Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 139

Eotoderm BMPs Epidermal

Organizer
Noggin, chordin, follistatin,
cerberus, Xnr3

Neural

Figure 3.16 BMP signaling and the specification of ectodermal fate (default model) in
Xenopus . Proposed model for the role of BMP signaling in the specification of the ectodermal
fate in frog embryos.
Source : From Muñoz-Sanjuán and Brivanlou (2002).

( Kimelman, 2006 ). Lack or elimination of VegT prevents formation of the mesoderm
( Kimelman and Bjornson, 2004; Kofron et al., 1999 ). Maternal VegT is responsible
for 90% of the mesodermal tissues ( Kofron et al., 1999 ).

Formation of the Neural Tube

Neural Induction
Neural induction follows gastrulation and leads to the formation of the neural tube
and the CNS. In terms of the commitment to neural fate, it begins before the gas-
trula stage and ends when gastrulation finishes ( Stern, 2004, 2005 ), and according
to Müller (1996) , the cascade of signals leading to formation of the CNS starts in the
uncleaved egg.
During neural induction, many maternal factors are still active in the embryo,
along the zygotic expression of genes. Given the maternal activation of the zygotic
gene expression and that neural induction and specification starts earlier during the
blastula stage ( Streit et al., 2000 ; Wilson and Edlund, 2001 ), neural induction is an
epigenetic process:

The development of the nervous system is a largely epigenetic phenomenon in
which events induce subsequent events in what is usually a highly orderly sequence.
Tierney, 1996

According to the default model for the neural induction, ectodermal cells in the
absence of BMPs and the presence of the BMP antagonists, noggin, chordin, and
follistatin, released by Spemann's organizer, most of the dorsal region of the embryo
adopts a neural fate ( Muñoz-Sanjuán and Brivanlou, 2002 ; Weinstein and Hemmati-
Brivanlou, 1999 ) ( Figure 3.16 ), and as much as 50% of the ectoderm is included in
the neural plate ( Gilbert, 2000 , http://www.ncbi.nlm.nih.gov/books/NBK10080/ ) .
Inhibition of BMPs is sufficient for neural induction, and in the absence of BMPs,
the CNS forms even without Spemann's organizer ( Reversade et al., 2005 ).
It is now understood that BMP inhibition is not sufficient for the neural induction
of ectoderm. In Xenopus , neural induction requires both weak FGF4 or embryonic