We provide evidence that prenatal removal of a rat forelimb results in both a disruption of the anatomical representation which would normally correspond to the forepaw and in an enlargement of the adjacent hindpaw representation in the brainstem and cortex. This enlargement, which in some cases is as much as 100%, only occurs following complete forelimb amputation on embryonic day 17 (E17) or earlier. This coincides with the age at which forepaw afferents first arrive in the brainstem, suggesting to us that the expansion is permitted in part because ingrowing hindpaw afferents are in the presence of cuneate cells which have never been previously innervated; in animals older than E17, the expansion is prohibited by either an intrinsic age-dependent change in the cuneate cells, or a change imposed upon them by forelimb afferents.
The number of cells in dorsal root ganglia subserving the expanded hindpaw areas does not differ from normal suggesting that the expansion of hindpaw territory within the brainstem reflects an increased terminal arborization by a normal complement of primary hindpaw afferents.
We interpret the cortical enlargement to be an upstream reflection of the brainstem events. In cortex, the enlargement seems to result from an invasion of the dysgranular cortex by thalamic afferents arising from the ventral posterior nucleus.