Factors affecting survival and outgrowth from transplants of entorhinal cortex

The purpose of this study was to examine injury-related effects on the survival and growth of entorhinal tissues transplanted to the adult rat entorhinal area. Embryonic entorhinal cortex was transplanted to the angular bundle region of adults either immediately, or 8-10 days, after severing specific host projections. Graft survival (Nissl stain) and connectivity (acetylcholesterase stain and retrograde labeling with wheat germ agglutinin-horseradish peroxidase) were examined two months post-transplantation. Grafts transplanted 8-10 days after severing the angular bundle were large and contained many cells which innervated the hippocampal formation. Grafts transplanted immediately after severing the angular bundle were small, did not integrate well with the host tissues, and failed to innervate the hippocampal formation. Grafts transplanted without producing any prior lesion, or following lesions which did not damage host entorhinal projections, were intermediate in size, but failed to innervate the hippocampal formation. The data demonstrate that: (1) introducing a delay between the lesion and implant surgeries can significantly enhance graft survival and the establishment of transplant-to-host projections, and (2) transplant-derived innervation of the host is significantly impaired when host homologous fibers are intact. These findings suggest that environmental factors, induced specifically by the destruction of host homologous fibers, are responsible for the differences in transplant survival and connectivity observed.