..It is not birth, marriage or death, but gastrulation which is

truly the most important time in your life..

Lewis Wolpert, 1986

Initial asymmetry in the amphibian egg

epidermisnervous system

Ectoderm

gutEndoderm

Mesodermblood, bone, heart, organs

1-2 mm in diameter

Cleavage in the frog egg

Inside the blastula

sea urchin blastula

Steps in early embryonic development

Multiple mitotic divisions

Gastrulation

Fate map of the Xenopus gastrula

Hans SpemannNobel Prize in 1935

Hilde Mangold

Spemann’s organizerHensen’s nodeEmbryonic shield

A-P patterning during neural induction

Retinoic acidFGFs

Experiments:

1) Dissociated cells

2) Dominant negative activin receptor

The BMP signaling cascade

Inhibitors of BMP signaling

Noggin-BMP4 Follistatin-Activin A

Different BMP antagonists

BMP-4 BMP-7 Activin

Noggin

Chordin

Follistatin

++

++

+

+

+

++ +++

-

-

Cloning of NogginSmith and Harland 1992

Chordin expression

Cerberus, Gremlin, Dan, Drm,Ogon/sizzled

The test: Injection of chordin induces a second axis

Depletion

Model for neural induction in amphibians

Things are not as simple as they seem....

Drosophila embryo

ventral neuroectoderm

Mechanisms are conserved (although inverted) in Drosophila

Chordin

BMP

Anti-neural(ventralizing)

Sog

DPP

Anti-neural(dorsalizing)

Coming up:

Tue 1/24 Class Discussion of paper(Wilson & Hemmati-Brivanlou, 1995)Lecture (Patterning)

Fri 1/27 Lecture (Patterning cont’d)Class Discussion of paper(Hynes et al., 2000)

Tue 1/31 QUIZ Presentation (patterning)Lecture (Neurogenesis)