269embryos however appear to have separately evolved ingression as the main mode of
internalization, although dorsal lip involution may be evolutionarily ancestral.
Reptiles (non-avians) represent a useful but little-studied intermediate case.
These embryos typically form a dorsal lip perpendicular to the axial midline in the
posterior of the blastoderm, through which chordamesoderm involutes. Lateral and
ventral cells ingress through a reptilian blastoporal plate, which resembles both the
urodele blastopore and amniote primitive streak (Bertocchini et al. 2013 ).
After its formation, the Hensen’s node begins to regress posteriorly and the prim-
itive streak closes in an anterior-to-posterior progression. Notochord and somites
form in the mesoderm behind the regressing node. At this point Hensen’s node and
surrounding cells likely comprise a pluripotent stem cell population that persists
throughout regression and into tailbud formation (Wilson et al. 2009 ). It is often not
discussed as such, but amniote primitive streak regression is likely an analogous
process to blastopore closure, with mediolateral intercalation behavior driving con-
vergent extension in the axial midline tissues and convergent behavior following
ingression at the primitive streak. For the mouse, mediolateral intercalation of
mesoderm has been established as a mechanism of axial elongation, and convergent-
extension behavior has been demonstrated to initiate in pre-somitic mesoderm cells
immediately after they exit the primitive streak (Yen et al. 2009 ).
In all vertebrates, the embryo lengthens and narrows along the AP axis at the culmi-
nation of gastrulation. The neural plate is visible in the dorsal ectoderm and the closed
blastopore represents the posterior pole of the embryo. Patterning of the AP and DV
axes is essentially accomplished during gastrulation. Although the left- right axis is thus
specified topologically, the cellular and molecular mechanisms that regulate left-right
asymmetry of the internal organs do not come into play until after gastrulation. These
are primarily mediated by a distinct ciliated structure that forms during late gastrula-
tion at the posterior end of the notochord along the gastrocoel roof plate, a part of the
archenteron lining (Blum et al. 2014 ). This structure is derived from surface mesoderm
in the late involuting dorsal lip and is homologous in many respects across vertebrates.
Monocilia in the posterior notochord region are thought to create a leftward fluid flow,
generating asymmetry in Nodal signaling in the left lateral plate mesoderm, leading to
morphological left-right asymmetry and the generation of internal bilateral asymmetry
in organs such as the heart, gut and lungs (also, see Blum et al. ( 2014 ) for review of
caveats in chick and pig, which lack nodal cilia (Gros et al. 2009 )).
6.6.4 Molecular Regulation of Axial Morphogenesis
The control of convergent extension has been extensively studied at the molecular
level in Xenopus and zebrafish embryos. A host of cellular and genetic experiments
have implicated components of the Wnt/PCP pathway as critical regulators of cell
shape, cell polarity and activity during convergent extension. In Xenopus, overex-
pression of certain beta-catenin-independent Wnts and dominant-negative
Dishevelled 2 (Xdd1; Sokol 1996 ) were sufficient to block axis elongation without
6 Vertebrate Axial Patterning: From Egg to Asymmetry