Fig. 1. Midline precursors MP4, MP5 and MP6, and the MNB originate sequentially from the En-positive midline regions. Labeling was by anti-En (red) and anti-HRP (green). (A) 28% embryo shows the pattern of En expression and the progression of neuronal differentiation across neuromeres. Neuromeres posterior to T3 are successively younger by about 1%. (B-G) Sequence of MP and MNB differentiation in 25-29% neuromeres. Stars indicate MP2 progeny. (B,C) 25% neuromere with MP4 and incipient MP5. (B) A relatively dorsal scan; (C) The scan in B together with underlying En-positive region. MP4 is the largest and most strongly En-positive cell at the midline. The incipient MP5 (arrow) is just posterior to MP4. (D) 26% neuromere. MP4 and MP5 express high levels of En. The incipient MP6 (arrow) lies ventral to MP5. (E) 27% neuromere. MP4 has divided to form two progeny (4, 4), anterior to MP5. (F) 28% neuromere. MP4 progeny (4, 4) have strong HRP label, indicating neuronal differentiation and one of the two siblings (on left) has decreased En expression. MP5 has divided to give rise to two progeny (5, 5). MP6 and the MNB with associated cap cell (cc) are side-by-side and posterior in the cluster. (G) 29% neuromere. MP6 has divided to give rise to two progeny (6, 6). One of the two MP4 progeny (4d) is more dorsal than its sibling and has decreased En expression. MP5 progeny (5, 5) are beginning neuronal differentiation judged by HRP-labeling, but MP6 progeny (6, 6) are yet unlabeled. The MNB is ventral to MP6 progeny. Scale bar: 100 µm in A; 15 µm in B-G.

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