Sonic Hedgehog
From Ting-Berreth and Chuong, 1996, Dev, Dynamics 207:157-170.
This is a piece of embryonic chicken dorsal skin,
hybridized with SHH probes. The midline is from
left lower corner to right upper corner, where the feather buds are more mature.
Toward the lateral sides, feather buds are younger. SHH is expressed in the
distal placodes, andlater in the marginal plate epithelia, cells associated
with cell death and become the space between barb branches.