Early dendritic protrusions of Fmr1 KO mice develop normally in vitro, except for a transiently elevated density at P4–7.

<p>(A) Low magnification view of L2/3 cells in an acute slice through the somatosensory cortex of a P11 WT mouse (maximum intensity projection of 40 optical slices, 3 µm apart). The cells were sparsely labeled with GFP via in utero electroporation and imaged with two-photon microscopy. The location of the pia is shown by a red line. The boxed region in blue is shown at higher magnification in panel B, right). (B) High magnification view of representative dendritic branches at the two postnatal ages examined. Images are best projections (∼5–11 optical sections, 1 µm apart). Dendrites were imaged every 60 seconds, but only a subset of time points is shown. From P5 to P11, thin protrusions that quickly appear and disappear (green and red arrowheads, respectively) are gradually replaced with more stable spines (yellow arrowheads), typical of mature dendrites, and which often have large swellings (heads) at their tips. (C) Length of dendritic protrusions at two different postnatal ages in WT and Fmr1 KO mice. Each gray square indicates a different dendrite from a WT mouse and each blue circle indicates a different dendrite from a KO mouse. Protrusion length did not change significantly during early postnatal development in WT or KO mice (p = 0.27 and p = 0.79, respectively). (D) Density of dendritic protrusions in WT mice increases by 43% between P4–7 and P8–P11 (^o p = 0.05). There was also a nearly significant trend towards higher protrusion density in older KO mice (^∓ p = 0.06). Compared to WT mice, dendrites in KO mice had a slightly higher density of protrusions at P4–P7 (* p<0.05). (E) Motility of dendritic protrusions decreases between P4–7 and P8–P11 in both WT and KO mice (* p<0.05 and ** p = 0.001, respectively). There were no significant differences in motility between WT and KO mice at either age. (F) Turnover ratio (TOR) of dendritic protrusions does not change significantly in WT mice (p = 0.15), but there was a trend toward lower turnover in older KO mice (^o p = 0.05). There were no significant differences in turnover between WT and KO mice at either age.