A model for the specification of ventral neuronal fates in response to Sonic Hedgehog (SHH).
A. The developing spinal cord is influenced by two major signaling centers that affect the development of its columns: Bone morphogenic proteins (BMPs) from the ectoderm, and later from the roof plate, affecting the specification of the dorsal (alar) columns, and SHH, arriving from the notochord, and later from the floor plate, affecting the specification of the ventral (basal) columns.
B. SHH diffuses dorsally, creating a signal gradient, where each SHH concentration generates a different neuronal cell fate, including development of motor neuron progenitors.
Abbreviations: SHH, Sonic Hedgehog; BMP, Bone Morphogenic Proteins; FP, Floor Plate; N, Notochord. pV0-pV3, four different classes of ventral interneuron progenitors; pMN, Motor Neuron progenitors.