TROPISM AND CORTICOSPINAL TARGET SELECTION IN THE RAT

Layer V pyramidal cells in the intermediate part of the cerebral cortex enter the lumbar spinal gray, but not the cervical spinal gray matter, during the first postnatal week. To study if the ingrowth of intermediate corticospinal axons into the lumbar spinal gray is guided by a diffusible tropic factor, we co-cultured explants of the intermediate part of the sensorimotor cortex and of the lumbar spinal gray matter in 3-D collagen gels. Using this test system, a target specific directional growth of cortical axons towards the lumbar spinal gray explant can be demonstrated in vitro. Retrograde labeling indicates that most labeled cell bodies were located in layer V of the cortex explant and were characterized by a pyramidal cell shape. Furthermore, axon behavior of retrogradely labeled neurons within the cortical explant is considerably affected by the presence of lumbar spinal gray target tissue. In contrast to lumbar spinal gray innervation, intermediate corticospinal tract axons do not enter the cervical spinal gray in vivo. Is it the inability of intermediate corticospinal tract axons to respond to cervical target-derived influences? In the current study we co-cultured explants of the intermediate cortex and cervical spinal gray matter in 3-D collagen gels. Our data indicate that in vitro axons from layer V neurons in the intermediate part of the cortex are capable of recognizing and responding to a diffusible factor released by the cervical spinal cord target area. This suggests that the failure of neurons within the intermediate cortex to enter the cervical spinal gray in vivo, is not due to their inability to respond to a target-derived diffusible factor, but probably regulated by other extrinsic factors.