Development of pterygian limbs (fins)

After a reminder of the important concepts developed in the second lesson, particularly concerning genetic and cellular approaches to chiridial limb development, this third lesson begins with a discussion of the difficulty of finding good model fish to study fin development. Next, the zebrafish is introduced and its pectoral fin development discussed in detail. While the early phase of this development resembles that presented previously in amniotes, the folding of the ectodermal layer and the formation of an exoskeleton in the most distal part clearly distinguishes the development of pterygian and chiridian limbs. The cellular origin of this exoskeleton, absent in tetrapods, is then discussed, as is its interaction with the endoskeleton, with the precise timing of the folding of the ectodermal layer perhaps adjusting the respective importance of these two types of bony parts.