Layer-specific sprouting of commissural fibres to the rat fascia dentata after unilateral entorhinal cortex lesion: a Phaseolus vulgaris leucoagglutinin tracing study.

After unilateral entorhinal cortex lesion commissural fibres to the inner molecular layer of the rat fascia dentata are said to sprout into the former termination zone of entorhinal afferents. This sprouting process has not yet been demonstrated at the level of individual fibres. In the present study, Phaseolus vulgaris leucoagglutinin tracing was used to analyse the commissural projection to the inner molecular layer in rats with longstanding entorhinal cortex lesions. In comparison with controls, the commissural fibre plexus in the inner molecular layer had expanded by 20-45 microns outwards on the side of the entorhinal lesion. Unexpectedly, only a small number of axons arising from the bulk of commissural fibres in the inner molecular layer left the main fibre plexus and entered the outer molecular layer. Thus, there was still a clearly recognizable border between the Phaseolus vulgaris leucoagglutinin-labelled commissural fibre plexus in the inner molecular layer and the unstained outer molecular layer. The few commissural axons invading the outer molecular layer rarely branched but formed multiple en passant boutons, and occasionally exhibited growth cones. The data indicate that only few commissural fibres appear to be able to sprout beyond the border of their appropriate layer suggesting that the characteristic laminar specificity of hippocampal afferents is largely retained following deafferentation.