3437.6.3.1 Nodal
Mutations in mouse and zebrafish demonstrate that nodal is required for mesoderm
formation. The primitive streak and Node fail to form in mouse nodal mutants
(Conlon et al. 1994 ). nodal mutants lack expressionof goosecoid (gsc) and brachy-
ury (T) expression, indicating that the prechordal plate and notochord are absent.
Mosaic analysis showed that nodal function is required in the visceral endoderm
(VE) for formation of prechordal plate mesoderm (Varlet et al. 1997 ). Similarly in
zebrafish, ndr-1/sqt and ndr-2/cyc single mutants display mild defects in the forma-
tion of axial mesoderm (Thisse et al. 1994 ; Heisenberg and Nusslein-Volhard 1997 ;
Erter et al. 1998 ; Sampath et al. 1998 ). ndr-1/sqt; ndr-2/cyc double mutants lack all
derivatives of the head and trunk mesoderm (Feldman et al. 1998 ). In the absence of
ndr-1/sqt and ndr-2/cyc, marginal cells that would normally populate the mesoderm
and endoderm of the head and trunk instead adopt ectodermal fates (Feldman et al.
2000 ). Nodal signaling is required in a dosage-dependent manner to pattern the
animal–vegetal axis (Dougan et al. 2003 ). Cells in the dorsal margin are more sensi-
tive to reductions in Nodal levels than cells in the ventrolateral margin (Dougan
et al. 2003 ; Harvey and Smith 2009 ). Expression of Nodal antagonists inhibits for-
mation of mesoderm and endoderm in a dosage dependent manner in both frogs and
fish, and blocks formation of the primitive streak in chicken (Piccolo et al. 1999 ;
Thisse and Thisse 1999 ; Agius et al. 2000 ; Bertocchini and Stern 2002 ). Mesoderm
forms normally in frog xnr1 morphants, suggesting that the xnr genes have redun-
dant requirements during mesoderm formation (Toyoizumi et al. 2005 ). These
experiments indicate that Nodal signals are the endogenous mesoderm and endo-
derm inducing signals in vertebrates, and that these signals act in a dosage depen-
dent manner to pattern the mesoderm and endoderm.
7.6.3.2 Activin/Vg1
Formation of axial mesoderm in frogs is disrupted when Activin B function is
reduced by injection of antisense morpholino oligonucleotides (Piepenburg et al.
2004 ). Similarly, expression of a dominant negative version of Activin in the
Medaka fish showed that maternally expressed Activin is required in a dosage
dependent manner for mesoderm formation (Wittbrodt and Rosa 1994 ). In zebraf-
ish, Activin regulates oocyte maturation, but its function in mesoderm formation
has not been directly tested (Tan et al. 2009a, b). This role of Activin in mesoderm
formation may not be conserved, however, since mesoderm forms normally in
activin null mutant mice, consistent with its late expression pattern (Matzuk et al.
1995 ). Depletion of Vg1/Gdf1 in Xenopus results in embryos with reduced head
structures, and a lack of notochord (Birsoy et al. 2006 ). Thus, Vg1/Gdf-1 is required
for dorso-anterior development in frogs. In zebrafish, depletion of maternal Dvr1
results in trunk defects of varying severity which have not been well characterized
(Ye et al. 2010 ; Li et al. 2012 ). Posterior mesodermal fates do not form when
Derriére/Gdf-3 activity was inhibited by overexpression of a dominant negative
Derriére/Gdf-3 ligand (Sun et al. 1999a). Thus in frogs, ActivinB, Vg1/Gdf1 and
7 Establishment of the Vertebrate Germ Layers