Extended Data Figure 10: Schematic representation of pectoral fin development and its relation to the fin-to-limb transition.

a, Induction of fin fold formation and subsequent actinotrichia formation are dependent on somitic mesoderm-derived apical fold inducing cells (AFICs) cells infiltrating the AER. After the induction of fin bud formation (1), a somite-derived lineage (green) will infiltrate the AER (yellow, 2). Subsequent fin fold (red) induction (3) is dependent on this process, as is the formation of actinotrichia (dashed lines, 3 and 4), which rely on the secretion of collagen directly from these cells (green, 3 and 4). b, Throughout evolution of the gnathostome lineage, the cells that regulate fin fold induction have been gradually lost as a necessary prerequisite for limb formation. Schematic representation of fin development including the transition from AER (yellow) to AF (red). The timing of somite-derived cells (green bars) infiltrating the AER coincides with the AER to AF transition. Relative numbers of somite-derived cells are represented by the height of the green bars. In cartilaginous fish, cells infiltrate the AER early and abundantly to remain in the fin fold throughout development. In ray-finned fish, infiltration into the AER is delayed and the number of cells infiltrating the AER is also reduced, but infiltration continues throughout development. The number of cells infiltrating the AER and their timing of residency is further reduced in lobe-finned fish, while finally in tetrapods there is an absence of somite-derived cells in the AER, and a lack of AF induction.