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 dorsal roots of the peripheral nervous system and the spinal cord. In rats older than 1 week, lesioned primary sensory axons regenerate within the 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 developed a culture model of the interaction of sensory axon growth cones with the intact DREZ, whereby dissociated dorsal root ganglion neurons from rats of various ages are grown on longitudinal cryosections of rat spinal cord, incorporating the DREZ and attached dorsal roots, from 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 the spinal cord depended on both their age and that of the spinal cord substrate. Neurites from neonatal neurons failed to cross either the P6 or adult DREZ, but a substantial proportion crossed the immature neonatal DREZ. Early embryonic neurites exhibited substantial crossing on both immature and adult DREZ. These findings strongly suggest that soon after birth, the normal mammalian DREZ acquires growth inhibitory activity that is recognized by the axons of postnatal but not early embryonic sensory neurons.