267Xenopus, but has several important distinctions. Mesoderm is derived from the sur-
face layer in urodeles (Vogt 1929 ). Axial mesoderm undergoes similar involution at
the dorsal lip in urodeles, but internalization in the ventral and lateral regions occurs
mostly by ingression of individual cells (Shook et al. 2002 ). This pattern of ingres-
sion has been referred to as a “bilateral primitive streak.” In urodeles, lateral and
ventral convergent forces are generated by the ingressing cells in these regions, as
these cells undergo apical constriction and bring additional cells into the ingression
zone (Keller and Shook 2004 ).
Interestingly, the urodele pattern of dorsal lip involution and lateral ingression is
conserved in more primitive organisms including cyclostomes and lungfish (Shook
and Keller 2008 ). It is likely that both the anuran and teleost mechanisms of primary
internalization by involution evolved independently within those lineages. These
changes in gastrulation may be correlated with the cellular organization of the ecto-
dermal moiety, occurring as a multilayered ectodermal “deep layer” of non- polarized
cells, covered by a superficial polarized epithelial layer in the anurans and teleosts,
as opposed to the typical “epiblast” of pseudostratified interdigitating epithelial
cells (Shook and Keller 2008 ).
6.6.2 Teleost Gastrulation
Similar morphogenetic events occur in the zebrafish/teleost gastrula, albeit in the
context of a different embryonic architecture. One of the main features of fish gas-
trulation is prominent epiboly, in which all three germ layers (as opposed to just
ectoderm) undergo extensive radial intercalation as the blastoderm expands to cover
the yolk cell. Epiboly in teleost fish is somewhat unusual, and involves interconnec-
tions between the yolk cell and the superficial epithelial layer of the blastoderm.
Epiboly begins just prior to gastrulation with YSL nuclei migrating vegetally in
advance of the blastoderm margin. Cytoskeletal perturbations and in vivo observa-
tions suggest that the YSL “tows” the blastoderm margin vegetally (Betchaku and
Trinkaus 1978 ; Solnica-Krezel and Driever 1994 ).
Internalization initiates by an involution-like “synchronized ingression” around
the blastoderm margin, forming a bilayered structure, the germ ring (Adams and
Kimmel 2004 ). Typical ingression is not thought to occur, as teleost gastrulation
lacks a classical epithelial-to-mesenchymal transition (EMT) and cell migration
characteristic of typical ingression movements (Shook and Keller 2008 ).
Internalizing cells migrate beneath the outer layer toward the animal pole, forming
an inner hypoblast (this is mesendoderm, and not homologous to the amniote hypo-
blast) and outer epiblast (ectoderm). Both layers are covered by a flattened superfi-
cial epithelium, the enveloping layer (EVL), which remains extraembryonic and
may provide structural support for morphogenetic movements (Kimmel et al. 1990 ).
On the dorsal side of the margin, involuting and single “delaminating” cells form a
thicker layer, constituting the embryonic shield (Montero 2005 ). Also dorsally, a
6 Vertebrate Axial Patterning: From Egg to Asymmetry