Regeneration of the septohippocampal pathways in adult rats is promoted by utilizing embryonic hippocampal implants as bridges

The ability of embryonic hippocampal tissue to promote regeneration of cholinergic axons in the septohippocampal system has been studied in adult rats. Strips of embryonic hippocampus, taken from 7–40 mm rat fetuses, were implanted into a 2–3 mm wide cavity which completely transected the septal cholinergic axons innervating the intrinsic hippocampus. The ingrowth of cholinergic fibres into the denervated host hippocampal formation was monitored by measuring the activity of the enzyme, choline acetyltransferase (ChAT), and by acetylcholine esterase (AChE) histochemistry. The results demonstrated a gradual, partial return of both ChAT enzyme activity and AChE-positive fibres in the initially denervated hippocampal formation of the adult recipient. Time-course studies indicated that this ingrowth progressed from the implant into the rostral tip of the host hippocampus, and continued caudally to cover the entire dorsal hippocampus by 3–6 months post-operative Although the regenerating AChE-positive fibres reached the hippocampal target in the recipient along abnormal routes, they reinnervated selectively the appropriate terminal areas within the host hippocampus and dentate gyrus, suggesting the presence of quite specific mechanisms to guide the regenerating axons back to their original targets. Lesions of the medial septum-diagonal band area of the host and horseradish peroxidase (HRP) injections into the host hippocampus, caudal to the implant, indicated that the origin of the regenerating axons was predominately from the ipsilateral ventral medial septum and diagonal band area of the host. The results provide evidence that axonal regeneration and reinnervation of a denervated target zone can be promoted by utilizing implants of embryonic CNS tissue to bridge a tissue defect between the target and the lesioned axonal stumps.