IntFIG 3: Embryonic development of the intestine.

A. DIC images of embryos are pseudocolored in the E2 and E16 stages. The remaining are graphic renditions. The diagrams are not to scale; in fact, the total volume of the entire intestine is almost equal to the volume of the E blastomere. (Based on Leung et al., 1999.) Following internal migration, Ea and Ep go through a left-right division followed by an anterior-posterior division, giving rise to E4. The left-right division provides the basis for the bilateral symmetry of intestine; Eal and Epl generate the left half, whereas Ear and Epr generate the right half of intestine. (Short blue lines) Sister cells in four-cell and E stages. In the E16 and 357-minute stages, shown are only the nuclei (blue balls) of the anteriormost cells. (Small curved arrows) Direction of cell intercalation; (asterisks) cells that divide further. The anterior pair divides dorsoventrally, whereas the posterior pair divides anteroposteriorly, bringing the total number of cells to 20. There are two 90° rotations of intestinal rings during embryogenesis except int I; The first one is a 90° clockwise rotation of int rings II-IV by 430 min (post first cell division) and the second is a 90° counter-clockwise rotation of int rings VII-IX between 430 min and hatching (Sulston and Horvitz, 1977; Mendenhall et al., 2015). As a result, the intestine has a left-handed twist of 180° around the longitudinal body axis in the region of the primordial gonad. This twist likely causes the displacement of the intestine slightly left of the midline in the anterior and slightly right of the midline in the posterior. Ventral cells in int V and/or int VI rings later get pushed to one (L or R) side by the growing uterus (not shown.) (Red lines) Adherens junctions surrounding the lumen.
B. Epifluorescent image of an adult transgenic animal expressing the reporter gene emr-1::GFP. The nuclei of the cells in int II-IV and int VIII-IX rings can be seen as located dorso-ventrally to each other, whereas the ventral int 5 cell (int 5L) is displaced to the left side by the embryo-filled uterus. (Image source: A. Mendenhall.) Magnification, 400x.

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