353(Fig. 7.10a). Furthermore, nodal is expressed, albeit at low levels, in the proximal
epiblast of cripto−/−;cryptic−/− double mutants. This indicates that the signal that
induces nodal expression acts independently of the Nodal co-receptor. This would
appear to rule out Vg1 and Nodal family members as the inducing signal. In the
chicken, cVg1 in the PMZ induces cNR-1 in the epiblast (Fig. 7.10b). In zebrafish,
Nodal signals in the extraembryonic YSL are required to induce ndr1/sqt and ndr2/
cyc in the overlying blastomeres (Fig. 7.9b).
7.8 Interpreting Activin/TGF-β Signals
In 1952, the famous mathematician and founder of computer science, Alan Turing
(1912–1954), built a model to explain how a reproducible pattern could emerge
from an initially homogenous population of cells (Turing 1952 ). His Reaction-
Diffusion model proposed that two diffusible factors, morphogens, could generate
reproducible patterns in embryos. The features of this model are that one of the
factors, called the Inducer, can induce its own expression as well as that of the sec-
ond factor, called the Repressor. The Repressor diffuses faster than the Inducer, and
inhibits production of the Inducer. Turing showed that over time, these two factors
form graded distributions that resolve into a stable pattern of peaks and troughs. The
pattern generated by this model can be changed dramatically simply by altering a
few parameters, such as the rates of production, degradation and diffusion of the
signals (Kondo and Miura 2010 ).
7.8.1 The Nodal Morphogen Gradient
Several lines of evidence in frogs, fish, and mice indicate that Nodal signals behave
as morphogens to pattern the germ layers. In all species examined, nodal genes are
expressed at higher levels in dorsal cells than in ventral cells (Zhou et al. 1993 ;
Jones et al. 1995 ; Joseph and Melton 1997 ; Rebagliati et al. 1998a; Takahashi et al.
2000 ; Skromne and Stern 2002 ). Dorsal cells have higher levels of activated Smad2
than ventral cells, indicating that they experience an elevated level of Nodal signal-
ing (Harvey and Smith 2009 ). Nodal proteins are diffusible and act directly on cells
at a distance (Chen and Schier 2001 ). Of the three nodal-related genes in zebrafish,
only Ndr1/Sqt acts over long distances and is considered a candidate morphogen. In
overexpression assays, Nodal signaling acts in a concentration dependent manner,
with high doses specifying endodermal fates and lower doses specifying ventral and
lateral mesoderm (Smith et al. 1988 ; Green and Smith 1990 ; Jones et al. 1995 ;
Agius et al. 2000 ). Conversely, inhibition of Nodal by overexpressing Antivin/Lefty
suggests that Nodal signaling is required in a dosage dependent manner to pattern
the dorsoventral axis (Agius et al. 2000 ; Thisse et al. 2000 ). Genetic analysis in the
mouse supports the idea that Nodal signals are also required in a dosage dependent
to pattern the epiblast (Robertson 2014 ).
7 Establishment of the Vertebrate Germ Layers