Salamander Limb Regeneration
If a salamander gets in a fight, it may surrender its tail to the enemy as a defense mechanism. After all, in a few weeks time, it can grow a new one. This is a pretty complex process, but in a nutshell, regeneration involves shuffling around the cells at the wound site and assigning them a new specialization.
Within in the first hours after getting a body part lobbed off, the salamander's epidermal cells in the area migrate to cover the open flesh. That layer of cells gradually thickens in the following days, forming the apical epithelial cap [source: Muneoka, Han and Gardiner]. Cells within the salamander's tissues called fibroblasts also congregate beneath that epidermal covering. Fibroblasts are undifferentiated, which means that they're free to become multiple types of cells, depending on which body part needs replacing.
After that initial phase, the blastema develops from the mass of fibroblasts; the blastema will eventually become the replacement body part. Researchers recently discovered that the expression of a protein called nAG kick-starts blastema growth [source: Kumar et al]. The blastema is sort of like a mass of human stem cells in that it has the potential to grow into various limbs, organs and tissues. But how does the salamander's body know what needs replacing? The genetic coding in the blastema contains a positional memory about the location and type of missing body part. That data is stored in the Hox genes in the fibroblast cells [source: Muneoka, Han and Gardiner].
While this is happening, capillaries and blood vessels are regenerating into the blastema. As the blastema cells divide and multiply, the resulting mass becomes a bud of undifferentiated cells. In order for that mound to become a full-fledged limb, tail or other body part, it must receive stimulation from nerves [source: Kumar et al]. However, when salamanders drop their tails, they lose not only flesh but also nerves. That means that nerve axon regeneration is happening at the wound site in tandem with tissue, bone and muscle regeneration.
From there, the cells differentiate and create the appropriate body part. As part of that positional memory in the fibroblast cells, the blastema knows to grow in the proper sequence to avoid defective regeneration. For example, if a salamander loses a foot at its ankle, the blastema will develop outward to form a foot instead of an entire leg.
With the salamander as the blueprint, scientists hope to someday engineer blastemas from human cells. Until then, our amphibian friends are still the reigning regenerators of the animal kingdom.