Quantification of layer 5 B (L5B) pyramidal cell dendritic spine density and turnover. (a) Example of spines in somatosensory cortex, imaged through a chronic cranial window 1 d after surgery and 20 d later. Examples of some spines that have disappeared (arrows) and appeared (arrowheads) over this period are indicated. (b) Spine densities vary from cell to cell under the cranial window (black (data from A.H., V.D.P., K.S., L.W.), red (data from R.M., C.P.-C.) and blue (data from T.B., S.B.H., M.H., T.K., T.D.M.-F.)) as well as in the naive cortex (green (data from A.H., J.C., G.K.)), and remain relatively stable on an average over several weeks of imaging. (c) Example of spines that have disappeared (arrows) and appeared (arrowheads) in visual cortex, imaged through a chronic cranial window 2 d after surgery and 12 d later. (d) Average survival fractions of spines. Open markers represent survival fractions in experiments where imaging was started immediately after the surgery, solid markers represent experiments that included a waiting period of 2 weeks after the surgery (black points represent data from earlier studies). (e) Turnover ratio (TOR) over 4 d imaging intervals. Imaging was started immediately after the surgery (red, same mice as in b) or after a 2-week waiting period (black and blue, data from different studies). TORs are, on an average, constant over extended periods of time (> 90 d). All experiments using animals were carried out under institutional and national guidelines.