Figure 7.7
The Inhibitory Field and Wavefront Model for creating a hexagonal lattice. See also App. 7.
a. Mature left eye disc. Vertical stripe is the morphogenetic furrow (MF). Compass (at right) gives axes in the fate map (cf. Fig. 7.1). The MF leaves a hexagonal lattice of ommatidial cell clusters (dots) in its wake. Oval (far right) denotes an inhibitory field wherein no R8 photoreceptors can arise. This field is supposed to be generated by a pre-existing R8 precursor ('8' or 'R8p') and possibly other R-cell precursors (e.g., 'x' = R2/5p) [2493]. The identity of the secreted inhibitor is unknown. Suspected candidates have included Argos [4035] and Scabrous (Sca) [179, 2352, 2885], but Argos cannot be acting alone since argos^null clones have normal spacing of R8s [204]. Secretion implies a cone-shaped gradient.

b, c. Imaginary slices of the columnar epithelium (magnified from box in a) at two times separated by ~1.5 h ('Δt'). The MF is a groove (black) that runs along the D-V axis. It moves from P to A (long arrows). R8 precursor cells (teardrop shapes) arise within 'intermediate groups' (not shown; cf. Fig. 7.9) at intervals along the MF [184] and found the ommatidia (cf. Fig. 7.6) [1048, 1279]. Gray stripes mark columns of nascent ommatidia, which are older (and hence more complete) as distance (and time) from the MF increases. Successive columns are out of register (shifted vertically) by 0.5 R8p interval -- thus forming a hexagonal lattice. The phase shift is attributable to rules that govern how new R8p cells arise: for a cell to become an R8p it must be (1) in the MF and (2) not in an inhibitory field. Thus, as the MF moves anteriorly (compare b to c), the first cells that become competent for the R8 fate are those that lie in the crevices of the preceding inhibitory fields. Fields must arise within ~1.5 h after R8p inception. How long they persist is not known. Here they are shown in only one column, though expression of Sca (and Ato) persists in R8p cells for ~4 columns [183, 1158, 4621]. Also unknown is whether the signals come from single R8p cells [2885], from nests of a few cells as shown here [2632, 4035], or from the intermediate groups whence R8p cells arise [186, 2461, 4208]. The signaling cells are presumably deaf to the inhibitor because they disable their receptors (or transduction pathway) before they start signaling.