The formation of functional epithelial tissues involves the coordinated action of several protein complexes, which together produce a cell polarity axis and develop cell-cell junctions. During the last decade, the notion of polarity complexes emerged as the result of genetic studies in which a set of genes was discovered first in Caenorhabditis elegans and then in Drosophila melanogaster. In epithelial cells, these complexes are responsible for the development of the apico-basal axis and for the construction and maintenance of apical junctions. In this review, we focus on apical polarity complexes, namely the PAR3/PAR6/aPKC complex and the CRUMBS/PALS1/PATJ complex, which are conserved between species and along with a lateral complex, the SCRIBBLE/DLG/LGL complex, are crucial to the formation of apical junctions such as tight junctions in mammalian epithelial cells. The exact mechanisms underlying their tight junction construction and maintenance activities are poorly understood, and it is proposed to focus in this review on establishing how these apical polarity complexes might regulate epithelial cell morphogenesis and functions. In particular, we will present the latest findings on how these complexes regulate epithelial homeostasis.

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Cell polarity is an essential feature of most eukaryotic cells, especially epithelial cells in multicellular animals. Polarity protein complexes that regulate epithelial organization have been identified. In this review, it is proposed to describe how the Crumbs complex acts in the process of cell polarity and epithelial organization. During the last decade, several partners of Crumbs, an apical transmembrane protein, have been identified and their direct or indirect associations with the cytoplasmic domain of Crumbs have been dissected. In addition, mutants of several of the genes encoding proteins belonging to the Crumbs network have been obtained in animals ranging from flies to mouse, which have led to a better understanding of their functions in vivo. These functions include polarity axis formation, stabilization of epithelial apico-lateral junctions, photoreceptor organization and ciliogenesis. Since human CRUMBS1 mutations are associated with retina degeneration, it has become essential to define Crumbs network and to understand exactly how this network acts in polarized cells, with a view to developing suitable therapeutic approaches for treating this severe degenerative disease.