Neuroscience/Developmental Neurobiology/Neural Induction

The primary neural tissue is derived from the ectoderm, one of the three primary germ cell layers - the other two are the mesoderm and endoderm.

The notochord (also called the chordomesoderm) releases the two signaling molecules noggin and chordin, which act as inductive agents. When the notochord releases these inductive molecules, neural ectoderm is formed from the overlying surface ectoderm. Noggin and chordin antagonize the effects of Bone morphogenetic Protein 4 (BMP-4) - a growth factor in dorsal ectoderm that must be inhibited for ectoderm to become neural.

Research into the notochord has played a key role in understanding the development of the central nervous system. By transplanting and expressing a second notochord near the dorsal neural tube, 180 degrees opposite of the normal notochord location, one can induce the formation of motoneurons in the dorsal tube.

The notochord also secretes a protein called sonic hedgehog homolog (SHH), a key morphogen regulating organogenesis and having a critical role in signaling the development of motoneurons. The secretion of SHH by the notochord establishes the ventral pole of the dorsal-ventral axis in the developing embryo. As shh is secreted a gradient is formed. Different levels of shh cause different types of cells to be formed in the developing embryo. Motoneuron formation generally occurs in the ventral neural tube, while the dorsal tube generally forms sensory neurons.