Figure 2. Loss of persistent spines in HD. A, Time line for imaging and definition of spine categories. Imaging days are represented by 41, 49, 57, 65, 73, and 81. B, Time-lapse images of a dendritic branch, from a control Thy1-GFP-M mouse. Notice the presence of two types of spines, persistent spines (blue arrows) and transient spines (green arrows). Persistent spines are larger and contain a head, which is clearly separated from the dendrite by a thin neck. The transient spines are thin, filopodia-like, and do not survive until the next imaging session (green arrows). C, Time-lapse image of a dendritic branch from R6/2 mice. Blue arrows indicate persistent spines, red arrows indicate lost persistent spines, and green arrows mark transient spines. D, Average fraction of persistent spines in control and R6/2 mice. Throughout the examination period, R6/2 mice show lower fraction of persistent spines compared with controls. E, Average fraction of transient spines in control and R6/2 mice, which is greater than twofold in R6/2 mice (∼38%) compared with controls (∼16%). F, Time-lapse imaging of a dendritic branch from R6/2 mice. Green arrows indicate transient spines. Note the presence of several transient spines that appear and disappear throughout the entire imaging period in the R6/2 mice. In the control mice (B, green arrows) very few transient spines appear during the imaging period. G, Changes in brightness of persistent spines before their complete loss in R6/2 mice. Yellow arrow indicates loss of fluorescence in the persistent spine. Note that over the course of the imaging experiment, this dendritic branch lost most of its spines. Significant differences between groups are indicated by asterisks. Data are presented as mean ± SEM. Means were considered to be statistically significant if p < 0.05. N = 13 mice/group. Scale bar, 2 μm.