AN IN-VITRO MODEL OF THE RAT DORSAL-ROOT ENTRY ZONE REVEALS DEVELOPMENTAL-CHANGES IN THE EXTENT OF SENSORY AXON GROWTH INTO THE SPINAL-CORD

The dorsal root entry zone (DREZ) forms the junction between the dorsa l roots of the peripheral nervous system and the spinal cord. In rats older than 1 week, lesioned primary sensory axons regenerate within th e dorsal roots but stop at the DREZ, and are thus unable to reconnect with the spinal cord. To analyze the causes of this failure, we have d eveloped a culture model of the interaction of sensory axon growth con es with the intact DREZ, whereby dissociated dorsal root ganglion neur ons from rats of various ages are grown on longitudinal cryosections o f pat spinal cord, incorporating the DREZ and attached dorsal roots, f rom neonatal, 1-week-old (P6), or adult animals. Neurites of all ages grew along the roots to the DREZ, where their ability to cross into th e spinal cord depended on both their age and that of the spinal cord s ubstrate. Neurites from neonatal neurons failed to cross either the P6 or adult DREZ, but a substantial proportion crossed the immature neon atal DREZ. Early embryonic neurites exhibited substantial crossing on both immature and adult DREZ. These findings strongly suggest that soo n after birth, the normal mammalian DREZ acquires growth inhibitory ac tivity that is recognized by the axons of postnatal but not early embr yonic sensory neurons.