This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Anatomy
Primitive streak is initiated and maintained by? 
Select and explain. 
A. Nodal gene, B. BMP4, C. FGF, D. Brachyury gene
A: Nodal gene
Answer: A: Nodal gene 
Explanation: 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.