Formation of feathers on chick foot scales: a stage-dependent morphogenetic response to retinoic acid

In the chick embryo, large scales (scuta) form on days 9-11 on the anterior face of the shank and the upper surface of the toes; smaller scales (scutella) appear on day 11 on the posterior face of the shank; most of the smallest scales (reticula) form on days 12 and 13 on the plantar surface of the foot. A single injection of 125 micrograms of retinoic acid (RA) into the amniotic cavity caused the formation of feathers on the foot scales in locations and percentages differing according to the age of embryos at the time of treatment. Injection at 10 days caused feathers to form on the feet in 57% of the embryos, almost all of which bore feathers on their scuta and scutella; a few reticula were occasionally affected. When treated at 11 days, 48% of embryos had feathered feet, the tarsometatarsal scutella and digital reticula being most frequently affected. Treatment at 12 days resulted in feathers on the feet of 15% of the embryos, all of which bore feathers on the reticula only, while the scuta and scutella were not affected. It was concluded that the foot skin regions which were affected by RA treatment were those in which scale morphogenesis was starting or about to start at the time of injection. These regions contained pre-placodal or placodal stages of scuta and scutella, and the pre-elevation or elevation stages of reticula.