Regulators of the Morphogenetic Furrow

Abstract

Unlike other imaginal discs, the Drosophila eye disc has a progressive pattern of differentiation. Photoreceptor clusters begin to form at the posterior margin of the eye disc in the third larval instar, and more anterior rows of clusters then differentiate in succession (Ready et al. 1976). Just prior to their differentiation, cells undergo an apical constriction and apical-basal contraction that produces an indentation in the disc known as the morphogenetic furrow (MF; Ready et al. 1976). Cells anterior to the MF divide actively and appear unpatterned. Just posterior to the MF, cells assemble into evenly spaced rosettes; slightly more posteriorly these transform into arcs, and the arcs then close to produce five-cell preclusters (Wolff and Ready 1991). Concurrently, these cells initiate a program of gene expression resulting in the appearance of neuralspecific markers in a defined sequence in the cells of each cluster (Tomlinson and Ready 1987). Cells in the MF are arrested in the G1 phase of the cell cycle (Thomas et al. 1994); posterior to the MF, cells excluded from the preclusters undergo one more round of division, the second mitotic wave, to generate the remaining cells of each ommatidium (Ready et al. 1976; Wolff and Ready 1991). This orderly and sequential pattern of differentiation, proliferation, and morphogenesis is organized by a set of signaling molecules that also direct many other developmental processes. The expression patterns and interactions of these molecules in the developing eye disc are shown in Fig. 1.