Formation of synapses in cerebellar explants by axons from co-cultured medulla.

Explants of cerebellum from E12-E14 chick embryos were co-cultured with pieces of medulla from the same embryos for periods of up to 4 weeks and examined by EM with or without prior transection of the fibre bundles that formed rapidly between the co-cultured explants. The cerebellar explants developed a rich and complex neuropil within which a variety of presynaptic endings could be recognised, including some resembling climbing fibre endings and some mossy fibre endings. Four to six hours after transection of linking fibre bundles about 5% of preterminal and terminal profiles were undergoing intense degeneration indicating their origin from cell bodies in the adjacent explant of medulla and showing that most of the synaptic contacts are of intrinsic origin. The degenerating terminals contained spherical synaptic vesicles and made Gray Type I synaptic contacts. They appeared to be predominantly small-medium sized en passant terminals from long, thin, branched preterminal axons, and most closely resembled climbing fibre terminals.