Crossed thalamo-cortical and cortico-thalamic projections in adult mice.

The crossed thalamo-cortical and cortico-thalamic connections of the mouse are drawn using the tracer wheat germ agglutinin-horseradish peroxidase. After injections in the frontal cortex of the right hemisphere cells labeled retrogradely and axons labeled anterogradely are observed in the thalamus ipsilateral and contralateral to the cortical injections. The retrograde and anterograde labeling in the contralateral thalamus is less intense than ipsilaterally and involves the mediodorsal, ventral medial, central medial, and paracentral nuclei. Crossed fronto-thalamic axons reach more lateral regions than those containing contralateral thalamo-frontal neurons. Our results demonstrate that the thalamo-cortical system of mice has a bilateral component. The functional significance of this pathway and analogies with crossed thalamo-cortical connections in other species are discussed.