Cleavage is faster in the pigmented animal pole. At this stage, the animal pole cells are fated to become epidermal tissue, the marginal zone will become mesodermal and neural tissue and the yolky vegetal cells are specified to an endodermal fate. Gastrulation is initiated at the future dorsal side of the embryo, underneath the organiser region. A blastopore is formed by the invagination of local endodermal cells. The animal pole cells undergo epiboly and converge at the blastopore.
Arrows indicate major cell movements.
Mesoderm induction Mesoderm tissue formation is fundamental to the patterning of the Xenopus embryo. The three signal model of mesoderm induction. A ventral signal is released by all vegetal cells, inducing ventral mesoderm in the overlying tissue. The Nieuwkoop centre releases a signal to induce the dorsal organiser mesoderm. Extensive research has been carried out to identify the growth factors and molecular interactions that constitute mesoderm-inducing signals.