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initial invagination at the blastopore (Keller and Shook 2004 ). Extension of this
cavity during gastrulation creates the archenteron. Internal vegetal rotation cell
movements in the presumptive endoderm pull the marginal zone vegetally and initi-
ate internalization by involution at the dorsal lip. Internalization spreads laterally
from the dorsal lip eventually forming a circular blastopore, with axial mesendo-
derm entering at the early dorsal lip and more paraxial and lateral mesendoderm
internalizing from the lateral and ventral blastopore (Keller and Shook 2004 ).
On the dorsal side, the anterior leading edge of the mesoderm (prechordal/head
mesendoderm) migrates as a cellular stream along the blastocoel roof (Winklbauer
and Nagel 1991 ; Winklbauer et al. 1992 ). This migration requires fibronectin fibrils
deposited by the blastocoel roof cells (Boucaut et al. 1985 ; Winklbauer and Nagel
1991 ; Nagel and Winklbauer 1999 ). Internalizing mesendoderm cells maintain what
is termed “tissue separation” with the ectoderm, and cannot intercalate back into the
ectoderm (Wacker et al. 2000 ). This segregation is evident by the presence of
Brachet’s Cleft between the mesendoderm and ectoderm/neuroectoderm
(Nieuwkoop and Florschütz 1950 ). An analogous cleft separating these tissue layers
is found throughout the vertebrates.
In Xenopus and other anurans, the majority of the mesoderm arises from deep
rather than superficial cells (Nieuwkoop and Florschütz 1950 ; Keller 1975 ).
Involution of the mesendoderm is thus internal in these species and provides the
main mechanical forces driving gastrulation. The mesendoderm and deep neural
ectoderm undergo radial intercalation just prior to involution, occurring first in more
anterior cells and later in posterior ones (Keller et al. 1985 ). Additionally, tissue
extension is biased in the anterior–posterior direction, likely because some initial
mediolateral intercalation occurs. After involution, cells become elongated perpen-
dicular to the axis of extension and exhibit bipolar protrusive behavior and undergo
mediolateral intercalation (Keller and Tibbetts 1989 ; Shih and Keller 1992b). This
behavior drives convergent extension of the marginal zone and is the major biome-
chanical force behind internalization and elongation of the axis. Mediolateral inter-
calation behavior begins in more anterior tissue as it involutes and spreads to more
posterior axial tissue as well as lateral mesoderm as the blastopore closes.
Closure of the blastopore at the ventral marginal zone is accomplished by con-
vergent thickening, a little understood behavior in which prospective posterior
somitic mesoderm converges into a pile of cells that do not extend, although they do
so later in neurulation (Keller and Danilchik 1988 ). In general, the process of con-
vergence can lead either to extension or thickening, or both, with the degree of each
outcome depending on the constraints applied by the surrounding tissue. In either
case, the progressive expansion of mediolateral intercalation behavior around the
blastopore and the linking of forces around the marginal zone is thought to create
“hoop stress” that pulls the blastopore lip over the yolk plug and internalizes the
endoderm (Keller et al. 2000 ; Keller and Shook 2004 ).
Urodele and anuran amphibians undergo significant differences in development
that have been long noted but perhaps not well appreciated. The process of gastrula-
tion in salamanders and newts is outwardly similar to the situation described in
D.W. Houston