Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 132

(A) (B)

Frizzled

Wnt

LRP5/6

LRP5/6

Frizzled

Dsh Dsh

GSK3 β GSK3 β

Axin APC Axin APC

β -catenin β -catenin

Degradation

TCF TCF

Figure 3.12 The canonical Wnt signaling pathway. (A) In the absence of Wnt, little
β-catenin is present in the cytoplasm because it is degraded as a result of phosphorylation
by the Axin/APC/GSK3beta complex; (B) Interaction of Wnt with Frizzled and LRP5/6
activates Dsh, which in turn inactivates the Axin/APC/GSK3beta complex, allowing β-catenin
to accumulate and enter the nucleus. Upon entering the nucleus, β-catenin interacts with the
transcription factor of the TCF family to activate gene transcription ( Hsieh, 2004 ).

with members of the T-cell factor/lymphoid enhancer factor (LEF/TCF) transcription
factor family to promote the activation of downstream target genes involved in the
dorsal axis specification. ( Figure 3.12B ). In the absence of Wnt, the trimer induces
degradation of β-catenin ( Hsieh, 2004; Soriano et al., 2001 ) ( Figure 3.12A ).
Another important embryonic pathway is the Wnt-calcium pathway, which seems
to regulate the formation of the early embryonic organizer through antagonism with
the Wnt-β-catenin pathway ( Slusarski and Pelegri, 2007 ).

Epigenetic Control of Early Development

Theodor Boveri, based on his embryological investigations, 110 years ago came to
the now generally accepted conclusion that early development from the zygote to the
blastula stage is controlled by extranuclear ( epigenetic , as it is known today) factors.
I would like to ascribe to the cytoplasm of the sea urchin egg only the initial and
simplest properties responsible for differentiation … it provides the most general
basic form, the framework within which all the specific details are filled in by the
nucleus ( Boveri, 1902 ) .
The term early embryonic development will be used here to describe the entire
period spanning from egg fertilization to the phylotypic stage.
In brooding oviparous animals, the egg genome is dormant and the egg fer-
tilization is triggered by epigenetic factors activated by the entry of the sperm. As
mentioned earlier, the first cleavage division and a varying number of subsequent