Development of the tetrapod limb (chiridian)

After a brief reminder of both the fundamental question posed in this lecture and the important notions developed in the first lesson, this second lesson begins with a discussion of the concept of homology and the different depths (structure, gene expression, fine mechanisms...) that need to be considered when trying to establish evolutionary correspondences between structures as different as the most distal parts of pteridian and chiridian limbs. Next, after some notions of classical embryology, an Abstract of the major stages in tetrapod limb development is presented, with particular emphasis on the genetic networks involved. Two more cellular approaches are then described. The first concerns the analysis of the sequences of cartilage cell condensations that lead to the organization of bone pieces, and the conservation of this sequence of events in amniote vertebrates. The second presents a recent technological development (RNA sequencing at single-cell level) applied to the chiridian limb, which now makes it possible to redefine cell lineages and their dynamics over time with very high analytical precision. Will these recent advances shed new light on the question of how the ancestral fin of a sarcopterygian fish was transformed into today's tetrapod limb?