Fig. 6. Loss of tcf3b function affects brain morphogenesis. All embryos are shown at 24 hours post-fertilization (hpf), rostral towards the left. (A) Alexa Fluor 594 phalloidin staining reveals forming rhombomere boundaries (arrowheads). (B) In a tcf3b MO-injected embryo, no boundaries are visible and ectopic tissue is present dorsally (arrows). (C) At higher magnification, cell shape changes are apparent at rhombomere boundaries (asterisks) in an uninjected embryo. (D) These features are absent in an injected embryo. (E) wnt1 is expressed at rhombomere boundaries (arrowheads). (F) This expression is disorganized following tcf3b MO injection, and ectopic expression is present medially (arrows). (G) mariposa is normally expressed at ventral rhombomere boundaries (arrowheads). (H) In injected embryos, mariposa expression is uniform and no boundary expression is visible. (I) en2 expression is present throughout the entire midbrain-hindbrain boundary (mhb) when viewed as an optical cross-section. Lateral expression is in mesoderm. (J) After tcf3b MO injection, the mhb fails to close at the dorsal (left) side. (K,L) When viewed dorsally, the rostrocaudal size and position of en2 expression is normal in injected embryos. (M,N) The positions of rhombomeres 3 and 5 are also normal, as marked by krox20 expression. (O,P) isl1 expression indicates that neurogenesis is grossly normal in tcf3b MO-injected embryos. ov, otic vesicle.