The major candidate molecule forming Nieuwkoop center is:
Dorsal vegetal cortex transport during cortical rotation accumulates Dishevelled, GBP and Wnt11 dorsally, inhibiting GSK-3 beta destruction complex, thereby stabilizing beta-catenin. Nuclear beta-catenin in dorsalmost vegetal blastomeres binds TCF3 activating homeobox genes Siamois and Twin, definitive markers of Nieuwkoop center. Ectopic injection of beta-catenin mRNA ventrally creates second Nieuwkoop center and double axis, while Wnt11 alone less sufficient. Beta-catenin acts as transcriptional coactivator essential determinant, not VegT or Noggin, establishing dorsal vegetal inductive center crucial for organizer induction via Nodal signals.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Beta-catenin as primary Nieuwkoop center candidate molecule.