CNS differentiations induced by culturing Ambystoma maculatum ectoderm attached to a glass surface (in Holtfreter’s saline solution) can be blocked by addition of UO126, a chemical inhibitor of the MAPK/Erk pathway. (A) Ectoderm cultured attached to glass can develop extensive neural differentiations. After the initial induction of CNS tissue, differentiations of secondary fields also take place, giving rise to olfactory placodes, retina, retinal pigmented epithelium, and lens (of which an enlargement is shown). (B) In the presence of UO126 CNS differentiations are blocked. The explants develop as atypical epidermis (which is called atypical because it contains small cavities containing keratinized cells). (C) Section of a sibling embryo at the same stage of development (9 days) to illustrate the normal histological appearance of CNS tissues. (D) Outside view of Ambystoma maculatum 9-day larva indicating the plane of section. Abbreviations: ba, balancer; CNS, central nervous system; ey, eye; g, gills; gm, gray matter; le, lens; me, mesencephalon; op, olfactory placodes; re, retina; rpe, retinal pigmented epithelium; te, telencephalon; v, ventricle; vm, white matter. Reproduced from , with permission.