Nieuwkoop Center

β catenin is located in the dorsal aspect of the blastula but overlaps with Nodal signalling which is located in the vegetal region of the egg (Fig IV). This overlap of Nodal and β catenin allows activation of the Siamois gene, creating the expression of the Siamois protein. This is a diffusible transcription factor and the cells expressing this molecule are defined as the Nieuwkoop center. The Nieuwkoop center is located in dorso-vegetal region of the blastula (FigV).

The Nieuwkoop center is involved in the dorsalizing the embryo. There are several experiments which show this:

  1. Division of the embryo into dorsal    and ventral halves;

  •  At 4 cell stage, the xenopus embryo is divided into dorsal and ventral halves.
  • Only the dorsal halve contains the nieuwkoop centre, therefore no dorsal signalling occurs in ventral region. 
  • As a result the ventral regions lack the dorsal and anterior structures e.g neural tissue.
  • Also the dorsal region does not recieve and ventral signals and therefore no ventral structures e.g. gut.
  • This shows that the nieuwkoop center is necessary for the dorsalising embryo.

2. Ectopic Nieuwkoop center.

 

  • Transplant a donar Nieuwkoop center to the ventral region of a reipient xenopus embryo.
  • During development of the embryo, and twinned embryo forms with 2 dorsal regions.
  • This shows that the Nieuwkoop center is suffiecent to induce the dorsal aspects of the embryo.

FIG IV: Nodal & Brachyury Expression.

 FIG IV: Nodal and Brachyury expression, creating a brachyury belt around a blastula.

  FIG V: Nieuwkoop Center. 

 

FIG V: Neiuwkoop center formation due to nodal and βcatenin overlap.