(A) Series of line drawings of co-culture experiments. Red dots in hypothalamic explants represent MCH perikarya; red lines represent MCH fibers revealed by immunofluorescence. Note that the innervation of the telencephalic and/or mesencephalic explants is independent of the size of these explants and of the presence of MCH perikarya close to the corresponding edge in the hypothalamic explants. (B) Newborn mice caudal hypothalamic explants were co-cultured with E11 to E14 embryonic basal midbrain and basal telencephalic explants. Data were obtained from at least 3 independent experiments for each embryonic stage. 12 to 19 basal midbrain and basal telencephalic explants were analyzed per stages and 9 to 17 tectal explants that were used as controls. Data indicate mean ±SEM. aaa: p≤0,001 mesencephalon vs tectum; bbb: p≤0,001 - b: p≤0,05 telencephalon vs tectum; **: p≤0,01 mesencephalon vs telencephalon; n.s.: not significant, Kruskal Wallis's test. (C–E) Photomicrographs of slices after two days in vitro and MCH immunofluorescence. Low (C) and higher (D,E) magnifications to illustrate that MCH neurons survived very well, and that perikarya and axons are clearly labeled and can easily be numbered. Scale bar in E: C = 60 µm; D, E = 30 µm.