Fig. 1. Chick neural plate border cells are specified as neural crest cells at the late blastula stage. (A) Ectodermal explants were isolated, separated form the hypoblast/endoderm, and cultured in vitro to the developmental equivalent of stage 10 before fixation, freezing and sectioning. (B) Different combinations of molecular markers define specific ectodermal fates. (C) Schematic representations of stage 2 embryos indicating the position at which medial (M, red box), prospective border [RB (rostral neural plate border), B (border) and CB (caudal border), green boxes] and lateral (L, blue box) explants were isolated. (D-H) Consecutive sections showing expression of molecular markers in explants cultured for 28 hours. (D) Stage 2 M explants (n>30) generated Sox1⁺ and Sox2⁺ cells, but no Snail2⁺, HNK-1⁺ or Ker⁺ cells. (E-G) Stage 2 B explants at different rostrocaudal positions (RB, n=23; B, n>30; CB, n=13) generated Snail2⁺ and HNK-1⁺ cells, but no, or only a few, Sox1⁺, Sox2⁺ or Ker⁺ cells. (H) Stage 2 L explants (n>30) generated Ker⁺ cells, but no Sox1⁺, Sox2⁺, Snail2⁺ or HNK-1⁺ cells. Data are represented as the mean ± s.e.m. Scale bar: 100 µm.