Patterning signals and proliferation in Drosophila imaginal discs.

Recent studies indicate that signaling pathways with well-known roles in patterning also directly regulate cell proliferation. During the differentiation of the retina, Hedgehog, Decapentaplegic, Notch and the EGF receptor regulate proliferation spatially through transcriptional regulation of string, dacapo, and as yet unidentified regulators of Retinoblastoma and Cyclin E/Cdk2 activities. In the developing wing, a novel response to discontinuities in Decapentaplegic signaling combines with concentration-dependent effects to achieve a uniform proliferation pattern in response to a Decapentaplegic gradient. Damage to growing tissues is repaired by transient Decapentaplegic and Wingless secretion from dying cells to induce compensatory proliferation. Diverse spatial patterns of fate specification and of proliferation can arise through distinct combinations of signaling pathways. Reminiscent of pattern formation, cell cycle effects of each signaling pathway differ in distinct developmental fields, making use of a variety of target genes.