The making of a fly leg: a model for epithelial morphogenesis.

Epithelial development dictates the shape of an organism. The metamorphic development of a Drosophila leg precursor into an adult leg is a well-defined example of epithelial morphogenesis that can be analyzed from the perspectives of genetics and molecular and cell biology. The steroid hormone 20-hydroxyecdysone induces and regulates the entire process. Mutants affecting Drosophila leg morphogenesis characteristically have short thick legs (the malformed phenotype) resulting from a failure to execute normal cell shape changes at a specific stage of development. Mutations that cause the malformed phenotype have already led to the identification and cloning of genes encoding transcription factors, a transmembrane serine protease presumably required for modification of the apical extracellular matrix, and components of the contractile cytoskeleton and adherens junctions. All of these products are required for the execution of normal changes in leg cell shape.