Organizer (Spemann organizer)

The dorsal lip of the blastopore (an invagination that encircles the vegetal pole of the embryo) becomes a signaling center for differentiation (see Fig. O20). The formation of the organizer is preceded by induction of the mesoderm cell layer, resulting in the expression of organizer-specific homeobox genes and transcription of genes coding for signal molecules. This is followed by recruitment of the neighboring cells into the axial mesoderm and neural tissues. Several of the nuclear genes responsible for the component of the organizer have been identified by genetic and molecular means. The organization is controlled by proteins such as Wnt that determine the body axis and anterior posterior development. Activin and fibroblast growth factor (FGF) involve signal reception to organize the formation of the mesoderm or by noggin that controls dorsal/ventral differentiation. Chordin affects the development of the notochord, and follistatin is an antagonist of