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Abstract
The Fragile X mental retardation syndrome is the largest source of inherited mental
retardation. The syndrome usually results from the transcriptional silencing of the
fragile X mental retardation gene (FMR1). To date the most prominent reported neuronal
abnormalities for the fragile X mental retardation syndrome include a higher density
of long thin spines similar to those found in sensory deprived and developing tissue,
suggesting a possible deficit in pruning of immature spines. Dendrites on spiny stellate
cells in the inner 1/3 of the barrel wall in layer IV of the rodent somatosensory
cortex have been shown to exhibit developmental pruning similar to that affecting
spines. To determine if FMRP plays a role in dendritic development, these neurons
were examined in two strains of adult FMRP knockout (FraX) mice. FraX mice in both
strains exhibited a greater amount of septa-oriented dendritic material, a morphology
consistent with pre-pruning status early in development. This observation suggests
that FMRP could be necessary for normal developmentally regulated dendritic pruning.