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

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-somite 18-somite

26-somite

22 hr

Zebrafish development

Early development

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Three germ layers

Side view

top view

Skin & nervous system

Muscle & cardiovascular

Gut and liver

Animal pole

Vegetal pole

Gastrulation simplified

Involution

Mesoderm & endoderm insideEctoderm outside

Dorsal-Ventral

Side view

top view

dorsal

dorsal

ventral

ventral

Involuting cell movements during gastrulation

Convergence-Extension

convergence

extension

Convergence-Extension

Convergence-extension

Cell fates

Dorsal viewCross section

dorsal

dorsal

Convergence-extension

Convergence-extension

Slow dorsal migration

Convergence Extension

Convergence-extension cell movements

Slow dorsal migration

Fast dorsal migration

Convergence Extension

Convergence-extension cell movements

Slow dorsal migration

Fast dorsal migration

Convergence Extension

Packing, inhibition of protrusions

Convergence-extension cell movements

Slow dorsal migration

Fast dorsal migration

Packing, inhibition of protrusions

Intercalation

Convergence Extension

Convergence-extension cell movements

Convergence-extension cell movements

Summary

Formation ofgerm layers

Convergence-extension

Formation oftissue boundaries

Cleavage

Early Development movies

Histone 2B GFP

Made RNA

Injected embryos at the 1-cell stage

Imaged with Digital Scanned Laser Light Sheet Fluorescence Microscopy(capture whole embryo image every 60-90s)

Produced a “digital embryo”

512-cell

Divisions symmetric prior to 512-cell

Divisions asymmetric after 512-cell

Radial waves just after 512-cell

Early Development Movies

Early Development Movies

Animal view Vegetal view

dorsal dorsal

dorsaldorsal

dorsal

dorsal

Early Development Movies

Slow-moving nucleus

Fast moving nucleus

Early Development Movies

dorsal

Early Development Movies

Two major Questions

• How are the germ layers established?

• What determines polarity?

Signaling from the YSL

Nodal mutant

wild-type sqt;cyc -/-

No endodermNo head & trunk mesoderm

Normally ~1550 cells internalize; in MZoep ~60 internalize

Germ layer patterning

YSL signal Sqt and Cyc Endoderm Head and Trunk Mesoderm

Tail mesoderm

Dorsal-ventral patterning

Shield(Organizer)

Bozozok

The Organizer

Shield(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