TRANSIENT PROJECTIONS FROM RAT OCCIPITAL CORTEX ARE ABLE TO RESPOND TO A SPINAL TARGET-DERIVED DIFFUSIBLE FACTOR IN-VITRO

Layer V pyramidal neurons in the occipital part of the rat cerebral co rtex project to both the cervical spinal cord and the tectum early in postnatal development. The occipito-spinal projection is transient and is subsequently withdrawn, while a permanent connection is maintained with the tectum. The withdrawal of the transient occipital corticospi nal axons may be due to their inability to respond to target-derived i nfluences. In the current study we co-cultured explants of the occipit al cortex and cervical spinal gray matter or tectum in 3-D collagen ge ls. Directional growth of the cortical axons towards either the cervic al spinal gray or tectal explant was observed. This indicates that the failure of neurons located in the occipital cortex to maintain collat erals within the spinal cord in vivo is not due to their inability to respond to a target-derived factor, but must be regulated by other ext rinsic factors.