HypFIG 9: Division of P cells.
All are epifluorescent images from the same strain as in HypFIG 6. Original magnification, 600x.
A. Lateral view. Newly generated seam cells elongate (arrows) over their hypodermal sisters and restore their mutual contact with their neighbors.
B. Approximately 9 hours after hatching, ventral view, slightly tilted to the left. After seam-cell descendants fuse to hyp 7, the P cell migrates into the ventral cord, starting from the anteriormost pair and proceeding posteriorly (Sulston, 1976). At the same time, P-cell pairs rotate by 90o to form a single row along the ventral midline and retract their lateral margins. For the P1/P2 and P11 pairs this rotation is biased, whereas for the others the rotation can occur in either direction (Sulston and Horvitz,1977; Delattre and Felix, 2001).
C. Late-L1 stage, ventral view, slightly tilted to the left. All P descendents, except P3p-P8p, either fuse to hyp 7 or become ventral cord motor neurons (arrows point to dissolving adherens junctions in these cells).
D. L2 stage, ventral view, slightly tilted to the left. The positions of the nuclei of P2p, P9p-P1p and P12pa that have already joined hyp 7 are shown by pink ovals drawn over the epifluorescent image. The nuclei of the hypodermal cells reside in the ventral hypodermal ridge (when P-cell divisions are completed, there are 12 hypodermal nuclei in the ridge between the retrovesicular ganglion and the anus). The newly born ventral cord motor neurons are situated next to the hypodermal ridge along the ventral midline (not shown). P3p-P8p still have more divisions to go through in L3 stage. (vm) Ventral midline.
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