Axis Formation in Amphibians How does a radially symmetrical egg develop the axes of bilateral symmetry? dorsal posterior anterior ventral

SP Gray crescent

Cortical rotation L R

2-cell Nuclei stained Nuclei not stained dorsal ventral β-catenin (TF) is localized in the nuclei of dorsal cells due to cortical rotation. gastrula

Canonical Wnt Pathway

Dsh binds to and blocks GSK3. GSK3 degrades β-catenin.

β-catenin activates a transcription factor cascade that establishes the inducing power of the dorsal lip.

Inject goosecoid mRNA Inject dominant inactive GSK3 in both blastomeres at 2-cell stage. GSK3 suppresses dorsal specification of ventral half. http://www.youtube.com/watch?v=BkKWApOAG2g

Unequal radial holoblastic cleavage

Fate Map of Xenopus laevis Where is the mesoderm?

Fate mapping Walter Vogt, 1929

Gastrulation movements Formation of endodermal bottle cells Convergent extension Crawling of lead cells of sheet

Epiboly Phayngeal endoderm moves to blastocoele ceiling and becomes leading (crawling) edge of forming archenteron.

Fibronectin stained yellow Early gastrula Late gastrula Injection with integrin binding fibronectin peptide (competitive inhibition)

Epiboly of ectoderm

Epiboly: cell division and convergent extension

The gray crescent is necessary for axial development baustück

The dorsal lip of the blastopore (the organizer) derives from the gray crescent and it has the power to ORGANIZE a second embryo. Thus is primary induction. Spemann and Mangold, 1924 http://www.youtube.com/watch?v=e3HcqGcXls4

Pigmented markers showed the NT developed from host ectoderm http://www.youtube.com/watch?v=e3HcqGcXls4

Depletion of EP-cadherin mRNA Why a blastocoele? 1. Keep animal cap away from inducing endoderm 2. Provide space into which involuting endoderm and mesoderm can move.

Where does the organizer come from Where does the organizer come from? The induction of the mesoderm by endoderm

Pieter Nieuwkoop: Nieuwkoop center

Mesodermal induction: Nieuwkoop Center acts in blastula

Maternal mRNA for Veg1 (white) tethered to the vegetal cortex

(Xenopus nodal related) TGF-β paracrine factor TGF-β paracrine factor Maternal mRNA for TF

The Organizer Initiates gastrulation (bottle cells) Autonomously differentiates into the notochord Dorsalizes the lateral mesoderm to form somites. Dorsalizes the overlying ectoderm to form a neural tube. The search for the inducer.

Is neural induction permissive, rather than instructive?

BMP induces ventral mesoderm and epidermis BMP induces ventral mesoderm and epidermis. Neural ectoderm and dorsal mesoderm and endoderm require that BMP be blocked! This is what the inducers do! Does this explain Holtfreter’s surprising results? Nodal Review figure 10.10 A

Find it: Chordin mRNA expression Chordin is a paracrine factor that blocks BMP-2 and BMP-4 from binding its receptor

Morpholino against chordin (and noggin and follistatin) Control Sox2 expression (neural marker) DevBio9e-Fig-07-28-0.jpg Morpholino against BMP-2, 4, and 7)

Noggin mRNA can “rescue” UV-irradiated embryos (belly pieces)

Regional specification Saxén and Toivonen (film)

Saxén and Toivonin. Neuralizing (head) and mesodermalizing (trunk and tail) gradients Please watch Saxen interview