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

Previous light microscopical studies have indicated that fibres from the ventrolateral thalamic nucleus (VL) establish direct axo-somatic and axo-dendritic presumed contacts with layers III and V neurones of the intact frontal cortex projecting to the striatum. Additional experiments provided evidence that this thalamo-fronto-striate pathway could be partly reconstructed by transplantation of embryonic frontal tissue into the damaged cortex. The present study was undertaken to validate these results at the ultrastructural 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 projecting to the striatum whereas an anterograde neurotracer (Phaseolus vulgaris leuco-agglutinin) was used to label within the transplant, axons and terminations arising from the VL. The same injection procedures were applied to intact adult rats (control). The distribution of retrograde and anterograde labellings within the intact cortex and within the graft was examined at light and electron microscopic levels to identify the synaptic contacts. Our findings showed that labelled contacts were less numerous within the transplant than within the intact cortex but their synaptic organization was similar: 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 help reconstruction of appropriate synaptic integration within the thalamofronto-striate system.