Cell-pattern disordering during convergent extension in Drosophila

Convergent extension is the cell-rearrangement process by which a developing embryo elongates to establish the head-to-tail body axis. In the early Drosophila embryo, this process occurs within a one-cell-thick epithelial layer. Using confocal microscopy, images were collected of the two-dimensional cell pattern at four stages during convergent extension in wild-type embryos and at one stage in two classes of mutant embryos. The cellular topology was analysed in terms of the statistical distribution p(n), the frequency of occurrence of n-sided cells. For wild-type embryos, the results demonstrate progressive cell-pattern disordering during convergent extension. The second moment (variance) of p(n) triples to 1.1 while the peak at p(6) drops from 0.65 to 0.38. The fraction of fourfold vertices (four edges meeting) increases from 2% to 8%. Quantitative analysis of interface orientations reveals that the initial degree of hexatic edge-orientational order essentially disappears during the course of convergent extension. The degree of cell-pattern disordering in the two mutants resembles distinct stages in the wild type and correlates with the extent of axis elongation.