Patterning and Gastrulation

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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

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