Fig. 6. Cleft palate in Msx1 mutants is caused by defective cell proliferation in the anterior portion of palatal shelves. (A-C) Palatal shelves from E13.5 Msx1^–/– embryo (B), like those from wild type (A) and Msx1^–/–/Tg embryos (C), fused and exhibited a disruption of the midline seam when placed in contact in vitro. Arrows indicate the remainder of the midline seam. (D-F) Cell proliferation, indicated by BrdU labeling, was significantly reduced in the E13.5 Msx1^–/– palatal mesenchyme (E), particularly in the mesenchyme immediately beneath the MEE (arrow), when compared with mesenchyme in wild-type palates (D). Cell proliferation was restored to normal levels in Msx1^–/–/Tg palatal shelves (F). (G) A summary of BrdU-labeling studies in the anterior and posterior portions of the palatal shelves of the E13.5 wild type, Msx1^–/– and Msx1^–/–/Tg embryos. The box in D represents the position and size of counting area (22.5 mm²), as determined by use of an ocular grid. Using Student’s t-test, P values were obtained by comparing numbers from Msx1^–/– or Msx1^–/–/Tg with those from wild type. T, tongue. Scale bars: 100 µm.