Mechanisms of septal lamination in the developing hippocampus revealed by outgrowth of fibers from septal implants. I. Positional and temporal factors.

Pieces of embryonic septal tissue were implanted into a cavity within the entorhinal or occipital cortices of neonatal rat hosts (2-3 days old). The initial outgrowth of fibers originating from the grafts and their pattern of termination within the host hippocampal formation was examined using a histochemical stain for acetylcholinesterase (AChjE). Fimbrial transections were performed in conjunction with the implant surgery to eliminate native septohippocampal fibers. When placed in the occipital cortex, septal implants innervate specific laminar zones within the host hippocampus and dentate gyrus; these are the same laminar zones that receive septal efferents in the normal animal. Septal implants placed in the entorhinal cortex also produce a pattern of AChE staining in the host hippocampus which is similar to that seen in the normal hippocampus. In the dentate gyrus ipsilateral to the implant, however, the unstained commissural/association (C/A) zone is expanded. Temporal characteristics of the outgrowth of fibers from the implant were studied in animals sacrificed 6, 8, 10 and 12 days after the implant surgery. AChE activity indicative of innervation from the implant is present in 50% of the animals sacrificed 10 days post-implant; by 12 days post-implant all but one animal displayed AChE reaction product in the dentate molecular layer. AChE-positive processes emanating from cells in the implant were oriented toward the hippocampus throughout the developmental period. The position of the afferent source and the time of arrival of the afferent fibers in the target area do not seem to be critical to the mechanisms underlying the development of specific connection patterns in the septohippocampal system. We have proposed that contact between a 'critical afferent' (C/A system in this case) and the target cell might initiate the events underlying the development of appropriate patterns of connectivity.