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6.7 Concluding Remarks
The overall conservation of axial patterning mechanisms in vertebrates has long
been recognized. However, detailed investigations at the cellular, molecular and
genetic levels are only now revealing the true extent of this conservation and are
also uncovering important differences. The broadest pattern of axis specification
involves the generation of animal–vegetal polarity in the oocyte followed by the
dorsal localization of determinants in the egg. This ab ovo specification mechanism
operates throughout the primitive vertebrates, reptiles and possibly the egg-laying
mammals. Based on various cell biological, genetic, and morphological observa-
tions, the basic pattern of axis generation is the involution of notochordal precursors
at the early dorsal lip/organizer and the later ingression of ventrolateral precursors
around the lateral blastopore, or “bilateral primitive streaks.”
ancestral sauropsid:
blastoporal platemodern bird/mammal:
primitive streakorganizer/dorsal mesoderm,
cell involutionventrolateral mesoderm,
cell ingression“primitive streak”
cell ingressionancestral ichthyopsid:
blastopore- size, yolk increase,
mercleavageoblastic
2. delaminating hypoblast ?loss of early organizer
induction?
by Nieuwkoop centerorganizer induction - primitive strby Nieuwkoop centereak induction
signaling molecules - organizer induction
by primitive streak cell involution and/or ingressionorganizer/dorsal mesoderm,
polonaise-like cell
movementsyolk plugMediolateral cell intercalation,
Wnt/PCP signalingMediolateral cell intercalation,
Wnt/PCP signalingFig. 6.14 Model for the evolution of axis formation in vertebrates. Highly generalized schematic
diagrams of different vertebrate embryos during early (upper panels) and middle (lower panels)
gastrulation. In the basal vertebrates (left panels; vegetal views), gastrulation initiates at the orga-
nizer with induction by the Nieuwkoop center (high/early Nodal signaling). The initial internaliza-
tion movements are through involution. By mid-gastrulation, ingression of mesendoderm
progresses around the vegetal cells (yolk plug) and forms the nascent blastopore. Involuted dorsal
mesendoderm undergoes convergent extension via mediolateral cell intercalation under the control
of Wnt/PCP signaling. During the evolution of amniotes (middle panels; top/dorsal views), eggs
increased in size and yolk content and began to undergo meroblastic cleavage. Organizer induction
by Nieuwkoop center molecules is retained in early gastrulation. Ingression proceeds through the
horizontal slit of the blastoporal plate/blastopore and does not circumferentially envelop the non-
cleaving yolk (not shown). In the evolution of modern birds and mammals, gastrulation initiates
with ingression at the primitive streak, which would be homologous to the later ventrolateral blas-
topore in ancestral forms. The organizer (Hensen’s node) is induced later by Nodal signaling from
the middle primitive streak. The heterochrony in the pattern of gastrulation morphogenesis and
organizer formation could result from several main events; the hypoblast/anterior endoderm segre-
gating from the epiblast as opposed to forming from cleaving vegetal cells, the loss of early orga-
nizer induction, the apparent emergent behavior of polonaise-like movements leading to primitive
streak formation and the relatively later induction of Hensen’s node by the primitive steak
D.W. Houston