THE SENSORY INNERVATION OF THE MOUSE SPINAL-CORD MAY BE PATTERNED BY DIFFERENTIAL EXPRESSION OF AND DIFFERENTIAL RESPONSIVENESS TO SEMAPHORINS

To better understand the regulatory processes underlying axonal pathfi nding we analyzed the embryonic expression of seven murine semaphorin genes by in situ hybridization. In the spinal cord, transcripts of all seven semaphorin genes were detected from Embryonic Day 11.5 (E11.5) onward and restricted to distinct regions at E15.5. Interestingly, sem E, F, and G mRNAs were in addition differentially expressed in the ven tricular zone of the telencephalon. In order to correlate these expres sion patterns to the behavior of different types of sensory afferents, we tested their response to recombinant semaphorin proteins. Specific subpopulations of dorsal root ganglion sensory neurons displayed a de velopmentally regulated differential response to Sem D. Whereas extens ion of both NGF- and NT-3-dependent neurites was inhibited by Sem D at E12.5, only neurites formed in the presence of NGF responded at E14.5 . This suggests that Sem D may be involved in preventing an early pene tration of the spinal cord by sensory afferents and subsequently shapi ng their lamina-specific termination.