Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 145

Salamander Chick Human

Egg
(to scale)

Blastula Human

blastocyst
(incorrectly
aligned)

Gastrula

Phylotypic
stage

Figure 3.20 Phylotypic stages of chordates and arthropods. The morphology of chordate
embryos, represented by salamander, chick, and human embryos, initially converges to
look more similar at the so-called phylotypic stage (or pharyngula stage) and subsequently
diverges. The high degree of similarity between organisms allows an unambiguous alignment
of stages near the phylotypic stage, but ambiguities in alignment can occur at other stages. We
argue that the illustrated and generally accepted alignment of the human blastocyst with the
blastula stages of amphibian and chick is incorrect.
Source : From O'Farrell et al. (2004) .

NCCs and neurons derive from the same precursor cells. NCCs differentiate in
the neural plate; that is, in the dorsal portion of the neural tube ( Figure 3.23 ). Prior
to leaving the neuroepithelium, they undergo the transition from epithelial to mesen-
chymal ( Barembaum and Bronner-Fraser, 2005 ), a transformation that is also typical
for many neuronal cells ( Baker and Bronner-Fraser, 1997 ). In the neural plate, NCCs
are hardly distinguishable from neuroepithelial cells, and when a part of the neural
tube is removed, the adjacent neural tube compensates for the NCCs of the ablated
portion Schwerson et al. (1993) .
The neural induction is determined by maternal cytoplasmic factors. Then, signals
from the neural plate stimulate the expression of the “neural crest specifiers” ( Slug/
Snail, AP- 2, FoxD 3, Sox 10, Sox 9, and Msl/ 2). These genes enable expression of the