350
7.7.2.2 Chicken
In the chicken, cNR-1 is expressed in the elongating primitive streak (Levin et al.
1995 ). COS cells expressing cVg1 induce an ectopic primitive streak when implanted
in the marginal zone (Seleiro et al. 1996 ; Shah et al. 1997 ). cNR-1 expression is
induced within 6 h after misexpressing cVg-1 (Skromne and Stern 2002 ). Formation
of the ectopic primitive streak is blocked by expression of a Nodal antagonist in the
area pellucida but not by a Nodal antagonist expressed in the marginal zone
(Bertocchini and Stern 2002 ). This indicates that Nodal signaling is necessary for
extension of the primitive streak, but is not required for inducing the primitive
streak. The chick hypoblast secretes Cerberus, an endogenous Nodal antagonist
(Piccolo et al. 1999 ; Foley et al. 2000 ). Surgical removal of the hypoblast results in
the formation of supernumerary primitive streaks expressing cNR-1 (Bertocchini
and Stern 2002 ). This data is consistent with a model in which endogenous cVg-1 in
the PMZ induces cNR-1 expression in the posterior area pellucida and initiates for-
mation of the primitive streak (Fig. 7.10b). cNR-1 activity in the nascent streak is
initially blocked by Cerberus expressed in the hypoblast. Elongation of the
a b ChickencVg1cNR-1MesodermFgf8MouseNodalgdf1,3Mesoderm
NodalFgfFig. 7.10 Mesoderm and endoderm inducing pathway in amniotes. (a) In the mouse, a signal from
the extraembryonic ectoderm induces Nodal expression in the epiblast. The Vg1 orthologues Gdf1
and Gdf3 act in parallel with this signal to induce Nodal expression (black arrow). Nodal expres-
sion in the epiblast establishes a self-reinforcing loop that expands Nodal expression throughout
the epiblast and induces Nodal expression in the visceral endoderm. Nodal expression in the vis-
ceral endoderm is required to induce mesendodermal fates, acting in parallel with Fgf signals. (b)
In the chick, Vg1 is expressed in a crescent in the PMZ and induces expression of cNR-1 in the
epiblast. cNR-1 induces a positive feedback loop that expands the Nodal expression domain as the
primitive streak extends. cNR-1 and Fgf8 act in parallel to induce mesodermal fates. In both the
mouse and chick, Fgf signals act as a chemoattractant, drawing epiblast cells toward the primitive
streak and repelling cells away from the primitive streak as they exit the streak
W. Tseng et al.