Tracheoles insure the proper supply of air to growing larval tissues. Late induction of the dominant negative form of Breathless blocks tracheole formation. A single fusion cell at the tip of each fusion branch expresses the zinc-finger gene escargot , leads branch migration in a stereotypical pattern and contacts with another fusion cell to mediate fusion of the branches. A high level of MAP kinase activation is also limited to the tips of the branches. Restriction of such cell specialization events to the tip is essential for tracheal tubulogenesis.
Notch signaling plays crucial roles in the singling out process of the fusion cell.
In addition, Notch is required to restrict activation of MAP kinase to the tips of the branches, in part through the negative regulation of Branchless expression. These fusion branches are capped with fusion cells that express Esg. High expression of DL mRNA and protein is expressed in the DT of stage-15 embryos. Cells in the tracheal primordium just after invagination expressed Dl uniformly. At stage 14, high Dl expression remains only in the DT, which has completed fusion.