TROPISM AND CORTICOSPINAL TARGET SELECTION IN THE RAT

Layer V pyramidal cells in the intermediate part of the cerebral corte x enter the lumbar spinal gray, but not the cervical spinal gray matte r, during the first postnatal week. To study if the ingrowth of interm ediate 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 i n 3-D collagen gels. Using this test system, a target specific directi onal growth of cortical axons towards the lumbar spinal gray explant c an be demonstrated in vitro. Retrograde labeling indicates that most l abeled cell bodies were located in layer V of the cortex explant and w ere characterized by a pyramidal cell shape. Furthermore, axon behavio r of retrogradely labeled neurons within the cortical explant is consi derably affected by the presence of lumbar spinal gray target tissue. In contrast to lumbar spinal gray innervation, intermediate corticospi nal tract axons do not enter the cervical spinal gray in vivo. Is it t he inability of intermediate corticospinal tract axons to respond to c ervical target-derived influences? In the current study we co-cultured explants of the intermediate cortex and cervical spinal gray matter i n 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 recogn izing and responding to a diffusible factor released by the cervical s pinal cord target area. This suggests that the failure of neurons with in the intermediate cortex to enter the cervical spinal gray in vivo, is not due to their inability to respond to a target-derived diffusibl e factor, but probably regulated by other extrinsic factors.