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

1156 Chapter 21: Development of Multicellular Organisms
pigmented
cytoplasm
of animal
pole
ANIMAL POLE
sperm
entry point
VENTRAL
DORSAL
plasma
membrane
cortex
yolky core
Wnt11 mRNA
Wnt11 mRNA
VegT mRNA
VEGETAL POLE
VegT mRNA
(A)
0.5 mm
(B)
Figure 21–14 The frog egg and its asymmetries. (A) Side view of a Xenopus egg photographed just before fertilization.
(B) The asymmetric distribution of molecules inside the egg, and how this changes following fertilization so as to define a
dorsoventral as well as an animal-vegetal asymmetry. Fertilization, through a reorganization of the microtubule cytoskeleton,
triggers a rotation of the egg cortex (a layer a few μm deep) through about 30° relative to the core of the egg; the direction of
rotation determined by the site of sperm entry. Some components are carried still further to the future dorsal side by active
transport along microtubules. The resulting dorsal concentration of Wnt11 mRNA leads to dorsal production of the Wnt11 signal
protein and defines the dorsoventral polarity of the future embryo. Vegetally localized VegT defines the vegetal source of signals
that will induce endoderm and mesoderm. (A, courtesy of Tony Mills.)
At one extreme, the egg is spherically symmetrical, MBoC6 and m22.68/22.14 the axes only become
defined during embryogenesis. The mouse comes close to being an example, with
little obvious sign of polarity in the egg. Correspondingly, the blastomeres produced
by the first few cell divisions seem to be all alike and are remarkably adaptable.
If the early mouse embryo is split in two, a pair of identical twins can be produced—two
complete, normal individuals from a single cell. Similarly, if one of
the cells in a two-cell mouse embryo is destroyed by pricking it with a needle and
the resulting “half-embryo” is placed in the uterus of a foster mother to develop,
in many cases a perfectly normal mouse will emerge.
At the opposite extreme, the structure of the egg defines the future axes of the
body. This is the case for most species, including insects such as Drosophila, as we
shall see shortly. Many other organisms lie between the two extremes. The egg of
the frog Xenopus, for example, has a clearly defined A-V axis even before fertilization:
the nucleus near the top defines the animal pole, while the mass of yolk (the
embryo’s food supply, destined to be incorporated in the gut) toward the bottom
defines the vegetal pole. Several types of mRNA molecules are already localized
in the vegetal cytoplasm of the egg, where they produce their protein products.
After fertilization, these mRNAs and proteins act in and on the cells in the lower
and middle part of the embryo, giving the cells there specialized characters, both
by direct effects and by stimulating the production of secreted signal proteins. For
example, mRNA encoding the transcription regulator VegT is deposited at the
vegetal pole during oogenesis. After fertilization, this mRNA is translated, and the
resulting VegT protein activates a set of genes that code for signal proteins that
induce mesoderm and endoderm, as discussed later.
The D-V axis of the Xenopus embryo, by contrast, is defined through the act of
fertilization. Following entry of the sperm, the outer cortex of the egg cytoplasm
rotates relative to the central core of the egg, so that the animal pole of the cortex
becomes slightly shifted to one side (Figure 21–14). Treatments that block
the rotation allow cleavage to occur normally but produce an embryo with a central
gut and no dorsal structures or D-V asymmetry. Thus, this cortical rotation is
required to define the D-V axis of the future body by creating the D-V axis of the
egg.
The site of sperm entry that biases the direction of the cortical rotation in
Xenopus, perhaps through the centrosome that the sperm brings into the egg—
inasmuch as the rotation is associated with a reorganization of the microtubules
nucleated from the centrosome in the egg cytoplasm. The reorganization leads to
a microtubule-based transport of several cytoplasmic components, including the
mRNA coding for Wnt11, a member of the Wnt family of signal proteins, moving