Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 140

Maternal Zygotic

chd
nog

CHD
NOG

Epidermis

β -catenin bmp

BMP/Smad1

Neural

fgf

FGF

Mesoderm

Veg-T

Blastula

Xnrs XNRs

Figure 3.17 A model for neural induction in Xenopus . Colored domains indicate prospective
tissues. Orange and purple dots indicate β-catenin and VegT protein distribution in blastula
nuclei, respectively. (Arrows are not intended to represent direct regulation. The goal of this
figure is to represent how the major molecular pathways known to affect early embryonic
patterning are integrated. Regulatory arrows shown in black are inferred in part or totally
from our work, while those shown in gray originate from other studies.) Neural specification
requires concomitant BMP inhibition and low FGF signaling acting in a BMP-independent
manner.

Source : From Delaune et al. (2005 ).

fibroblast growth factor (eFGF) and BMP inhibition. eFGF serves as both a neural
inducer and as a BMP inhibitor and seems to be a conserved initiator of neural speci-
fication among chordates ( Delaune et al., 2005 ). This finding led to the development
of the combinatorial model of neural induction ( Figure 3.17 ).
The fact that transplants from Spemann's organizer and Hensen's node can induce
the formation of neural tubes in other species' embryos indicates that the neural
induction is highly conserved in vertebrates.

The Primary Neurulation

After the maternal-zygotic transition and at the beginning of gastrulation, 12 neural
fate stabilizing (NFS) transcription factors, products of a number of maternal and
zygotic mRNAs and neurogenic mRNAs expressed in response to anti-BMP signals
released by the BCNE center and the organizer, are detected in the dorsal ectoderm.
These maternal and zygotic NFSs interact in a gene regulatory network ( Figure
3.18 ), initiate the formation and patterning of the neural plate and the neural differ-
entiation. Among these genes, foxD 5 plays a crucial role as an upstream signal upreg-
ulating the function of the neuroectodermal genes gem , sox 11, geminin , and other
genes. In turn, foxD 5 expression is induced by FGF signals ( Rogers et al., 2011 ).
Several of the genes upregulated by foxD 5 , in turn, feed back to downregulate its
expression, thus contributing to the medial-lateral patterning of the neural plate for
maintaining a progenitor neural cell population and preventing the neural differentia-
tion of the neuroctoderm ( Rogers et al., 2009; Yan et al., 2009 ).