo into the future body parts?
The maternal gene products lead to the formation of signaling centers on the
vegetal and dorsal sides of the embryo. The dorsal signaling center in particular
has a special place in the history of developmental biology. Experiments in the
early twentieth century identified it as a small cluster of cells, located on the dorsal
side of the amphibian embryo, with an extraordinary property: when the cells
were transplanted to an opposite site, they could trigger a radical reorganization
of the neighboring tissue, causing it to form a second whole-body axis (Figure
21–29). The discovery of this signaling center, called the Organizer, led the way
to a pioneering analysis of the chain of inductive interactions that establish the
framework of the vertebrate body.
In contrast to the Drosophila syncytial embryo, the fertilized frog egg undergoes
rapid cleavage divisions that result in an embryo consisting of thousands of
cells. Patterning must therefore be mediated by extracellular signal molecules
V
E
N
TR
A
L
epidermis
ANIMAL
somites
blood kidney heart
endoderm
VEGETAL
nervous
system
D
ORSAL
notochord
Figure 21–28 Blastula fate map in a
frog embryo. The endoderm derives
from the most vegetal blastomeres (yellow),
the ectoderm MBoC6 from n22.215/22.28 the most animal (blue),
and the mesoderm from a middle set
(green) that contributes also to endoderm
and ectoderm. Different cell types
derive from different positions along the
dorsoventral axis.
graft small group of
cells into host embryo
Figure 21–29 Induction of a secondary
axis by the Organizer. An amphibian
embryo receives a graft of a small cluster
of cells taken from a specific site, called
the Organizer region, on the dorsal side
of another embryo at the same stage.
Signals from the graft organize the behavior
of neighboring cells of the host embryo,
causing development of a pair of conjoined
(Siamese) twins. See Movie 21.4. [After
J. Holtfreter and V. Hamburger, in Analysis
of Development (B.H. Willier, P.A. Weiss
and V. Hamburger, eds), pp. 230–296.
Philadelphia: Saunders, 1955.]