Fig. 3. Eda pathway expression in tissue sections. In situ hybridization to tissue sections from embryos at day 7.5 (A,B), day 8 (C-F) and day 10.25 (G,H) of incubation. (A) Eda expression is detected in the epidermis of the presumptive feather tract at the extreme right of the section, but drops below the level of detection in the more developed epidermis of the medial tract (left). Note the lack of signal in the epidermis where it has separated from dermis (blue arrowheads). Expression is observed throughout the dermis at this stage, with the exception of the extreme left, where a dermal condensation has formed (red arrowhead). (B) Edaradd expression is observed in an adjacent section throughout the tract epidermis, although expression is augmented in the epidermal placodes (arrowheads). (C) Edar expression is largely restricted to epidermal placodes as buds are formed. (D-F) Adjacent sections show placodal expression of Edaradd (D) overlying dermal condensations that express Bmp4 (E) but do not express Eda (F). Eda expression is observed in the interfollicular dermis. (G,H) Adjacent sections show Eda expression persists in the interfollicular dermis at lower levels and is highly expressed in the posterior distal mesenchyme of the feather bud, while Edaradd expression is confined to the epidermis.