Figure 2. Convergent extension in the anteroposterior axis between pre-L1 and L2.
(A) Mid stage embryo with pre-denticles. The seven rows of pre-denticles (1–7) are indicated. At first all pre-denticles are found on the posterior boundaries of the appropriate rows. However, in later embryos pre-denticle rows 1 and 4, the two rows that will make denticles pointing forwards, are now situated near the middle of the cells. This suggests that some movement of the pre-denticles may be part of polarity reversal. Also it may be relevant that cell lines I and IV of the embryo are the only lines that make extra lines of cells in the larva, and therefore contribute to convergent extension. Labelling for (A–E): The pre-denticles are labelled with utrp::GFP and the cell outlines with DE-cad::GFP (A and B) or DE-cad::tomato (C–E). (B) Late stage embryo, before moulting to L1 but after actin pre-denticles have gone. The pattern is similar to the earlier embryo, with two lines of cells between the tendon cells (II and V). (C) Mid stage embryo showing the marked portion of the epidermis. The rectangle demarcates a segment in the anteroposterior axis and the region between the pair of ventral sensorial papillae (p1) (Dambly-Chaudière and Ghysen, 1986) in the mediolateral axis. The total number of cells and the numbers along the axes were counted, see (F). (D) The pre-L2 stage. By this stage the cells have rearranged and extended in the anteroposterior axis: the number of cells in that axis has increased from ca 14 to 18 cells per segment. The fixed rectangle has changed shape dramatically but contains the same number of cells as in the embryo (Table 1). This change of dimensions is due to convergent extension that involves cell rearrangement as well as alterations in the shapes of cells. (E) The pre-L3 stage. The pattern of cells and the shape of the fixed rectangle resembles that in the pre-L2. (F) Quantitation of the evidence for convergent extension: Boxplots (Frigge et al., 1989) of the number of cells enclosed by fixed pattern landmarks remains the same while these cells extend in the anteroposterior axis and narrow in the mediolateral axis. Pairwise comparisons using t tests with Bonferroni adjustment (ns = not significant, *** = p<0.001).