341of a secondary primitive streak in a chick embryo when it is expressed in the lateral
region of an epiblast that lacks the hypoblast, which secretes Nodal antagonists
(Bertocchini and Stern 2002 ). Of the three teleost nodal-related genes, two, nodal-
related- 1 (ndr-1)/squint (sqt), nodal-related-2 (ndr-2)/cyclops (cyc), are expressed
in the presumptive mesendodermal cells (Rebagliati et al. 1998a; Fan and Dougan
2007 ). ndr-1/sqt expression begins in dorsal marginal cells and the dorsal YSL, fol-
lowed by transient expression of both ndr1/sqt and ndr2/cyc around the entire mar-
gin, including the YSL (Erter et al. 1998 ; Feldman et al. 1998 ; Fan et al. 2007 ).
Subsequently, ndr1/sqt expression is lost from the margin, persisting only in the
NEM/Dorsal Forerunner Cells, while ndr2/cyc continues to be expressed in the
involuting axial mesoderm (Rebagliati et al. 1985 , 1998a). Thus, dorsal marginal
cells are continuously exposed to Nodal signals, while ventrolateral cells are only
transiently exposed to Nodal. ndr1/sqt is also expressed maternally (Feldman et al.
1998 ). The third nodal-related gene, nodal-related- 3 (ndr-3)/southpaw (spaw), is
expressed after gastrulation and is not involved in mesoderm formation (Long et al.
2003 ). In Xenopus, six genes, xnr1-6, were cloned based on homology to nodal
(Jones et al. 1995 ; Joseph and Melton 1997 ; Takahashi et al. 2000 ). All of the xnr
genes are expressed in the presumptive mesoderm in a graded fashion, with highest
levels in the Organizer, and can induce mesoderm in a dosage dependent manner.
Mouse Nodal induces mesoderm and endoderm in a dosage-dependent manner in
both frog and fish embryos (Jones et al. 1995 ; Toyama et al. 1995 ). Ndr-1/Sqt,
Ndr-2/Cyc, and each of the Xenopus nodal genes have similar activities when over-
expressed (Jones et al. 1995 ; Joseph and Melton 1997 ; Erter et al. 1998 ; Feldman
et al. 1998 ; Rebagliati et al. 1998b; Takahashi et al. 2000 ; Aoki et al. 2002 ). The
exception is Xnr-3, which lacks a conserved cysteine in the mature ligand domain,
and acts as a neural inducing signal rather than a mesoderm-inducing signal in ani-
mal cap assays (Smith et al. 1995 ). The ability of Ndr-3/Spaw to induce mesoderm
has not been tested, but its strong homology to Ndr-1/Sqt suggests it should have a
similar activity. The late expression pattern of Ndr-3/Spaw, however, eliminates it as
a possible endogenous mesoderm-inducing signal.
7.6.2 TGF-β Signal Transduction Pathway
Activin, Vg1, Derriére and Nodal are all members of the TGF-β superfamily, named
after its founding member, Transforming Growth Factor-β. TGF-β proteins are syn-
thesized as precursor proteins that undergo extensive post-translational modifica-
tions and covalent dimerization before the final cleavage event that releases the
mature ligand from the C-terminus. These ligands bind to transmembrane receptors
on the surface of a responding cell and activate their serine/threonine kinase activity.
This initiates a signaling cascade that results in activation of specific target genes.
There are two types of TGF-β receptors: Type I receptor and Type II receptor
(Massague 1992 ). When a TGF-β ligand binds to the type II receptor, the ligand-
bound type II receptor has higher affinity to the type I receptor and forms a complex
7 Establishment of the Vertebrate Germ Layers