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Patterning and Gastrulation
Readings
y Chapter 19, p.442 In the fruit fly, to p.446 (end of the chapter)y Chapter 43, p.926-932 (Section 43.2 on gastrulation)
Some maternal determinants activate embryonic gene expression
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The Drosophila embryo is sequentially patterned by a cascade ofsegmentation genes
y Gap genes organizeareas on the
anterior-posterior
axis.y Pair rule genes
divide embryo into
units of two
segments each.
y Segment polaritygenes determine
segment boundaries
and back-front
orientation.
y Homeotic genesidentify where to
place antennae,
legs, and wings.
Some ofthe maternal effect genes are transcription
factors that activate gap gene expression
y Three gap genes shownTwo gap gene products overlap, defining a third
region
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y Combinatorial control: A unique combination of transcription factorsresults in a unique combination of gene expression and unique
cellular fate
Gap gene products activate pair rule
genes
y Two pair-rule genes,each expressed in
seven alternate stripes
Pair-rule gene products activate
segment polarity genes
y Segment polaritygenes define an
anterior or posterior
compartment in
each segment
Homeotic Genes
y Activated by pair-rule & segment polarity genesy Control specific identity of segments
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y Gastrulation involves massive cell movements that start at thedorsal lip of the blastopore
y The three primary germ layers are madey Cells from various regions of the blastula come into contact with
new neighbours
The three germ layers and their derivatives
y Ectoderm: skin, brain, spinal cordy Mesoderm: bones, muscles, blood cellsy Endoderm: gut, lungs, pancreas, thyroid
Gastrulatio
n in an am
ph
ib
ian emb
ryo
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-catenin, in combination with signals from the vegetal cells, induce formation ofthe
primary embryonic organizer in the region ofthe gray crescent
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y The goal of this figure is to appreciate how signaling moleculesinteract to produce different combinations of signals that guide cells
into paths of determination and differentiation.
Spemann and Mangolds blastopore transplant experiment
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y The transplanted dorsal lip of the blastopore induces a second siteof gastrulation and organizes tissue into a second embryo
y -catenin is sufficient to initiate a primary organizerThe large egg yolk in reptile and bird eggs causes the
blastula to form a flattened disc of cells (the blastodisc)
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y Epiblast forms the embryoy Hypoblast contributes to extra embryonic membranesy The Hensens node is equivalent to the dorsal lip of the blastoporey Signaling molecules identified in the frog organizer are also
expressed in Hensens node