Complexities of Crumbs function and regulation in tissue morphogenesis.

Establishing and maintaining epithelial polarity is crucial during development and for adult tissue homeostasis. A complex network of evolutionarily conserved proteins regulates this compartmentalization. One such protein is Crumbs, a type I transmembrane protein initially shown to be an important apical determinant in Drosophila. We discuss recent studies that have advanced our understanding of the function and regulation of Crumbs. New findings obtained in flies and fish, reporting homotypic interactions of the extracellular domain and retromer-mediated recycling, shed light on the regulation of Crumbs levels and activity. These results - obtained in different organisms, tissues and developmental stages - point to more complex functions and regulation than previously assumed.