During Drosophila embryogenesis, single C1 cells in the dorsal ectoderm secrete Spitz, which activates epidermal growth factor receptors (EGFRs) on neighboring cells, causing them to delaminate and migrate to form six-cell clusters of secretory cells called oenocytes. Oenocyte precursor cells do not divide; thus, Brodu et al. were surprised to find that immunohistochemical analysis of phosphorylation of the extracellular signal-regulated kinase Rolled (a marker of EGFR activation in oenocyte precursors) revealed only three oenocyte precursors surrounding C1. Using four-dimensional confocal microscopy to track cells at different developmental stages, the authors found that the oenocytes within a cluster arose from two three-cell waves of precursor cell delamination. Precursor cells formed two concentric three-cell rings surrounding C1, with the inner ring delaminating 45 minutes before the outer ring. High-threshold EGFR signaling targets (including phosphorylated Rolled and the mRNA for Argos, a secreted protein that binds Spitz and inhibits low-level EGFR signaling) were apparent only in the inner ring, becoming detectable in the outer ring when the inner ring delaminated; low-threshold targets were apparent in both rings. In fly strains that over-secreted Spitz throughout the oenocyte precursor zone, additional three-cell delamination cycles took place, with the number of cells in mature oenocyte clusters peaking in multiples of three. In contrast, constitutive activation of the EGFR, or of its effector Ras1, inhibited pulses of delamination, so that the characteristic three-cell peaks in oenocyte number were not apparent. Fly strains lacking argos similarly lacked pulsatile delamination. Thus, the authors conclude that a combination of cell movement and inhibitory intercellular signaling underlie the pulsatile response.
V. Brodu, P. R. Elstob, A. P. Gould, EGF receptor signaling regulates pulses of cell delamination from the Drosophila ectoderm. Developmental Cell 7, 885-895 (2004). [Online Journal]