New Insights on Neural Circuit Refinement in the Central Nervous System: Climbing Fiber Synapse Elimination in the Developing Mouse Cerebellum Studied With Serial-Section Scanning Electron Microscopy

Abstract

For many vertebrate animals, the nervous system undergoes a drastic rewiring between birth and adulthood. This process, known as synapse elimination, is essential to normal development, but little is understood about the mechanisms that make it work, especially in the central nervous system. One reason for this lack of information is that the techniques traditionally used to probe neuronal connectivity are limited in the kinds of data they can capture. In this work, I have used high-throughput electron microscopy to circumvent these limitations and study the initial phase of the synaptic reorganization that occurs in the developing cerebellum, by tracing out the synaptic connections between climbing fibers and Purkinje cells in early postnatal mice. My analysis shows that many more climbing fibers innervate developing Purkinje cells than previously estimated, and that these climbing fibers preferentially add synapses onto Purkinje cells that they already strongly innervate, so that by the end of the first postnatal week, typically one or at most a few climbing fibers dominate each cell. In addition, my findings show that whereas some climbing fiber axons are adding many synapses, weak inputs persist. These results argue that the eventual dramatic phase of climbing fiber synapse elimination does not occur until a dominant climbing fiber is designated for each Purkinje cell. Finally, I present a new morphological feature of immature Purkinje cells that I discovered through my reconstructions.