Transcript The Classics

Vertebrate Embryonic Patterning 1
Amphibian Gastrulation: Classical
Observations & Experiments
Amphibian Development
• Fertilization initiates cortical rotation
• Cortical rotation relocalizes determinants
• These determinants act to specify the
dorsal-most regions of the embryo
Establishing Asymmetry
Localized mRNAs
Early Cleavages in Xenopus
gray
crescent
Tiers
A
B
C
D
Fate Map of Xenopus Blastula
Primary Tissue Layers
• Ectoderm
– skin, neural
• Mesoderm
– notochord, somites, blood, kidney
• Endoderm
– gut, prechordal plate
Gastrulation Movements
Xenopus Gastrulation Movements
Gastrulation (again)
Xenopus
laevis
Classic Experiments
• Hans Spemann (1903)
• Constriction of cleavage stage embryos &
fertilized eggs
• Demonstrated asymmetry in fertilized egg
Embryo Constriction Experiment
Egg Constriction
Experiment
Constriction of the egg becomes the 1st
cleavage plane. Constriction of egg has to
bisect the gray crescent in order for
twinning to occur. If division occur
perpendicular to this, only ½ of the egg
develops into an embryo.
Fate Map of Gray Crescent
• dorsal blastopore lip
• point where gastrulation begins
• What is it about the gray crescent that
makes it so important?
Spemann’s
Specification
Experiments
Presumptive neural plate ectoderm in
the early gastrula was specified as
ectoderm, but undetermined.
Later gastrula neuroectoderm was
determined to a neural fate.
Dorsal Lip Transplantation
Perturbations of Axis Formation:
Making Two Dorsal Axes
Cool Perturbations of Normal
Development
• Ventralization:
– UV irradiation of vegetal pole shortly after
fertilization
– blocks cortical rotation by crosslinking
cytoskeletal proteins
– DAI Scale – rating of 0 to 5 of dorsal inhibition
• 0 is normal, 5 is completely ventralized
• embryo develops as a lump of endoderm &
ectoderm
More Cool Perturbations
• Anterior/Dorsalization:
– LiCl treatment
• increases the amount of anterior/head tissue at the
expense of posterior-dorsal mesoderm & ventral
tissues
• Posterior/Ventralization
– Retinoic Acid treatment
• decreases anterior/head structures
Implications
• Strategies for cloning genes involved in
dorsal/ventral specification
– Subtractive library screens between dorsalized
and ventralized embryos
– Functional library screens
• Rescue of ventralized embryos
• Induction of secondary axis
Properties of Dorsal Lip Region
• Autonomously specified
• Dorsal lip never respecified by tissue at
location of transplantation
• Donor cells always contribute to notochord
(mesoderm)
• Secondary neural tube forms from recruited
host cells
• Donor lip cells organize host cells
Mesoderm Induced by Vegetal Cells
Nieuwkoop’s Experiment
Nieuwkoop’s Center Rescues Dorsal Axis
in UV & Duplicates Axis in Wild-type
Inducing Centers in the Blastula
“Spemann”
(General)
Fate Map of Organizer and
Nieuwkoop’s Center