Feather formations

The individual feathers in the skin of a developing chicken embryo are arranged in a precise array within a series of so-called 'tracts'. The images above show nascent feather buds in the legs of embryos at 8–9 days of incubation. Buds are added sequentially across the forming feather tract, as shown by the range of developmental stages in the image at top left. (In all three upper images, buds are marked by the expression of the signal molecule Sonic hedgehog, labelled dark purple.) The uniform spacing of buds is achieved by lateral inhibition between buds. In other words, as an inducing signal spreads through the tract to initiate feather formation, pre-existing buds prevent surrounding cells from forming buds. Bone morphogenetic protein 2 (BMP-2), which is expressed in the nascent bud, is a crucial mediator of lateral inhibition. The effect of inappropriate BMP-2 expression on feather patterning can be seen in the image at top right, which shows the leg of an embryo in which patches of BMP-2 expression have been created by infection with a transducing retrovirus. Forced expression of BMP-2 prevents bud formation in these regions. (Buds formed next to areas of suppression are normally spaced but not arranged in a precise lattice because they lack a pre-existing row of buds by which to determine their position.) In a normal chick of the same age (top, centre) the feather buds form straight rows. BMP-2 might work in part by regulating the expression of a fibroblast growth factor (FGF) receptor. The two images at lower left and lower centre show the expression of FGF receptor 2, which is expressed in the skin between feather buds and suppressed in the buds. In the normal leg shown at lower left, it is possible to draw straight lines through each file of buds. In the leg shown at lower centre, BMP signaling has been inhibited in patches, resulting in enlarged and fused buds. The image at lower right shows what happens to the expression of BMP-2 itself in a leg treated with the BMP inhibitor. The altered spacing between buds results in the appearance of branched files of buds.