In species capable of regrowing limbs and organs, such as salamanders and zebrafish, tissues form a blastema at the site of regeneration. This mass of cells recapitulates much of the behavior of embryonic development, including the complex signal interactions that steer the construction of replacement tissue. How exactly are the correct structures produced? Researchers hope that understanding the underlying processes will enable the inducement of similar cellular activity to heal injuries and age-damaged tissues in our species. That better understanding may also have other applications, such as in ongoing efforts to find a robust way to build complex blood vessel networks to support engineered tissue, which at the moment is one of the limiting factors preventing the creation of entire organs from a patient's own cells:
When parts of the zebrafish tailfin are injured by predators, or are experimentally amputated, the lost tissue is replaced within three weeks. Zebrafish fins consist of a skin that is stabilized by a skeleton of bony fin rays; similar to an umbrella that is supported by metallic stretchers. Fin rays are formed by bone-producing cells, the osteoblasts. In order to rebuild an amputated fin, a large number of new osteoblasts have to be formed by cell divisions from existing osteoblasts.
Retinoic acid is required to regulate the addition of bone material in growing fish. During regeneration, mature osteoblasts have to revert to an immature osteoblast precursor, which enables the switch from bone synthesis to cell division. The switch requires retinoic acid levels to drop below a critical concentration. However, upon amputation the tissue beneath the wound initiates a massive bout of retinoic acid synthesis that is required to mobilize cell division in the fin stump. How do mature osteoblasts circumvent this dilemma? The answer: osteoblasts that participate in regeneration transiently produce Cyp26b1, an enzyme that destroys and inactivates retinoic acid. Protected by this process, osteoblasts are able to rewind their developmental clocks, thus turning into precursor cells that contribute to a pool of undifferentiated cells, the blastema. Cells in the blastema pass through a number of cell divisions to provide the building blocks for the regenerated fin.
However, these cell divisions are supported by high concentrations of retinoic acid, which poses the next predicament: The reversion to become a mature osteoblast is inhibited by high levels of retinoic acid. Connective tissue in those areas of the blastema from which new mature osteoblasts eventually emerge produces the retinoic acid killer Cyp26b1. This lowers the local concentration of retinoic acid, so that osteoblast precursors are again able to mature and produce new fin rays. Other parts of the blastema, which replenish the supply of cells needed for regeneration to occur, continue to produce retinoic acid. "This is an elegant mechanism that ensures a gradient of cells experiencing high and low levels of retinoic acid. This allows two processes to run in parallel during regeneration: Proliferation for the production of all cells that replace the lost structure and redifferentiation of osteoblasts where the skeleton re-emerges."
How is the exact shape of the fin skeleton regenerated? In order to form new fin rays, newly formed osteoblasts have to align at the correct positions. Osteoblasts are ultimately guided to target regions by a signaling protein called Sonic Hedgehog. This is produced locally in the epidermis, a skin-like layer that covers the fin and the blastema. However, signal production only occurs in locally restricted cells that are free of retinoic acid. Such epidermal cells produce Cyp26a1, an enzyme that is functionally similar to Cyp26b1. Lastly, it emerged that osteoblasts themselves exert a piloting function for other cell types, particularly mesenchymal cells and blood vessels that also have to be directed to appropriate destinations during the rebuilding process. "The re-emergence of the skeletal pattern relies on a navigation system with interacting parts. Initially, retinoic acid is inactivated where new rays are to form. This allows the local production of a signal that pilots immature osteoblasts to areas where existing fin rays are to be extended. Interestingly, over the course of regeneration other cell types in the blastema are informed by osteoblast precursors to respect the boundaries between emerging fin rays."