Early placental development in mice involves patterning of the chorion into distinct layers, though little is understood regarding the interactions that regulate its organization. Here we demonstrate that keratin aggregates found in Mrj−/− chorionic trophoblast cells are associated with abnormal cell morphology, collapse of the actin cytoskeleton, E‐cadherin and β‐catenin misexpression and extracellular matrix (ECM) disorganization. Accordingly, Mrj−/− trophoblast cells in vitro are nonadherent and display erratic migratory behavior. These cells also fail to differentiate into syncytiotrophoblast cells since Rhox4b expression, a marker of syncytiotrophoblast progenitors, was maintained and Gcm1, Synb, and Syna expression failed to increase. This differentiation defect was not solely attributable to E‐cadherin misexpression or ECM disorganization. However, plating Mrj‐deficient cells on exogenous laminin‐511 normalized their cell behavior. Lastly, we show that Mrj−/− chorions at embryonic day 8.5 have expanded Rhox4b expression domains and do not form normal layers of gene expression suggesting that chorion patterning requires Mrj. Developmental Dynamics 240:2505–2519, 2011. © 2011 Wiley Periodicals, Inc.