Fig. 1. Eyelid development of the mouse at embryonic day 11.5 (E11.5) (A-A''), E13.5 (B-B''), E15 (C-C''), E16 (D,D'), and E17.5 (E,E'). The upper and lower eyelids are at the left and right, respectively. (A'-E',A''-C'') Higher magnifications of the eyelid primordia shown in A-E, respectively. (A) The ocular surface epithelium forms small grooves (arrows), and eyelid primordia (arrowheads) begin to emerge. (A',A'') The epithelial cells at the bottom of the groove have become cuboid (arrowheads). (B) The arrowheads indicate eyelid protrusions. (B',B'') The eyelid epithelium has become two-layered, with the leading edge cells yet to be formed, while the eyelid dermis starts proliferating. (C-C'') The protruding epithelial ridge has formed at the leading edge (arrow in C' and C''). (D,D') The suprabasal epithelial cells between the two lids meet and fuse to form an epithelial bridge. (E,E') After epithelial fusion, the ocular surface epithelia stratify and differentiate, while the upper and lower lid mesenchyme (m) have extended towards the junctional area and faced each other. co, cornea; ep, epidermis; hf, hair follicle; le, lens; lld, lower eyelid; p, periderm; uld, upper eyelid. Scale bars: A and B'-E', 100 µm; A',A'', 50 µm; B-E, 200 µm.