AbstractMammalian submandibular gland (SMG) development leads to the establishment of highly organized secretory acinar and nonsecretory ductal epithelial cells. The ability of maturing salivary epithelial cells to attain their differentiated state has been shown to depend, in part, on interactions between extracellular matrix (ECM) proteins and their integrin receptors. In a search for key regulators of salivary cell lineage, we have studied α3β1 integrin, a receptor for the basement membrane protein laminin, by characterizing embryonic day 18 (E18) SMGs isolated from mice carrying a targeted mutation in the α3 integrin gene. Transmission electron microscopy studies showed that the mutant SMGs exhibited an aberrant differentiation phenotype with defects in the apical‐basal polarity axis and in the basement membrane. Based on immunohistochemistry and Western blot analyses, the α3β1‐deficient SMGs had altered expression and/or localization of several ECM and adhesive molecules, including laminin β1, fibronectin, α5 integrin, and E‐cadherin. These changes correlated with alterations in the activation state of Ras‐extracellular signal‐regulated kinase (ERK), as well as the expression and/or localization of Cdc42 and RhoA, two Rho GTPases that regulate the organization of the actin cytoskeleton. We conclude that α3β1 is required for normal salivary cell differentiation and that its absence affects multiple components of adhesive complexes and their associated signalling pathways. © 2001 Wiley‐Liss, Inc.