Developmental pattern and structural factors of dendritic survival in cerebellar granule cells in vivo.

Granule cells (GCs) in the cerebellar cortex are important for sparse encoding of afferent sensorimotor information. Modeling studies show that GCs can perform their function most effectively when they have four dendrites. Indeed, mature GCs have four short dendrites on average, each terminating in a claw-like ending that receives both excitatory and inhibitory inputs. Immature GCs, however, have significantly more dendrites-all without claws. How these redundant dendrites are refined during development is largely unclear. Here, we used in vivo time-lapse imaging and immunohistochemistry to study developmental refinement of GC dendritic arbors and its relation to synapse formation. We found that while the formation of dendritic claws stabilized the dendrites, the selection of surviving dendrites was made before claw formation, and longer immature dendrites had a significantly higher chance of survival than shorter dendrites. Using immunohistochemistry, we show that glutamatergic and GABAergic synapses are transiently formed on immature GC dendrites, and the number of GABAergic, but not glutamatergic, synapses correlates with the length of immature dendrites. Together, these results suggest a potential role of transient GABAergic synapses on dendritic selection and show that preselected dendrites are stabilized by the formation of dendritic claws-the site of mature synapses.