Primitive streak is initiated and maintained by?

The primitive streak itself is initiated and maintained by expression of Nodal, a member of the transforming growth factor-b (TGF-b) family. Once the streak is formed, Nodal upregulates a number of genes responsible for formation of dorsal and ventral mesoderm and head and tail structures. Another member of the TGF-b family, bone morphogenetic protein 4 (BMP4), is secreted throughout the embryonic disc. Ref: Langman's Medical embryology 10/e, p 52-53.