The thalamo-fronto-striate system: ultrastructural evidence of appropriatesynaptic integration of embryonic neurones grafted within the frontal cortex of newborn rats

Previous light microscopical studies have indicated that fibres from the ve ntrolateral thalamic nucleus (VL) establish direct axo-somatic and axo-dend ritic presumed contacts with layers III and V neurones of the intact fronta l cortex projecting to the striatum. Additional experiments provided eviden ce that this thalamo-fronto-striate pathway could be partly reconstructed b y transplantation of embryonic frontal tissue into the damaged cortex. The present study was undertaken to validate these results at the ultrastructur al level. Several months after the transplantation of fetal frontal tissue into the damaged frontal cortex of newborn rats, a retrograde neurotracer ( subunit b of the cholera toxin) was used to label the grafted neurones proj ecting to the striatum whereas an anterograde neurotracer (Phaseolus vulgar is leuco-agglutinin) was used to label within the transplant, axons and ter minations arising from the VL. The same injection procedures were applied t o intact adult rats (control). The distribution of retrograde and anterogra de labellings within the intact cortex and within the graft was examined at light and electron microscopic levels to identify the synaptic contacts. O ur findings showed that labelled contacts were less numerous within the tra nsplant than within the intact cortex but their synaptic organization was s imilar: asymmetrical synaptic axo-dendritic and axo-somatic contacts. This synaptic articulation is probably supplied by a thalamic excitatory input. These results provide ultrastructural evidence of the capacity of a frontal cortical transplant placed in damaged frontal cortex of newborn rats to he lp reconstruction of appropriate synaptic integration within the thalamo-fr onto-striate system.