AXONS FROM THE OLFACTORY-BULB TRANSPLANTED INTO THE HIPPOCAMPAL-FORMATION SHOW AXON PREFERENCES AND FORM TRANSPLANT-TO-HOST SYNAPSES SIMILAR TO THOSE OF NORMAL AFFERENTS

Using a mouse Thy-1 allelic system (an AKR strain of Thy-1.1 was used as the host and a BALB/c strain of Thy-1.2 as the graft), characterist ics of transplant-to-host axon projection and synapse formation of the olfactory bulb (OB) were studied in the hippocampal formation by immu nohistochemical and ultrastructural examinations. Thy-1.2-positive axo n projections were most prominent in the dentate molecular layer (DM), although there was a labeled-axon-poor layer in the innermost part of the DM. Some of the axons in the DM entered the hippocampus proper, f orming two clear borders between the two axon-rich outer layers and th e inner axon-poor central layer. These results suggested that axons fr om the transplanted OB extended into the hippocampal formation, showin g axon preferences. In the ultramicroscopic observation of the DM, thr ee different synaptic patterns could be distinguished among the transp lant-to-host synapses. These three synaptic patterns are similar to th ose of the synapses formed by perforant path fiber terminals in the DM , indicating that the transplanted OB forms fiber connections with the dendrites of the dentate granule neurons similar to those of normal a fferents.