Fig. 4. MP number increases in the absence of Notch signaling. (A-D') Ventral views of (A-B') wild-type and (C-D') Dl³/Dl³ mutant stage 14 Drosophila embryos. (A,A',C,C') Single segments stained for Tkr (magenta) and Tbh (green). (A,A') The mVUM6 (blue arrowhead) was Tkr⁺, whereas mVUM4 (black arrowhead) and mVUM5 (yellow arrowhead) were Tkr^-. Dotted ovals outline the mVUMs. (C,C') In Dl, Tkr expression was absent indicating that the excess Tbh⁺ cells were not mVUM6s. (B,B',D,D') Single segments stained for Cas (magenta) and Tbh (green). (B,B') There are three Tbh⁺ mVUMs in each segment: mVUM4 (black arrowhead) was Cas^lo, mVUM5 (yellow arrowhead) was Cas^hi, and mVUM6 was Cas^- (blue arrowhead). (D,D') Excess Tbh⁺ cells in Dl mutants were Cas^lo, indicating that they were mVUM4s. (E-H') Sagittal views of single segments of (E) wild-type and (F-H') Dl³/Dl³ mutant embryos. Midline cells are defined as MPs based on their presence at stage 10 and relatively large size. (E) At mid-stage 10, there is a single Odd⁺ (magenta) MP1 (arrowhead). (F) In Dl, the number of Odd⁺ MP1s (bracket) was increased. (G,G') Dl mutant embryo at two focal planes, 8 µm apart, showing three dividing cells (arrowheads 1-3) in close proximity, stained with anti-phosphohistone H3 (PH3, magenta). (H,H') In Dl, there is an increase in Odd⁺ (magenta) Cas⁺ (blue) MP1 neurons (magenta bracket), Odd^- Cas^- MP3 neurons (white bracket), and Odd^- Cas⁺ MP4 neurons (blue bracket).