Lorna 356 Lectures 5,6,7

One- the dorsal lip

At the 16 cell stage a nucleus from one side escapes to the nuclear free half, and then proceeds to begin cleavage divisions on this second side. Eventually, two normal embryos form.

That the early amphibian nuclei are the same, and that each is capalbeof forming an entire embryo.

Abnormal differentiation of the blastomere which does not recieve the grey crescent. Ectoderm, mesoderm and endoderm are still present but no dorsal structures are, leading spemann to term this mass of cells the "belly piece"

Dorsal lip, committed to invaginate the blastule (and thus initiate gastrulation i think)

He Nieuwkoop center is created by the cytoplasmic rotation that occurs during fertilization (see Chapter 7). When this rotation is inhibited by UV light, the resulting embryo will not form dorsal-anterior structures such as the head or neural tube

The Xbra protein is a transcription factor that activates the genes that produce mesoderm-specific proteins.

Animal pole – epidermis and central nervous system Vegetal pole – endoderm (gut, lungs) Marginal zone – mesoderm

Vegetal cells secrete TGFbeta like factors such as Vg1 and VegT, a transcription factor that instructs the endoderm to secrete activin and Nodal proteins (all TGF-b family members).

The organizer is a group of cells that is fated to become dorsal mesoderm, and that can “instruct’ overlying tissues about how to behave. The steps in the formation of the organizer are to:- 1) tell the cells that they are dorsal, and 2) to tell the cells that they are mesoderm, and 3) to give the organizer cells inducing properties

The beta catenin signal and the nieuwkoop center signals.

β-catenin continues to accumulate preferentially at the dorsal side throughout early cleavage, and this accumulation is seen in the nuclei of the dorsal cells ; Schneider et al. 1996; Larabell et al. 1997). This region of β-catenin accumulation originally appears to cover both the Nieuwkoop center and organizer regions. During later cleavage, the cells containing β-catenin may reside specifically in the Nieuwkoop center (Heasman et al. 1994a; Guger and Gumbiner 1995).

β-catenin is necessary for forming the dorsal axis, since experimental depletion of β-catenin transcripts with antisense oligonucleotides results in the lack of dorsal structures

The injection of exogenous β-catenin into the ventral side of the embryo produces a secondary axis. β-catenin is part of the Wnt signal transduction pathway and is negatively regulated by the glycogen synthase kinase 3

GSK-3+axin+APC complex binds Beta catenin )(armadillo) and degrades it via phosphorylation. However, dorsally, Dishevelled binds GSK-3, allowing beta catenin to accumulate.