Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 150

Figure 3.24 A model for the steroid regulation
of salivary gland programmed cell death.
b FTZ-F 1 induces the diap 2 death inhibitor in the
salivary glands of late prepupae. The prepupal
pulse of ecdysone then represses b FTZ-F 1
and induces E 75 A and E 75 B , which function
redundantly to repress diap 2. The prepupal
ecdysone pulse also directly induces rpr
transcription, as well as the transcription factors
encoded by BR-C and E 74 A . BR-C is required
for both rpr and hid transcription, and E 74 A is
required for maximal levels of hid expression.
Source : From Jiang et al. (2000) .

PTTH

Ecdysone

EcR/USP

E75A
E75B

β FTZ-F1 BR-C E74A

rpr

diap2 hid

Larval salivary gland
cell death

Left-Right Asymmetry

Around the beginning of the twentieth century, Spemann and his coworkers observed
that partial or complete ligature of embryos of two newt species caused the nor-
mal asymmetric placement of inner organs to invert ( situs inversus ) in 50% of the
embryos. Shortly thereafter, in 1921, Hilde Wilhelmi, based on her experimen-
tal results with ligature experiments, noticed that laterality defects were correlated
with ablations on the left side of the embryo at mid-neurula stage and concluded that
“situs inversion in general was explained by the fact that the left side of the germ
has something that the right half does not have” ( Blum et al., 2009 ). Seven decades
later, Levin et al. (1995) discovered what “the left side of the germ has.” They found
that a molecular cascade responsible for the establishment of the L-R asymmetry in
respect to AP and DV axes, was activated on the left side of chick embryos. In their
model, the left-right specification in chick embryos results from an interaction of the
products of genes for activin, Shh, and NR1 (nodal-related 1).
Considerable progress has been made in recent years in understanding the ulti-
mate source of the signals that trigger the cascade of gene activation to establish the
laterality of various organs in the vertebrate body. It was proposed, in 2001, that cilia
“on the node, or organizer, of the gastrulation-stage mouse embryo” cause a left-
ward movement of the fluid and induce the left-right asymmetry ( Brueckner, 2001 ;
McGrath and Brueckner (2003) ) during neurulation pIn Xenopus as well, the source
of the forces moving the fluid leftward is generated by the cilia of the gastrocoel
roof plate (GRP) or its equivalents, the Kupfer vesicle in fish and the ventral node
in mices. The flow transports a morphogen to the left side of the ciliated epithelium
and, from there, to the left lateral plate mesoderm (LPM), creating an asymmetric