Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 135

xNorrin

β -catenin
BCNE center

xNorrin activity

xNorrin

BMP4

Nodal-related

Nieuwkoop center

Wnt11 Wnt11 activity

Egg Midblastula Blastula

Figure 3.13 A model of dorsal specification in Xenopus . During oogenesis, maternal
xNorrin and Wnt 11 are localized to the animal and vegetal poles, respectively. After
fertilization, both mRNAs are enriched on the dorsal side, leading to two localized activity
domains: the BCNE center and the Nieuwkoop center. xNorrin in the dorsal animal cells helps
to specify neuroectoderm fate by activating a Wnt/β-catenin signaling domain, the BCNE
center, and also participates in antagonizing the Nodal-related signal from the vegetal half
and the bone morphogenetic protein (BMP) signal from the ventral side. Wnt11 in the dorsal
vegetal domain is required for β-catenin activation in all dorsal regions, including in the
BCNE center. Yellow: xNorrin and BCNE center. Purple: Wnt11. Red dots indicate stabilized
β-catenin. Green: Nieuwkoop center.

Source : From Xu et al. (2012) .

for early neuroectoderm specification and for expression of the zygotic β-catenin
( Xu et al., 2012 ).

Gastrulation—Formation of Embryonic Layers

The generalized spherical blastula soon develops into a new embryonic structure
known as the gastrula . The term was coined by Ernst Haeckel (1834-1919) due to
the gastrula's resemblance to a pouch (from ancient Greek gastros , stomach). The
zygotic genome is now functional, although, depending on species, various propor-
tions of maternal factors are still present and active. Many cells begin to differentiate
and move around in specific patterns. Their differentiation is correlated to changes
in the patterns of gene and mRNA expression. Despite the activation of the zygotic
genome, many maternal factors are still necessary for gastrulation to proceed.
Among these maternal factors are the maternally provided serotonin ( Colas et al.,
1999 ) and maternal Xsmad 1 RNA , whose translation is necessary for early gastrula-
tion, as suggested by the failure of Xenopus eggs depleted of maternal X smad 1 RNA
to gastrulate ( Miyanaga et al., 2002 ). The maternal interferon regulatory Factor 6
mRNA ( irf 6) recently was added to the number of maternal factors necessary for
gastrulation in Xenopus and the zebrafish Danio rerio . Depletion of this mater-
nal factor in Xenopus and the expression of a dominant negative variant of Irf6 in