Synapses formed by ectopic corticofugal axons: an electron microscopic study of crossed corticorubral projections in kittens.

The corticorubral projections in newborn kittens are bilateral, while the projections are unilateral in adults. We addressed the question whether or not the crossed corticorubral projection in kitten forms synaptic contacts in the red nucleus. The neurons in the sensorimotor cortex of the kitten were labeled by Phaseolus vulgaris-leucoagglutinin or biocytin. Electron microscopic observations revealed that corticofugal axons form synapses both in the contralateral and the ipsilateral red nucleus; most of them were on small postsynaptic profiles, possibly dendritic spines or distal dendrites.