Gastrulation It is not birth marriage or death

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

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

Somitogenesis 3 -somite 6 -somite 18 -somite 26 -somite 22 hr

Zebrafish development

Early development ?

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

Gastrulation simplified

Involution Mesoderm & endoderm inside Ectoderm outside

Dorsal-Ventral Side view ventral dorsal top view

Involuting cell movements during gastrulation

Convergence-Extension convergence extension

Convergence-extension

Cell fates dorsal Cross section Dorsal view

Convergence-extension

Summary Cleavage Formation of germ layers Convergenceextension Formation of tissue boundaries

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 Tail mesoderm Sqt and Cyc Endoderm Head and Trunk Mesoderm

Dorsal-ventral patterning shield

Bozozok

The Organizer

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

Mutants in the dorsal-ventral pathway Snailhouse (snh) = bmp 7 mutant Chordino (din) = chordin mutant

Bmp regulates cell movement High Bmp Low Bmp

Summary

Xenopus egg

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

Gastrulation in Xenopus vegetal pole view

Fate map

Cross sectional view

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?

Veg. T is a transcription factor

Veg. T

Veg. T activates the transcription of the Xnr genes

Veg. T

Formation of the dorsal side

Formation of the dorsal side b-catenin is a transcriptional activator

Creation of the early gastrula

The role of the Organizer

The role of the 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 O BMP promotes ventral fates Some organizer signals (Chordin, Noggin) inhibit BMP signals and promote dorsal fates