scored actual spine numbers rather than immunostained for synaptic markers. In addition, we used mouse instead of human APP to generate sAPP to validate the importance of sAPPα on synaptic morphology. Furthermore, we did not observe such rescue effects on spine number from sAPPβ conditioned medium. The different effects on spine number between sAPPα and β are intriguing as this supports the concept that a trophic domain is encoded in the 17 amino acids at the C-terminus of sAPPα that is lacking in sAPPβ (Mattson, 1997). Finally, we used a new 3-dimensional imaging technique to study dendrite and spine morphology more precisely and also systematically analyzed the spine volume and spine types in young and old APP−/− mice. This analysis has been shown to be much more accurate than the use of Golgi staining which is restricted in the level of resolution of the spines analyzed, as well as by the fact that spine number in Golgi preparations can only be reliably assessed in the x-y plane and omits the spines in the z-plane, thus resulting in a significant underestimation of spine density (Duan et al., 2003). Nevertheless, the results of our study support the hypothesis that APP is crucial
