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Figure 3.25 Possible routes of signal transfer from the GRP to the left LPM. Signal(s)
generated by leftward flow at the GRP could travel through the endoderm (yellow), mesoderm
(red), notochord and ectoderm (blue), or archenteron (black), or between endoderm and
mesoderm (white) to reach the left LPM (green, outlined by dashed line). Abbreviations : bp,
blastoporus; n, notochord; s, somite.
Source : From Blum et al. (2009) .
distribution of the morphogen and expression of a “leftward cascade” ( Blum et al.,
2009 ; see Figure 3.25 ).
The nodal flow model of the left-right asymmetry is conserved across vertebrates
at both the molecular and organ levels ( Oteíza et al., 2008 ), and it was a feature in
the common ancestor of the vertebrates ( Schweickert et al., 2006 ).
According to another model, laterality is determined by maternal ATP 4 mRNA,
which enables the passage of the maternal and/or embryonic serotonin to particu-
lar blastomeres before gastrulation. However, their asymmetric distribution in the
zygote and embryo remains controversial ( Walentek et al., 2012 ). The Xenopus
fertilized egg contains considerable amounts of maternal serotonin, which declines
during early development, only to rise again as a result of neurally derived seroto-
nin secretion after the formation of the neural tube. In Xenopus , downregulation of
serotonin expression ( Beyer et al., 2012 ) prevents the flow and asymmetry, and the
blockage of serotonin receptors (the mediators of serotonin function) alters the nor-
mal left-sided expression of Xnr 1 ( Xenopus nodal-related 1) and the downstream
X-lefty ( Fukumoto et al., 2007 ). ATP4a and serotonin converge to activate the Wnt
cascade, the former by inducing expression of the Foxjl and the latter by inducing
formation of superficial mesoderm and ciliated GRP ( Beyer et al., 2012; Shook
et al., 2004 ). Both models of the establishment of laterality rely on epigenetic fac-
tors: on the asymmetric distribution of maternal serotonin and ATP4a and on the
directed leftward movement of cilia of the GRP cells.
We close this brief review with an interesting example of the neural regulation
of the laterality of organs in the big-clawed snapping shrimp ( Alpheus heterochelis ).
This shrimp has a pair of chelae (claws) that initially develop symmetrically, with
similar size and morphology, but by the sixth juvenile stage, they start differentiating
into a large snapper claw for defense and display on one side, and a much smaller
claw for feeding and burrowing on the other. Denervation of limbs leads to the
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