Figure 4. Some FBM neurons migrate rostrally following inactivation of Celsr1 in r3 and r5.

A, E, F, Dorsal views of E12.5 embryos visualized live for GFP (A) or processed for Tbx20 ISH (E, F). GFP expression in r3 and r5 in a Krox20^Cre; ROSA26^mT/mG embryo (A) labels the expression domain of Cre used for the r3/r5-specific knockout of Celsr1 function (r3/r5-cKO). B-D, 30-μm coronal sections of an E10.5 wild type hindbrain processed for anti-GFP (green) and anti-Isl1 (red) immunostaining to mark Cre expression (GFP), and FBM neurons (arrow), respectively. The light red background signal (asterisk) is due to mTomato expression in all tissues of the ROSA26^mT/mG mouse where Cre is not active. In r3 (B) and r5 (D), Cre activity is ubiquitous, including in floor plate cells. In r4 (C), Cre is inactive except for a few cells along the pial surface, which was seen in all sections (5/5 embryos). E, F, In control embryos (Krox20^Cre; Celsr1^fl/+ or Krox20^Cre; Celsr1^KO/+, 18/18 embryos), Tbx20 labels trigeminal BM neurons (nV) in r2 and r3, and FBM neurons (nVII) migrating caudally from r4 to r6 (E). A small but significant number of FBM neurons migrate rostrally into r2 in several r3/r5-cKO embryos (F, white arrowheads) (Krox20^Cre; Celsr1^KO/fl or Krox20^Cre; Celsr1^fl/fl, 3/12 embryos).