Emu dermis has low density and is not permissive to tract patterning.

(A) Epidermal-dermal recombinations between CAG-GFP chicken and emu skin. E6.5 chicken and E14 emu dorsal skins were recombined and cultured for a period of 96 hours. Chicken epidermis and emu dermis do not form feather primordia, whereas emu epidermis recombined with chicken dermis does. Scale bar: 500 μm. (B) GFP detection of chicken cells in sections of recombined skin, stained with DAPI. Scale bar: 50 μm. (C) and (D) Immunodetection of TWIST1/2 protein (a nuclear marker of mesenchymal cells, brown) in the lateral and medial regions of the dorsal skin of embryonic stage HH23, HH26, and HH29 (C) chicken and (D) emu embryos. Scale bars: 50 μm. (E) Quantification of mesenchymal cell densities across the dorsal region of E4.5 (HH23), E5.5 (HH26), and E6.5 (HH29) chicken embryos. (F) Mesenchymal cell density quantification across the dorsal region of emu embryos from E9 to E20. Density increases markedly after E15. The numerical values for E and F can be found in S8 Data. E, embryonic day; GFP, green fluorescent protein; HH, Hamburger Hamilton stage.