Figure 1. Pocket bridge formation and pocket closure in the wild type and in mab-20 and vab-1 mutants.

(A) Schematic of epidermal pocket closure in minutes post first cleavage (PFC) of the zygote. Ten plexin band cells span the open pocket. Cell births and movements were followed using Pplx-2::gfp. Plexin band cells of the same color are sisters. Rearrangement of plexin band cells during pocket closure are as photographed in panel B. (B) Ventral view of the pocket region of a wildtype embryo with anterior at top right. The ventral midline (dashed line) and relative positions of expressing identified P cells (circles, closed circles are P9/10) and plexin expressing cells on the surface of the open ventral pocket (colored squares) are indicated. Developmental time at 20°C is indicated in each frame. The approximate size of the open pocket (dashed circle) reflects the advance of P cell extension toward the midline. At 290’ PFC, Pplx-2::gfp is expressed in the P cells. At 300’-310’ PFC, blast cells on the surface of the open pocket express Pplx-2::gfp. At 320’-350’ PFC expressing blast cells divide and sister cells remain attached to form a contiguous band of plexin expressing cells (plexin band), which spans the open pocket (when sporadically expressing yellow cells of Figure S1 are included). The midline distal pairs (red and orange squares) form the pocket bridge by migrating toward the midline over intervening expressing scaffold cells (green, yellow and blue squares). This rearrangement requires protrusions from the presumptive bridge cells that extend over scaffold cells toward the ventral midline in two stages (330’ open arrowhead) (see summary in Figure 2). P9/10 cell migration then occurs along the forming and mature pocket bridge. At 390’ PFC, P cell migration stops when opposing leading edges meet at the midline. At 400’ PFC, contralateral marginal cells align and suture and the embryo begins to elongate and turn. (C) Plexin band cells form a nearly contiguous band of cells by 340’ PFC in vab-1(dx31) null as well as in kinase dead embryos (not shown). The non-sister cell gaps (left open arrowhead at 320’) are usually closed later by unknown mechanisms that might involve small filopodia-like projections (solid arrowhead at 370’). (D) Skewed P cell leading edges (see 370’ inset) were detected in 2 of 5 mab-20(ev574) mutant embryos examined. Scale bar in B for panels B-D is 10 μm.