Crumbs, a transmembrane protein with a short cytoplasmic domain and very large extracellular domain, was identified as an apical polarity determinant in Drosophila that is required for epithelial integrity. In mammals the Crumbs family consists of three members, Crumbs1, Crumbs2 and Crumbs3. Here I will show that mammalian Crumbs family is not essential for establishing epithelial polarity in the early mouse embryo and that the earliest function of the Crumbs family in mammals is the regulation of gastrulation by Crumbs2. Mutations in human CRUMBS1 have been associated with Retinitis pigmentosa and retinal degeneration. Most mutations lie in the extracellular domain of Crumbs, whose function still remains unclear. Here I show that the extracellular domain of Crumbs2 is O-glucosylated by an enzyme Protein O-glucosyltransferse 1, which regulates its membrane localization. The apical membrane localization of Crumbs2 is essential for its function in mammalian gastrulation. The gastrulation phenotype in Crumbs2 mutants results in the accumulation of cells at the primitive streak. Although Crumbs proteins are expected to be required to maintain the apical domain of the cell, mammalian Crumbs2 is required for cells to lose apical adherens junctions and to facilitate cell delamination during gastrulation epithelial-to-mesenchymal transition. The requirement of Crumbs2 in epithelial organization and cell delamination at the primitive streak is locally non-cell autonomous, as Crumbs2 mutant cells can delaminate, migrate away and be incorporated into all lineages when surrounded by wild-type cells. Further, we find that Crumbs2 is required in the epiblast adjacent to the primitive streak to maintain epithelial integrity, but only when neighboring cells are undergoing cell rearrangements such as the gastrulation EMT, as the local disorganization of cells in the epiblast does not occur in the absence of the primitive streak. Thus I propose that Crumbs2 is part of a global system that maintains epiblast integrity while cells delaminate during the EMT in mammalian gastrulation.