Extracellular regulators and pattern formation in the developing Drosophila retina

The compound eye of Drosophila melanogaster is composed of about 800 similar facets (or ommatidia) each of which contains 20 cells (8 photoreceptor neurons and 12 accessory cells). The fly's vision depends on the precise geometry of these components, and the developmental systems that produce pattern and cell fate have become models for nervous system development in general. The development of this pattern is progressive and appears to involve several levels of induction and inhibition, mediated by diffusible signals. The analysis of genes identified by mutation has revealed that several diffusible factors act in the early patterning steps of the eye. Genes that encode these factors include hedgehog, decapentaplegic, wingless, spitz and scabrous.