Figure 1. Palatal fusion in vivo (A,B,C) and in vitro (D,E,F) examined in transverse sections stained with hematoxylin and eosin. (A) At E13 (n = 3), palatal shelves (ps) lie vertically on both sides of the tongue (T). (B) At late E14 (n = 5), the shelves are elevated horizontally and juxtaposed. The MEE formed the MES (arrow). (C) At E15 (n = 3), the MES was disrupted, and epithelial islands were observed (arrows), which completely disappeared at E16 (unpublished observations). The mesenchyme from both shelves merged at the midline (asterisk), and lateral mesenchymal cells condensed (stars). (D) At 30 hrs in culture (n = 5), the MEE cells formed the multilayered MES (arrow). (E) At 48 hrs (n = 5), MES thinned and was disrupted (arrow), allowing mesenchyme to merge across the midline. (F) At 60 hrs (n = 3), MES disappeared, and the palate was completely fused (asterisk). (G) Diagram summarizing time-frame of the fusion processes in vivo (bold lines) and in vitro (dotted lines). Scale bars: A,B = 500 µm; C,D,E,F = 200 µm. ps = palatal shelf, t = tooth bud, N = nasal surface, O = oral surface, T = tongue.