Fig. 9

Model: Bmp activity gradient specifies convergence and extension movement domains. Thresholds of Bmp activity (dashed gray lines) specify three CE movement domains. (A) In WT, high ventral Bmp levels specify NCEZ (red). An intermediate level of Bmp activity specifies increasing convergence and extension, where cells initially converge and extend slowly (light blue) and then at a faster speed (dark blue). The transition point from slow to fast convergence is unclear (question mark; light blue to dark blue). In contrast, low levels of Bmp activity specify generous extension with limited convergence (yellow). In DV patterning mutants, these movement domains are altered according to changes in the Bmp activity gradient. In ventralized (B) din and (C) boz din, the NCEZ is enlarged with increased Bmp activity, while convergence and extension are reduced in lateral regions. The domain of increasing lateral convergence and dorsal convergent extension are also reduced in din and may be absent in boz din double mutants. (D) However, in sbn, low levels of Bmp activity around the circumference of the embryo promote strong extension and low convergence in all regions. The distribution and degree of these morphogenetic movements shapes the early segmentation embryo. (E) In WT, NCEZ cells move to the tailbud, while lateral movements sweep cells toward the dorsal domain, where CE drives the elongation of the axis (yellow). (F, G) In din and boz din, due to reduced lateral convergence and extension, fewer cells contribute to the anterior structures, while expanded NCEZ creates enlarged tailbud, increasing posterior structures at the expense of head and trunk. (H) In sbn, circumferential CE movements elongate the An-Vg axis, and may eventually impair the extension of the embryonic axis. Anterior end of head (long arrow); posterior end of axis, tailbud (short arrow). Scale bar, 100 μm.