Rapid growth of parallel fibers in the cerebella of normal and Staggerer mutant mice

The growth of cerebellar granule cell axons was examined by placing focal implants of 1,1′,dioctadecyl‐3,3,3′,3′‐tetramethyl‐indocarbocyanine perchlorate (DiI) in the cerebella of normal and staggerer mutant mice at a series of developmental ages between postnatal day 2 (P2) and P30. Parallel fibers contacting the implant site were brightly labeled by the fluorescent dye, as were the associated granule cell bodies located principally in the internal granule layer. The extent of parallel fiber labeling in the molecular layer and the distance from the implant to the most extreme labeled granule cells were measured in sectioned material. Two additional measures describing the distribution of labeled granule cells about the implant site suggest length bounds for most parallel fibers. Parallel fiber growth is surprisingly rapid; all measures approached peak values at P3–P5, only a few days after the earliest postmitotic granule cells differentiate and migrate. At intermediate ages (P8 and P10), parallel fiber lengths appeared to decrease transiently. At later ages (P15 and beyond), the measures of fiber length increased to their mature values. These values differed little from lengths measured at P3–P5, suggesting that most parallel fiber growth occurs within a few days of cell birth. At early and intermediate ages, parallel fiber lengths in staggerer mice were comparable to controls, suggesting that an interaction with normal healthy Purkinje cells is not essential for parallel fiber outgrowth. J. Comp. Neurol. 389:642–654, 1997. © 1997 Wiley‐Liss, Inc.