Neurofascin B Regulates Myelin Targeting during the Critical Period of Sheath Formation and Retraction(A–A″′) Single time-point images of an individual oligodendrocyte in a wild-type animal as its myelinating processes exhibit several distinct dynamic activities over time: 1=sheath formation, 2=slow sheath growth, 3=rapid sheath growth, 4=exploratory process retraction, 5=engagement with a cell body, 6=retraction from a cell body, 7=retraction from an axon. Scale bar, 10 μm.(B and B′) Single time-point confocal images of an individual oligodendrocyte in a wildtype animal as it is forming myelin sheaths at the beginning (A) and toward the end (A′) of the critical period. Scale bar, 10 μm.(C and C′) Single time-point confocal images of an individual oligodendrocyte in a nfascb MO-injected animal as it is forming myelin sheaths at the beginning (A) and toward the end (A′) of the critical period. Note that a cell body has become myelinated during this time (arrows). Scale bar, 10 μm.(D–D″′) Time series showing myelination of a cell body in a nfascb MO-injected animal. Scale bar, 10 μm.(E–E″′) Time series showing a myelinating process being retracted from a cell body in a control. Scale bar, 10 μm.(F) Number of myelin sheaths generated on axons per oligodendrocyte during time-lapse analyses of control and nfascb MO-injected animals (control mean 13.50 ± 2.91 SD, n = 20 oligodendrocytes from 13 animals, nfascb MO mean 12.35 ± 3.30 SD, n = 34 oligodendrocytes from 17 animals, p = 0.2041, t test).(G) Graph showing the duration between the first formed and the last formed myelin sheath by individual oligodendrocytes in control and nfascb MO-injected animals (control mean 3.55 ± 1.45 SD, n = 13 oligodendrocytes from 10 animals, nfascb MO mean 4.11 ± 2.30 SD, n = 29 oligodendrocytes from 21 animals, p = 0.4249, t test).(H) Time of cell body myelination during time-lapse of control (none were myelinated) and nfascb MO-injected animals. Cell bodies are myelinated by Nfasc B-depleted cells during the same period as axons.(I) Number of myelin sheaths retracted from axons per oligodendrocyte during time-lapse of control and nfascb MO-injected animals (control mean 2.81 ± 1.60 SD, n = 16 oligodendrocytes from 13 animals, nfascb MO mean 2.81 ± 1.27 SD, n = 26 oligodendrocytes from 17 animals, p = 0.7463, Mann–Whitney test).