˙Determination of the axis

Oogenesis: Vg-1 and others' mRNA and has been synthesis for maternal gene products translation later.

Egg: Stopped at metaphase II. The animal / vergetal poles has determined the anterior-posterior axis. Presenting large amount of maternal gene mRNA.

Fertilization: Unknown mechanism triggered the cortical rotation opposite the site of sperm entry and forms there the signaling center. Sperm entry site determine the dorsal-ventral axis and the anterior- posterior axis.

Cortical rotation: The plasma membrane and the cortex opposite the site of sperm entry rotates toward it.The area called gray crescent will be the Nieuwkoop center, determining the dorsal side of embryo.

Activation of Vg-1 protein precursor as the class 1 & 2 signals for mesoderm induction.

Ventralized embryo: UV treated embryo with blocked cortical rotation goes on abnormal development.

Nieuwkoop center: Localized maternal effect - cell adhesio and Wnt signal pathway (blocking beta-catenin degredation activity of GSK-3) , thus beta-catanin presented in nuclie on the dorsal side as a transcription factor. Ventralized embryo: Isolation of the ventral side cell after the first cleavage lead to abnormal development.

˙Germ layer determination

The fate of cells were not determined.

Requirement of the induction of dorsal mesoderm and nervous system.

˙Induction of mesoderm

The animal cap cell competence: Respond to the signal for a limited time. Induction is complete by the time gastulation start. Gene expression of differentiation is not linked to the time of mesoderm induction.

Community effect: A suffeciently large number of animal cap cell must be present for induction of muscle differenciation to occur.

Class 1 signals: Vg-1(TGF-beta)
Mesoderm-inducing effect on animal cap cells. Rescuing the ventralize development induced by UV.

Class 2 signals: [morphogene?]
Vg-1(TGF-beta), VegT, beta-catenin, activin: Inducing nodal- related zygotic genes expression at the ventral mesoderm. Proteins transcribed as the signals toward the dorsal mesoderm.

Class 3 signals: Noggin, Chordin, frizbee,...
Signals from the Spemann organizer as dominant-negative receptors that block the action of ventralizing factors / class 4 signals.

Class 4 signals: BMP-4 (express uniformly throughout the late blastula), Xwnt-8 (exressed in the future mesoderm)
As ventralizing factors.

Spemann organizer:
1. To ventral mesoderm --- dorsalization of mesoderm and specify antero-posterior axis: Head or tail.
2. To ectoderm above --- neural induction of ectoderm: Two signal model (chordin and noggin) or Alternative model (FGF, Wnts and retinoic acid)?
3. To endoderm below --- anteriorization of endoderm