Dorsal-ventral patterning: a view from the top.

The generation of dorsal interneurons in the spinal cord is dependent upon specific signaling pathways and the subsequent establishment of progenitor domains mediated by cross-repressive interactions of different groups of transcription factors. These events lead to the implementation of specific differentiation programs that direct the development of distinct dorsal interneuron subtypes. Recent studies have taken advantage of complementary gain and loss-of-function studies in the chick and mouse to clarify the in vivo roles of transforming growth factor beta signaling, basic helix-loop-helix and homeodomain transcription factors in dorsal interneuron development. The challenge now lies in identifying the precise molecular mechanisms involved and applying these insights to understanding how more ventrally located dorsal interneurons are specified.