Fig. 1. The expression of BMP pathway factors during feather primordium formation. (A-D) The left and right femoral tract of the same HH30 embryo were compared; the numbers correspond to the order of appearance of the rows. The second row (R2) had only two feather primordia expressing BMP2 (A), whereas five expressed BMP7 (B). The additional primordia are indicated with red arrows and those also expressing BMP2 with black arrows. The distal border (yellow arrow), which preceded the appearance of R3, delimited the femoral and zeugopodial tracts and already expressed BMP7 (B) but not BMP2 (A). In another embryo, R4 expressed BMP2 on the left side (C, arrow), whereas follistatin was expressed only in three rows on the right (D). The continuous line (D, arrow), contiguous to R3, is the distal border of the tract, which expressed follistatin and not BMP2. (E-G) ID gene expression in the dorsal tract of HH30 embryos, analyzed by in situ hybridization with RNA probes. (E) There was a decrease of ID1 expression from the newly formed (black arrow) to the older primordia (red arrow). (F) ID3 transcripts showed stable expression in all the primordia. (G) ID4 expression exhibited early expression in the entire primordia (black arrow), and later expression restricted to a peripheral ring (red arrow). Scale bars: in A-D, 1000 µm; in E-G, 800 µm.