2656.6 Axial Morphogenesis
The physical form of the vertebrate axis is brought about during gastrulation. During this
process, the presumptive mesoderm and endoderm germ layers are internalized and
undergo a set of morphogenetic events that elongate the axis in the anteroposterior direc-
tion and establish dorsoventral (mediolateral) organization in the mesoderm. Vertebrate
embryos undergo gastrulation in one of two basic ways, depending on the architecture of
the embryo. In spherical anamniote embryos, internalization occurs through the blasto-
pore, which circumscribes the marginal zone, and forms the archenteron (primitive gut
tube). In the flat blastoderm of amniote embryos, internalization generally occurs
through the primitive streak in the posterior of the embryo, forming a trilaminar embryo
with the endoderm remaining open to the yolk and forming a tube secondarily.
In recent years, the cellular mechanics of vertebrate gastrulation have been studied
in great depth using live imaging approaches. Surprisingly, the underlying cell and
tissue mechanics and their molecular regulation are extensively conserved, despite
differing patterns and timing of gastrulation movements. Largely based on the semi-
nal work of Trinkaus, Keller, Stern and others, a common set of vertebrate gastrulation
movements and cell behaviors have been identified. These have been extensively
reviewed in recent years (Keller 1986 ; Keller and Shook 2004 ; Stern 2004 ; Solnica-
Krezel 2005 ; Tam et al. 2006 ; Shook and Keller 2008 ; Solnica-Krezel and Sepich
2012 ) and fall into three main categories, internalization, convergent extension and
epiboly. Together, these processes form a “mosaic of regional processes” (Keller
1986 ) that are exquisitely integrated to accomplish the feat of gastrulation.
Mesendoderm internalization typically initiates by invagination (in-pocketing) in
a tissue layer (the epiblast), followed either by involution as an in-rolling sheet of
cells over a lip (Einrollung; Vogt 1929 ), or by ingression of individual cells.
Invagination and ingression proceed by apical constriction of epiblast cells, and in the
case of ingression, epithelial-to-mesenchymal transition and subsequent cell migra-
tion. Convergent extension is a thinning and narrowing of a tissue and occurs by
mediolateral intercalation of closely packed cells or by migration of cells toward a
midline. Cells in the mesendoderm undergoing convergent extension typically elon-
gate along the mediolateral axis and exhibit bipolar protrusive behavior using lateral
lamellopodia. The traction generated by this bipolar activity is thought to facilitate
cell intercalation. Epiboly is a thinning and spreading/extension of a tissue and
involves radial intercalation to form a thinner cell layer, as well as cell shape changes.
In spherical embryos, epiboly is essential for the ectoderm to spread and cover the
yolk mass during gastrulation. Uniquely in teleost embryos, epiboly encompasses all
germ layers and the entire embryonic blastoderm covers the yolk cell.
6.6.1 Amphibian Gastrulation
Gastrulation in amphibians initiates at a latitudinal slit at the boundary of equatorial
and vegetal regions, forming the dorsal lip of the incipient blastopore. Cell internal-
ization begins with apical constriction of the so-called “bottle cells,” creating the
6 Vertebrate Axial Patterning: From Egg to Asymmetry