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teleost-specific population of epiblast cells, the dorsal forerunner cells (DFCs)
detach and migrate vegetally ahead of the dorsal lip, in association with the margin
of the EVL (Cooper and D'Amico 1996 ). Their function at this stage is unknown,
but later these cells will form the Kuppfer’s vesicle, which functions in left-right
asymmetry. Also, similar to the process in anurans, convergent extension elongates
the axial mesendoderm as a result of bipolar protrusive behavior and well as dor-
sally directed migration of more lateral cells (Solnica-Krezel 2005 ). Teleost thus
appear to have independently evolved an amphibian-style method of gastrulation as
a solution to concentrated yolk and discoidal cleavage in the egg.
6.6.3 Amniote Gastrulation
In the mammals and birds, gastrulation initiates at the primitive streak, a groove
formed by internalizing cells along the posterior axial midline. Anterior–posterior
patterning, and thus the positioning of the primitive streak towards the posterior, is
controlled by the anterior migration of the hypoblast/AVE (Sect. 6.5.1). Mesoderm
precursors are initially found throughout the posterior epiblast but migrate toward
the posterior end prior to primitive streak formation. In the chick, this occurs
through the double-vortex polonaise-like cell movements (see Sect. 6.5.1) and simi-
lar mechanisms have typically been thought to occur in mammals. However, mouse
embryos appear to lack these large-scale cell movements in the epiblast (Williams
et al. 2011 ), possibly owing to a smaller number of cells at the equivalent stage or to
constraints of the cup-shaped architecture. Alternatively, the cell movements posi-
tioning the streak may occur in the visceral endoderm layer in mouse (Weber et al.
1999 ). Polonaise-like movements appear to occur in rabbit embryos, which have a
flat morphology similar to chick embryos (Halacheva et al. 2011 ). Interestingly
though, in this case, a more localized extreme intercalation cell movement event
(“processional cell movements”), predominates (Halacheva et al. 2011 ). These
observations indicate that overall primitive streak positioning in amniotes can result
from many different patterns of cell movements depending on the species.
Internalization through the primitive streak involves massive epithelial-to-
mesenchymal transition (EMT) and ingression of individual cells to form definitive
mesoderm and endoderm (Stern 2004 ). Early ingressing mesoderm contributes
mainly to pharyngeal and cardiac mesoderm anteriorly, as well as extraembryonic
mesoderm posteriorly. Definitive endoderm ingresses by mid-streak stages and
intercalates into and displaces the hypoblast/visceral endoderm. The primitive
streak elongates and progresses anteriorly (distally in the mouse egg cylinder) until
mid-gastrulation (late streak stage), approximately to the level of the future hind-
brain, and begins to form the anatomically distinct Hensen’s node at the anterior tip.
Internalization at Hensen’s node in many birds and mammals, including humans,
resembles that of the amphibian dorsal lip, with invaginating bottle cells and involu-
tion of notochord and somitic mesoderm (Shook and Keller 2008 ). Chick and mouse
D.W. Houston