Early events in retinoic acid-induced ptilopody in the chick embryo

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Abstract
Intra-amniotic injection of 125 μg of retinoic acid to 10-day old chick embryos causes the formation of feathers on the scales of the anterior face of the tarsometatarsus. The early effects of retinoic acid (RA) on the chick foot integument have been studied between 12 h and 72 h following RA injections by two methods. Firstly, sequential fixation in glutaraldehyde and then osmium tetroxide to follow the early changes at the macroscopical and ultrastructural levels. Secondly, sequential grafts of contralateral samples on to chorioallantoic membrane (CAM) of nontreated chick embryos to test their morphogenetic performance and to determine the minimum time for RA to take effect. Results show that during the first 24 h RA causes morphological changes of both epidermal and dermal cells in almost half of the injected embryos. In particular, the dermal-epidermal junction is transformed from scale-type into feather-type. However, the development of grafted samples shows that feather morphogenesis is irreversibly undertaken only 24 to 48 h after the treatment. At this stage, roundish feather-like placodes are formed instead of the normal rectangular, scale placodes. The scales, the formation of which has been temporarily inhibited, resume their development between 48 h and 72 h after the the injection, proximally to the feather buds, so that feathers are finally carried by the distal tips of the scales.