Induction of corticospinal target finding by release of a diffusible, chemotropic factor in cervical spinal grey matter

Abstract

The outgrowth of corticospinal tract axons in rat spinal cord primarily occurs during the first postnatal week. Axons originating from a group of layer V pyramidal cell bodies situated in the anterior part of the cerebral sensorimotor cortex project mainly to the cervical gray matter (Joosten et al., Dev. Brain Res., 36 (1987) 121–130). By co-culturing explants of the anterior part of the sensorimotor cortex and of cervical spinal gray matter in 3-D collagen gels, a target-specific directional growth of cortical axons towards the cervical spinal gray explant could be demonstrated. After retrograde filling with the fluorescent tracer 1,1-dioctadecyl-3,3,3′,3′-tetramethyl indocarbocyanine perchlorate (DiI), in vivo as well as in vitro, most of the DiI-labelled cortical neurons were located in layer V of the cortical explant, and were characterized by a pyramidal shape. These data suggest that the cervical spinal gray matter target area becomes innervated by corticospinal axons through the release of a diffusible chemotropic factor.