Development and elimination of presynaptic elements on polylysine-coated beads implanted in neonatal rat cerebellum

Polylysine-coated sepharose beads were implanted in the cerebellum of neonatal rats and examined at 3 hr, 3 days, 7 days, 14 days, and 21 days after surgery. Previous studies at 5 or 8 days after implantation showed that axons formed neuronal swellings that appeared to be presynaptic elements, with the bead surface in the position of a postsynaptic element. Results reported here show that no beads at 3 hr had presynaptic elements, whereas the number of beads with presynaptic elements increased to high levels at 3 and 7 days but dropped to low values at 14 and 21 days after implantation. Presynaptic elements were seen on beads regardless of their distance from cerebellar tissue except at 3 hr, when no axons were seen in the implant, indicating that axons first grew into the implant and then formed presynaptic elements. The morphological measurements of presynaptic elements on beads at 3 to 7 days after implantation showed increases in area and number of synaptic vesicles, which then decreased at 14 and 21 days after implantation. These results show that axons can grow into implants of polylysine-coated beads and form presynaptic elements that do not survive with increased time after implantation. The survival of presynaptic elements on beads can be used as a model for investigations into regeneration of axons and presynaptic elements in the injured brain.