Download Analysis of Biological Development

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Axis Formation in Animals
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Questions
• How are axes specified during development?
- How are the poles of an axis
determined?
- How is the orderly array (pattern) of
structures along the axis specified?
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Body Axes and Planes
- Anteroposterior
- Dorsoventral
- Left-right
- Proximodistal
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Developmental Stages in the Life Cycle
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Cytoplasmic Determinants
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Cell Fate Determination (Time and Place)
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• The fate of a cell is the sum of all
structures that the cell or its descendants
will form at a later stage of normal
development.
• A fate map is a diagram of an organism at a
specific stage of development, indicating the
fate of its component cells or regions at a
later stage.
Fate Map of the Xenopus
Embryo
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Development of Isolated Blastomere
Tiers in Xenopus
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Mesoderm Induction
in the Xenopus Blastula
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Mesoderm Induction
• Mesoderm forms as a result of an interaction
between animal and vegetal cells.
• Induction increases complexity.
• The proximity of inducing and responding cells is an
effective guarantee that the structures for which
they are fated will arise next to each other and at
the same time.
• The ability of the responding tissue to react to an
inducer by changing its determined state is called its
competence.
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Determination of the Dorsoventral
Axis in Amphibians
As a rule, the gray crescent develops opposite the
sperm entry point, and the gray crescent is a reliable
predictor of the future dorsal side.
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Surgical Experiments for Dorsalization
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Model of Germ
Layer Formation
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Dorsaliztion of
Ventral Marginal
Zone
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Effect of Dorsoventral Polarity on
Mesoderm Induction in Xenopus
- Dorsal mesoderm at the early gastrula stage occupies
the dorsal lip of the blastopore.
-This material induces overlying ectoderm to form brain
and spinal cord.
-The dorsal blastopore lip is known as Spemann’s
organizer.
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Blastopore Formation in Amphibians
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Spemann and Mangold Experiment
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Questions?
• What is the nature of the factor(s) that are
segregated into dorsal blastomeres and impart
a dorsal bias on them?
• Which molecules act as inducing signals to
persuade marginal cells to form mesoderm
instead of ectoderm?
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Criteria
• The molecule must be present in the embryo
at the required concentration and in the
predicted region.
• Blocking the action of molecule in vivo must
interfere with its proposed biological
function.
• It must be active in rescue and/or
heterotopic transplantation experiments
with whole embryos.
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ß-catenin is a
likely candidate
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Mouse A-P Axis formation
• Yamamoto et al., Nodal Antagonists Regulate
Formation of the Anteroposterior Axis of
the Mouse Embryo. Nature 428: 387-392
(2004).
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Mouse Development
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Transformation of the Proximodistal Axis
of the Blastocyst into the
Anteroposterior Axis of the Embryo
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Mouse Gastrulation
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Cell Movements and Molecular Signals
Controlling Axis Formation
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The Expression Domains of Hex
and Other Genes
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Predicted Location of the AVE
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Question?
• How is DVE migration achieved?
• How is the direction of DVE migration
determined?
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Asymmetric Expression of Lefty1
and Cerl before DVE Migration
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• Lefty1 and Cerl expression domains at E5.5 were
shifted towards the future anterior side of
embryos, whereas the Hex expression domain was
apparent at the distal tip of the same embryos.
• Such asymmetric expression of Lefty1 and Cerl
might thus be expected to generate differential
Nodal activity in the DVE along the A-P axis, with
lower Nodal activity on the future anterior side.
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Ectopic Expression of Lefty1 and Cerl
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Patterns of VE Cell Proliferation
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• Nodal signaling promotes cell proliferation in the
VE and the asymmetry in proliferation along the
A-P axis is due to differential Nodal signaling.
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Nodal Antagonists Inhibit and Nodal
Promotes VE Cell Proliferation
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• The DVE migrates away from the side where
VE proliferation is stimulated by ectopic
expression of Nodal.
• The DVE migrates towards the side where
proliferation of VE cells is inhibited by
Lefty1 and Cerl.
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Anteroposterior Patterning Defects in
the Absence of Lefty1 and Cerl
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Conclusion