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Fig. 1 FBMNs undergo a change in velocity upon contact with the Laminin-containing ventral basement membrane. (A) A maximum intensity projection of a confocal Z-stack of a tg(hoxb1aBAC:mRFP1); tg(isl1:GFP) double transgenic embryo at 48 hpf reveals that most Isl1:GFP positive neurons in both r6 and r7 are also hoxb1a:RFP positive, indicating that they were born in r4. (B) A single XY section of a tg(zCREST1:memb-mRFP1);tg(krox20BAC:GFP) double transgenic embryo at approximately 24 hpf showing the measurement from the posteriormost extent of the FBMN cell body to the r4-r5 boundary (white bar). (C) Plots of the extent of migration of single FBMNs versus time elapsed generated from timelapse movies of a tg(zCREST1:memb-mRFP1);tg(krox20BAC:GFP) double transgenic embryo starting at approximately 24 hpf. In the left graph, points are best fit by a bilinear model rather than a single line, indicating that the FBMN undergoes a single change in velocity while the right graph is equally well fit by either model, indicating a constant velocity. (D) Plots of extent of migration versus time re-centered so that the origin of the graph corresponds to the point of contact with the r4-r5 boundary (negative timepoints and distances indicate migration through r4 while positive timepoints and distances indicate migration through r5). FBMN#1 to FBMN#7 are those best fit by a bilinear model while FBMN#8 is an example of one best fit by a single line. CP, changepoint, the point at which the slope changes in the bilinear model (red circle); v1, initial slope; v2, slope after changepoint. (E-G) Reconstructed YZ sections through 24-hpf isl1:GFP embryos stained for Laminin (E) or Laminin (red) and α Efnb2a (yellow) (F) reveal that FBMNs undergo a ventral migration in r4 and come into close apposition with the basement membrane just after crossing the r4-r5 boundary, after which point their migration is purely posterior. (G) The angle of the ventroposterior early migration with respect to the basement membrane (θ) is measured by defining a right triangle with vertices at the middle of the anteriormost cell, the point of contact with the basement membrane and the basement membrane as one of the legs.