Fig. 4. Disruption of actin cable assembly results in pouches with aberrant morphology, owing to a failure in proper proximodistal elongation. Effects of cytochalasin D treatment on pouch morphogenesis analysed through (A-C) Bmp7; (D,F) Pax1; or (G,H) Dlx2 in situ hybridization. (A,B) Left- and right-hand side views of the same embryo that had cytochalasin D injected into the pharyngeal cavity at stage 11+, prior to pouch formation, and the embryo allowed to develop to stage 15–. As the chick embryo turns, gravity causes cytochalasin D to accumulate over the left-hand side (LHS) tissue, which results in aberrant pouch morphology on that side (A) but not on the right-hand side (RHS) (B). LHS pouches have an open diamond shape compared with the narrow, slit-like pouches on the RHS or in the DMSO control (C) embryo treated in the same manner. (D) Side view of an embryo where cytochalasin D was injected into the vicinity of the first pouch (1pp) at stage 15–, and allowed to develop to stage 19. (E) High magnification of the contralateral first pouch, which did not receive an injection of cytochalasin D, displaying normal slit-like morphology. The dots outline the contours of the pharyngeal endoderm. (F) High magnification of a first pouch that did receive a cytochalasin D injection showing a contorted morphology of the pharyngeal endoderm along the proximodistal axis. Again, the dots outline the contours of the pharyngeal endoderm. (G) Side view of an embryo, treated with cytochalasin D by injection into the pharyngeal cavity at stage 12 and allowed to develop to stage 18 and (H) an embryo similarly treated with DMSO at stage 13 and allowed to develop to stage 19. In both G and H, Dlx2 expression shows the arches have been properly populated with neural crest cells. OV, otic vesicle. Anterior is towards the left.