Fig. 5. Determination and differentiation of syncytiotrophoblasts in Map2k1^-/- placentas. (A-F) The functional differentiation of chorioallantoic trophoblasts in syncytiotrophoblasts was assessed by Gcm1 in situ hybridization (red signal) in E8.5 (A,B), E9.5 (C,D) and E10.5 (E,F), wild-type (A,C,E) and Map2k1^-/- (B,D,F) placentas. Gcm1-positive cells were detected as early as E8.5 in wild-type and Map2k1^-/- placentas, with no major difference between the genotypes. However, at E9.5 and E10.5, Gcm1 labeling remained restricted to the chorioallantoic interface in Map2k1^-/- specimens (D,F; arrowheads), whereas in control placentas, Gcm1-positive cells invaded the labyrinth to line the maternal sinuses (C,E; arrows). (G-J) Alkaline phosphatase activity in E10.5 wild-type (G,I) and Map2k1^-/- (H,J) placentas. In wild-type specimens (I), cells surrounding the maternal blood sinuses produced high levels of alkaline phosphatase activity (arrowheads). There is a correlation between the punctuated localization of alkaline phosphatase activity (J; arrowheads) and the Gcm1 expression in the chorioallantoic interface of Map2k1^-/- placentas (F; arrowhead). Scale bars: 100 µm in A-H; 50 µm in I,J.