We next quantified dendritic spine density from apical dendrites in the microinjected neurons in CA1 (Fig. 3A) because our previous analysis detected morphological changes in apical dendrites only. Spine number was decreased by a small but significant 15% in old APP−/− mice compared to old APP+/− mice (Fig. 3B). In young mice, however, there was no reduction in spine density in young APP−/− compared to young APP+/− mice (Fig. 3B). In young mice, however, there was a non-significant trend in reduction in spine density in young APP−/− compared to young APP+/− mice (Fig. 3B, p > 0.05). Further analyses showed no changes in three types of dendritic spines (thin, stubby, and mushroom) classified using the Rayburst-based shape analysis algorithm (Rodriguez et al., 2006). Thus, the reduction in dendritic spine density in old APP−/− mice apparently extended through all spine subtypes (Fig. S2 and Table 1). There was also no difference in mean spine volume for all spines and subtypes in old APP−/− mice compared to controls. However, we did find a significant difference in spine volume in old APP−/− mice compared to young mice (p = 0.02, old APP−/− vs. young APP−/−).
