Bilateral thalamocortical projection in hedgehogs: evolutionary implications

In adult hedgehogs with large unilateral cortical deposits of fluorescent somatopetal tracers, labelled perikarya were found not only in the ipsilateral but also contralateral thalamus. An exceptionally large number of contralaterally labelled neurons was seen in the ventrolateral nucleus, also at a considerable distance from the midline. Deposits of one of two different tracers in the frontoparietal cortex of each hemisphere appear to label different perikarya in each ventrolateral nucleus. This projection to the contralateral cortex in hedgehogs does not resemble thalamo-cortical connections in either adult or developing brains of other mammalian species. Among amniotes, only in pigeons have contralateral projections from the thalamus to the telencephalon been described. The somatosensorimotor system of hedgehogs may be the only known mammalian remnant of primitive vertebrate thalamocortical organization. Whether primitive or derived, the bilateral thalamocortical projection in hedgehogs shows that hedgehog brains cannot be uncritically taken to represent brains of primate ancestors.