HypFIG 4: Ventral enclosure and elongation of the hypodermis (based on Chin-Sang and Chisholm, 2000; Simske and Hardin, 2001).
A&B. Ventral enclosure takes place between 310 and 360 minutes after first cell cleavage, at 20oC, and proceeds in three steps.
A. Ventral views.
B. Lateral views. In the first step, the anterior leading cells (white stars) extend actin-rich filopodia toward the ventral midline over a substrate of neuronal precursor cells (red circles). In the second step, the leading cells meet at the midline and form adherens junctions, whereas cells posterior to them become wedge shaped and extend toward midline, creating a ventral pocket (white arrow). In the last step, the ventral pocket closes, possibly through a purse-string type contraction mechanism (gray circular arrows). Simultaneously with ventral enclosure, the anterior hypodermal cells (hyp 4, hyp 5) migrate from where they are born toward the anterior part of the embryo (not shown) (Labouesse, 1997). As the embryo reaches the comma stage, before the dorsal cell fusions start, ventral and anterior areas are covered completely. Hypodermal cells that will become part of hyp 6 syncytium are marked by thin stripes, whereas those that will become part of hyp 7 syncytium on the ventral side are marked by textured coloring. Dorsal hypodermal cells of hyp 7 are shown in brown and seam cells in orange. Anterior hypodermal cells are not shown. Black arrowheads point to the excretory pore cell (middle image) or excretory pore (bottom images).
C. Elongation takes place after the embryo is covered and sealed by hypodermis. Contraction of circumferential actin filaments, especially within the seam cells, causes elongation of the embryo, thus increasing its length approximately fourfold and decreasing its thickness about threefold (see top and bottom epifluorescent images in HypFIG 3E).
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