lec 10 - germ layer formation and gastrulation

what are the 3 germ layers

exoderm, mesoderm, endoderm

gastrulation

major tissue movements reorganize germ layers into their correct locations

ectoderm final transformation

skin

neurons

mesoderm final transformation

muscles

blood

skeleton

kidneys

endoderm final transformation

intestines

alveoli

thyroid gland

why is xenopus good

• eggs are big

• can develop externally

• easy to inject, perform tissue transplants, or fate map with dye

• good model for vertebrate development - much of the genes are conserved

what is the critical step that triggers germ layer formation

egg polarization

describe the initial egg poles

top - animal pole

• pigmented, contains nucleus

bottom - vegetal pole

• yolky, contains maternal mRNA

• signals to animal pole to activate differentiation

vegetal cells become

endoderm

animal hem cells become

ectoderm

equatorial animal cells close to the marginal zone become

mesoderm

how does the blastula form

rapid cell division with short growth phases

mid-blastular transition

after 12 divisions, the embryo makes its own mRNA

molecular mechanisms of germline formation

1. veg hem activates the T-box TF VegT

2. VegT activates transcription of Xnr genes → creates Nodal

3. Nodal diffuses upwards to the animal hem - creates a concentration gradient

4. high nodal = endoderm, med nodal = mesoderm, low nodal = exoderm

establishment of the neiukwoop center and the spearmann organizer

1. egg is fertilized - symmetry breaking event

2. dorsal side develops opposite to side of sperm entry

3. cotical layer rotates 30 degrees towards dorsal region

4. dorsalizing factors (ex: Wnt11 and Dishevelled) are relocated along microtubules

5. Wnt pathway activation → increased beta-cat in dorsal area

6. area where beta-cat/Wnt signaling and Nodal overlap becomes the neiukwoop center

7. the neiuwkwoop center induces the spearmann organizer on dorsal side

high nodal + high beta cat →

low nodal + low beta cat →

• organizer

• ventral mesoderm

describe the animal cap assay

• anim hem cells are cultured with beads containing Nodal

  • no Nodal → exoderm (marker = keratin)

  • low nodal → mesoderm (marker = brachyury)

  • high nodal → organizer (marker = goosecoid)

activity of gradient is modified by Wnt signaling

exoderm marker

keratin

mesoderm marker

brachyury

organizer marker

goosecoid

the mesoderm gets different traits based on

nodal concentraion + interpretation of Wnt signaling across DV axis

_____ + _____ initiate gastrulation

targets of organizer-specific TFs

brachyury

why does high Nodal + active Wnt (beta-cat) induce organizer - ie celss with a particular transcriptional signature

organizer genes like goosecoid are only transcriptionally active via both a Nodal downstream effector (ex Smad 2/4) and a Wnt/beta-cat downstream effector (ex TCF)

spearmann organizer becomes

blastopore/dorsal lip

gastrulation begins with

the inward movement of cells through an invagination of the blastula called the blastopore/dorsal lip

gastrulation

initial invagination defines ___ extremity

by continuing to move inward, the ___ extremity is defined

posterior

anterior

describe the blastrula → gastrula process

1. bottle cells move into the interior of the gastrula to form the dorsal lip

2. sheets of mesoderm cells move through the lip

3. ectoderm cells spread around the outside of the gastrula via epiboly

4. the blastocel cavity shrinks - a new cavity (the archenteron, primitive gut) forms and becomes lined by endoderm

5. a second population of bottle cells creates the ventral lip of the blastopore - mesoderm contributes to reproductive tissues

6. embryo becomes surrounded by ectoderm; endoderm lines future gut; mesoderm lies between

12-day-old human embryo

• implanted into uterine wall

• embryo has separated into top epiblast and bottom hypoblast (similar to xenopus animal and vegetal)