Sonic Hedgehog functions by localizing the region of proliferation in early developing feather buds.

Feathers are formed following a series of reciprocal signals between the epithelium and the mesenchyme. Initially, the formation of a dense dermis leads to the induction of a placode in the overlying ectoderm. The ectoderm subsequently signals back to the dermis to promote cell division. Sonic Hedgehog (Shh) is a secreted protein expressed in the ectoderm that has previously been implicated in mitogenic and morphogenetic processes throughout feather bud development. We therefore interfered with Shh signaling during early feather bud development and observed a dramatic change in feather form and prominence. Surprisingly, outgrowth did occur and was manifest as irregular, fused, and ectopic feather domains at both molecular and morphological levels. Experiments with Di-I and BrdU indicated that this effect was at least in part caused by the dispersal of previously aggregated proliferating dermal cells. We propose that Shh maintains bud development by localizing the dermal feather progenitors.