Fig. 4. Midline precursors and GMCs each produce one En-positive and one En-negative neuron. (A) En expression in neurons from MP4, MP5 and MP6, in 28% to 35% neuromeres (n=36). Vertical bars show the number of neurons (left y-axis) in individual neuromeres as the sum of neurons with no detectable En (white) and at least some En expression (gray). Downward arrows indicate neuromeres having one additional MP sibling that had not yet differentiated neuronal markers (not included in bar total). Chart bars are rank-ordered, first by the total neurons present, and then by the number of En-positive neurons. Filled circles above each bar indicate percent development of neuromere in which neurons were tallied (right y-axis). (B,C) Population curves for midline neurons in T3 (B) and A1 (C) neuromeres, including neurons from MP4-6 and MNB lineages. En-positive neurons (black circles) and En-negative neurons (white circles) are equal in number through 45% embryogenesis. After 45%, the number of En-positive neurons continues to increase, but the number of En-negative neurons remains steady or decreases. (D,E) Embryos injected with BrdU at 35% (D) or 55% (E) were allowed to develop a further 48 hours (see text). Data obtained from T3 and A1 neuromeres were pooled, as there was no population difference. For injections at 35%, n=22 neuromeres counted, of which 11 were T3, and 11 were A1. For injections at 55%, n=17 neuromeres counted, of which 10 were T3, and 7 were A1. BrdU-labeled neurons were counted by type, and the number of En-positive neurons in each neuromere (x-axis) was plotted against the number of En-negative neurons in the same individual neuromere (y-axis). Where more than one neuromere had identical counts of the two types, the points are annotated by that number.