Figure 1. Diagram of collateral sprouting in the septal nucleus, as described by Raisman and Field. (a) Lateral septal nucleus neurons receive an afferent input from the hippocampus via the fimbria, whose axons terminate exclusively on dendrites. A second projection, via the medial forebrain bundle (MFB), originates in the ventral tegmental area and lateral hypothalamus and terminates on both dendrites and somata of lateral septal nucleus neurons. (b) After lesioning the MFB, fimbrial axons undergo collateral sprouting and project to the somata as well as the dendrites of lateral septal nucleus neurons. From Seil FJ (1989) Neural Regeneration and Transplantation. Frontiers of Clinical Neuroscience, vol. 6. New York: Alan R Liss, with permission.

Figure 2. Diagram of collateral sprouting and laminar reorganization in the adult hippocampus, as described by Cotman, Lynch and co-workers. (a) Granule cells in the hippocampus receive a dense input from the ipsilateral entorhinal cortex (Ipsi EC, solid lines) and from the hippocampal CA4 pyrimidal cells (CA4 C/A, solid lines) bilaterally. In addition, there is sparse input from the contralateral entorhinal cortex and septal nuclei (Contra EC and septum, dashed lines). Entorhinal cortex fibres project to the outer 75% of the granule cell dendritic tree, while CA4 fibres project to the inner 25%. (b) After destruction of the ipsilateral entorhinal cortex, CA4 fibres, septal fibres, and fibres from the contralateral entorhinal cortex all undergo collateral sprouting and form new synapses on granule cell dendrites, replacing lost synapses. CA4 fibres extend their projection to the inner 35% of the dendritic tree by sprouting partially into the entorhinal fibre zone. From Seil FJ (1989) Neural Regeneration and Transplantation. Frontiers of Clinical Neuroscience, vol. 6. New York: Alan R Liss, with permission.