Figure 3
Sites of BrdU incorporation in the pilidium. All: anterior left, apical up; purple = Hoechst, yellow = BrdU, aa = anterior axil, pa = posterior axil, ao = apical organ, la = lappet, st = stomach, br = buccal ridge. (A, B) Young pilidia (four days old) incubated for 24 hours in BrdU, then fixed immediately, lateral (A) and oral (B) views. (A) shows the nearer-to-lens half of the larva; ( A’ ) projection of entire larva; ( A” ) other half of the larva, to illustrate bilateral symmetry of labeled populations. ( A 1 to A 3 , B 1 to B 3 ): three to five section subsets emphasizing specific features. The most prominent labeling is in anterior and posterior axils and apical organ; other specific sites include a mesenchymal cell beneath the lappet ciliary band (arrowhead 1), a single cell at the end of the buccal ridge (arrowhead 2), a single cell on either side of the stomach entrance (arrowhead 3), and two to three mesenchymal cells behind the buccal ridges (arrowhead 4). (C) 9-day-old pilidium with nascent cephalic imaginal discs, labeled with six-hour BrdU treatment on day 6. ( C 1 ) Projection of seven 1-μm sections including the buccal ridge; note the row of BrdU-labeled cells alongside unlabeled nuclei. ( C 2 , C 3 ) grayscale and false-colored substacks (11 1-μm sections) of anterior axil ( C 2 ) and cephalic disc beneath ( C 3 ). Some cells retain bright labeling (arrowhead in C 2 ); others, especially in the disc, have diluted the BrdU substantially (arrowhead in C 3 ). (D) 20-day-old pilidium labeled with six-hour BrdU pulse on day 6; projection through half the larva nearer to the lens. Axillary BrdU labeling is shown on grayscale insets ( D 1 ) (anterior) and ( D 2 ) (posterior). A constellation of few bright epithelial cells surrounds the much-diluted cluster of axillary cells and their progeny, including the cephalic imaginal disc (cd).