Fig. 2. Elongation of the nematode embryo. (A-C) The dorsal surface of the nematode embryo shows intercalation of dorsal hypodermal cells (green), which elongates the dorsal aspect of the embryo, giving it a comma shape (lateral view, G). B is an enlargement of A: the basal surfaces of the intercalating dorsal hypodermal cells show medially directed protrusions. (D-F) Ventral views and (G-J) lateral views of ventral closure, during which the ventral hypodermal cells (red) move across the underlying neuroblast cells (purple) and meet in the midline. (K-M) Hypodermal-mediated elongation beyond the comma stage involves a circumferential actin microfilament cytoskeleton (yellow) in the dorsal (green) and ventral (red) hypodermal cells, and a circumferential contraction of the lateral or seam hypodermal cells (blue). In regions overlying muscles (K and enlargement), trans-epithelial attachments (TEAs) develop and connect the underlying matrix and muscle (purple) to the overlying cuticle (embryonic sheath). TEAs consist of fibrous organelles (FOs) - electron-dense plaques similar to hemidesmosomes on the ECM/muscle side and on the embryonic sheath side of the hypodermis; FOs are connected by intermediate filaments that span the hypodermal cells. Anterior is to the right in A-J. (A-J) Adapted, with permission, from Simske and Hardin (Simske and Hardin, 2001); (K-M) adapted, with permission, from Ding et al. (Ding et al., 2004).