Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 141

FoxD5

Geminin Zic2

Sox11

Zic1
Zic3
SoxD

Sox2
Sox3

Xiro1
Xiro2
Xiro3

bHLH neural differentiation genes

Figure 3.18 A gene regulatory network results in the stabilization of neural fate. After
these 12 NFS genes are induced, FoxD5 upregulates Geminin, Sox 11, and Zic 2 expression.
These three genes regulate each other, and together (bracket), they differentially affect the
expression of the other eight NFS genes. Geminin and Sox 2 /Sox 3 directly regulate each other,
and increased Zic 1 upregulates Sox 2 and SoxD . FoxD 5, Geminin, Sox 11, and Zic 2 maintain
neural ectodermal cells in an immature, stemlike state, whereas Zic 1, Zic 3 , SoxD , and Xiro 1-3
promote the onset of neural differentiation.
Source : From Rogers et al. (2009) .

Both the notochord and the prechordal plate, at the anterior end of the notochord,
contribute to the induction of the neural plate. Their formation is also epigenetically
determined and regulated by maternal factors. In ascidians, for example, primary
notochord cells derive from four blastomeres in the anterior marginal zone of the
vegetal hemisphere of the 64-cell embryo ( Nishida, 2002 ). It is noteworthy that a
maternal factor mys (misty somite) continues to be active during the formation of
the neural tube as a determinant of the boundaries of somites, transient embryonic
structures from which axial skeleton and associating muscles develop in vertebrates
( Kotani and Kawakami, 2008 ).
In humans, the neural plate starts in the form of a flat sheet of about 100,000
cells, and it is believed that all human neurons and glial cells originate from the neu-
ral plate ( Dowling, 2007 ). Once the neural plate forms, its edges elevate dorsally
until they close at the midline to form the roof plate and the neural tube.
This mechanism of formation of the neural tube is known as primary neurulation . The
secondary neurulation is another mechanism of the formation of the neural tube. In this
case, the neural plate forms a medullary cord, which hollows out to form the neural tube.