1. Understand the molecular mechanisms underlying early embryonic development in vertebrates. 2....
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1. Understand the molecular mechanisms underlying
early embryonic development in vertebrates.
2. Explain, in general, how organizers function to
pattern the forming axes of the early embryo.
3. Appreciate the conservation of molecular
mechanisms controlling body plan development in
different organisms: the case of homeotic genes.
4. Colinearity of the homeotic genes in man.
Learning OutcomesLearning Outcomes
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Developmental processes occurring during vertebrate development
Axes formation-Signalling centres
Left right asymmetry
Anterior-posterior axis formation
OutlineOutline
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Animals must be specified in three dimensions
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The germ layers are created during gastrulation
Lecture E01
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The germ layers form different tissues
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Basic morphogenic processes are similar between animals
Gastrulation in a fly
FlyBase
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Development in vertebrates is based on cell-cell interactions:
groups of cells called organizing centres emit instructive signals that induce and pattern surrounding tissues.
The concentration gradient of the (signal) morphogen induces multiple cell choices. (E05)
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Organisers are involved in body axis formation in vertebrates
Signalling centres instruct surrounding cells to form tissues
Node graft
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Two headed cow...
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Genetic determinants involved in body axis formation in mammals
The major signalling centre in vertebrates is the node
Node
Chicken Human
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Question: How does the node pattern?
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Genetic determinants involved in body axis formation in mammals
Organisers ‘pattern’ surrounding cells and tissues by secreting
signaling molecules (proteins)
Node cells secretes nodal and noggin and FGF
FGFNodal
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Cells signalling through transmembrane receptors
SHCGrb2
SOSRAS
RAF
MEK
MAPK
FGF
FGFR Extracellular
Intracellular
P
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Genetic determinants involved in body axis formation in mammals:Neural tissue
Signalling centres instruct surrounding cells to form tissues
Overlying tissues form a neural tube
Node or FGF protein
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Gradients of secreted proteins produce the different germ layers
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Left-right asymmetry of internal organs
LungsHeartGut loopingLiver
http://mekhala.blogspot.com/2007_11_25_archive.html
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Left-right patterningasymmetric signalling from the nodeThe expression of genes on the left side of the embryo leads to a cascade of gene expression and morphogenic changes
Nodal
Pitx2
Nodal
chickGut looping, heart looping
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In situ hybridisations of left-right asymmetry genes
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Node and ciliaHow to break symmetry
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anterior
posterior
R L
The node spins
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Loss of left-right asymmetry leads to disease
Situs inversus
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•Named for mutations thatrevealed existence
•Bithorax – part of haltere on 3rd thoracic segment is transformed into part of a wing
•Antennapedia – dominant mutations transform antennae into legs
•Homeotic mutation is the transformation of one segment into another related one
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Homeotic genes
Colinearity: location on the chromosome corresponds to the spatial expression pattern
3’5’
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Temporal and spatial colinearity: order of Hox genes on the chromosome follows the antero-posterior body axis.
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Veraksa, Del Campo & McGinnis. 2000. Mol. Genet. Metab., 69, 85-100.
How do we get anterior-posterior axis: the HOX Genes!!
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Combinations of Hox genes specify the development of the anterior-posterior axis
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Hox gene expression follows the somite bondaries
Embryonic structures Adult organs
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Film of somitogenesis
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When Something Goes Wrong…
Lumbar vertebra
Thoracic vertebra
Extra rib
The function of Homeotic genes in mammals is similar to in flies: the KO of hoxc8 in mouse causes an homeotic transformation: the first lumbar vertebra forms a rib.
*
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Summary: patterning of the vertebrate axial body plan
the four Hox gene complexes are expressed along the antero-posterior axis
Hox gene expression establishes positional identity for mesoderm, endoderm, and ectoderm
gastrulation and organizer activity
mesoderm develops into notochord,somites, and lateral plate mesoderm
somite develops into sclerotomeand dermomyotome
mesoderm induces neural plate from ectoderm
notochord patterns neural tube
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Polydactyly
Diseases associated with Hox gene mutations
1. Hand-foot-genital syndrome (Hox A11-13 deletion)
2. Synpolydactyly (HoxD13 deletion)
3. Cleft palate4. Brain abnormalities5. Leukemia (Hox D4)6. Retinoic acid, which causes
birth defects, affects Hox genes
Teratology Lecture
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Hox genes and vertebrate segment identity
•Hox gene mutations lead to subtle phenotypesWhy??
•Hox genes are used over and over again in the developing embryo
>>>Multiple phenotypes, multiple cancers
Reference book: Developmental Biology, Gilbert