Figure 4.

Progenitors in appendage regeneration. Axolotl limb and zebrafish fin regenerate through the formation of undifferentiated blastema (depicted in green for axolotl and yellow for zebrafish), which generates the differentiated cell types of the regenerating structure. Blastema cells are fate-restricted, which yields differentiated progenies of the same lineage from which they are born. The exact progenitors might differ between species, as progenitors for regenerating muscles are derived from differentiated muscles in newts, but from the satellite cells in case of axolotl. Although lineage-restricted blastemal progenitors are also described in zebrafish fin regeneration, osteoblasts (red cells) ablated before the amputation can still yield regenerating osteoblasts. This indicates that other cell types in the blastema can also differentiate to osteoblasts when required.