Patterning and Gastrulation

8/3/2019 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

Documents

Patterning and Gastrulation