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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).
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