Gastrulation It is not birth, marriage or death, but gastrulation, which is truly the most important event in your life Lewis Wolpert

17 hours in the life of a zebrafish embryo

Blastula Stages 1- cell2-cell 4-cell 8-cell 32-cell 512-cell 1000-cell Transcription begins sphere dome 30% epiboly 4 2/3 hrs

Gastrula Stages 50% germ ring shield 75%90% bud 10 hr

Somitogenesis 3-somite 6-somite18-somite 26-somite 22 hr

Zebrafish development

Early development ?

Three germ layers Side view top view Skin & nervous system Muscle & cardiovascular Gut and liver

Gastrulation simplified

Involution Mesoderm & endoderm inside Ectoderm outside

Dorsal-Ventral Side view top view dorsal ventral

Involuting cell movements during gastrulation

Convergence-Extension convergence extension

Convergence-extension

Cell fates Dorsal view Cross section dorsal

Convergence-extension

Summary Formation of germ layers Convergence- extension Formation of tissue boundaries Cleavage

Two major Questions How are the germ layers established? What determines polarity?

Signaling from the YSL

Nodal mutant wild-type sqt;cyc -/- No endoderm No head & trunk mesoderm

Germ layer patterning YSL signal Sqt and Cyc Endoderm Head and Trunk Mesoderm Tail mesoderm

Dorsal-ventral patterning shield

Bozozok

The Organizer

Bmp promotes ventral fates Organizer signals are BMP binding factors: Noggin, Chordin Bmp signals promote ventral fates (skin, ventral mesoderm) and oppose dorsal fates (neural, dorsal mesoderm)

Mutants in the dorsal-ventral pathway Snailhouse (snh) = bmp7 mutant Chordino (din) = chordin mutant

Bmp regulates cell movement High Bmp Low Bmp

Summary

Xenopus egg

Cleavage of Xenopus embryo Midblastula transition (MBT) occurs at 12th cleavage = start of zygotic transcription Animal pole view

Gastrulation in Xenopus vegetal pole view

Fate map

Cross sectional view

Gastrulation Gastrulation begins dorsally and spreads ventrally

Gastrulation in Xenopus

Two major Questions How are the germ layers established? What determines polarity?

Nieuwkoop’s experiment 1967

Nodals Xnrs induce mesoderm (and endoderm)

Why are the Xnrs expressed vegetally?

VegT VegT is a transcription factor

VegT

VegT activates the transcription of the Xnr genes

VegT

Formation of the dorsal side

 -catenin is a transcriptional activator

Creation of the early gastrula

The role of the Organizer

Organizer patterns the mesoderm and converts epidermis to neural

What are the Organizer signals? Inhibitors of Bmp signaling: –Noggin, Chordin Inhibitors of Wnt signaling: –Dickkopf, Frzb

Function of Bmp BMP promotes ventral fates O Some organizer signals (Chordin, Noggin) inhibit BMP signals and promote dorsal fates