Model of Drosophila midgut MET. (a) Serpent is down-regulated in endoderm cells independently of contact with the mesoderm. (b) The mesoderm secretes Wb. αPS1 localizes to the side of the endoderm that contacts the mesoderm. This defines the basal side of the cells and is required for the cells to establish apicobasal polarity and form a monolayer of columnar-shaped cells. (c) αPS3 localizes to the apical side of midgut cells, and this is required for the robust localization of E-Cad and Baz to the apical side of the cell. Finally, secretion of LanA by the midgut cells acts to reinforce αPS1 basally and αPS3 apically, and thereby ensure robust polarity.