All Things Stem Cell

Salamanders have the amazing ability to re-grow a limb after it has been cut off. It is thought that by better understanding this regenerative ability, researchers will be able to apply this knowledge to humans and improve wound healing. Recently it was reported that salamander limb regeneration may occur in a different way than was previously thought; in short, the severed limb may not need pluripotent stem cells to regenerate, as was believed, but only multipotent or unipotent stem cells, stem cells with relatively restricted fates.

In salamanders, when a limb is severed the resultant limb bud undergoes a distinct process to regenerate the lost limb. The epithelial layer quickly spreads across the amputation site, closing the wound within 24 hours (Mescher, 1996). This epithelial layer thickens and becomes what is referred to as the wound epithelium (WE). As the immune system responds to the injury, macrophages and neutrophils arrive to clean up the wound site beneath the WE. The existing injured tissues and cells are broken down as well as the extracellular matrix, which is made up of proteins that surround cells to hold them together and stimulate normal cellular functions. It was thought that at this time in the regenerative process other resident cells below the WE become multipotent mesenchymal stem cells (MSCs) (see Figure). These eventually form a mass of MSCs called a blastema (Mescher, 1996; Brockes and Kumar, 2005). The blastema was thought to contain a homogenous group of pluripotent stem cells that had “dedifferentiated” or “redifferentiated,” meaning they had reverted back from their committed fates to function as very potent stem cells in order to recreate the limb. The WE stimulates the cells in the blastema to proliferate, making new cells and extracellular matrix, though more than is required for simple repair; the WE signals the blastema cells to regenerate the entire lost limb (Mescher, 1996; Kragl et al., 2009).

Limb regeneration in the salamander after limb amputation (time course going from the top down). Shortly after the limb is amputated, the epithelium layer covers the exposed limb bud, forming the wound epithelium (WE). A group of stem cells collects below this layer, forming the blastema (at the tip of the bud). The WE signals the stem cells below it to rebuild the limb, recreating the limb from the point of injury out towards the hand. The final regenerated limb is indistinguishable from the original.

Recently, Elly Tanaka’s group showed that multiple different groups of stem cells with relatively limited fates, being only multipotent or unipotent, may actually regenerate the salamander limb, in contrast to the previously held belief that one homogenous group of pluripotent stem cells was responsible (Kragl et al., 2009). The group labeled the major tissue types in the limb (using green fluorescent protein [GFP]) to track the tissue types during regeneration. While the blastema does appear histologically homogenous, they found it may be made up of all the different tissue types found in the complete limb; all the cells in the blastema may be types of progenitor cells that can become only one or two specific adult tissue types. In short, many different cell types may coordinate to recreate the limb (Kragl et al., 2009).

Only one tissue cell type in the blastema was able to differentiate into a cell of a different tissue layer in Tanaka’s experiments. Specifically, Tanaka’s group reported that precursor cells for muscle, epidermis, cartilage, or Schwann cells (neural cells) could only create muscle, epidermis, cartilage, or Schwann cells, respectively; each of these cell types was limited to become its own cell type. The dermis tissue layer was the only cell type found to be able to become more than one fate; the dermis could become both dermis and skeleton/cartilage (but not muscle or Schwann cells). Dermis and cartilage have a common developmental origin in the mesoderm, which helps explain why the dermis layer cells could become both of these cell types. Additionally, the group investigated whether these progenitors know their proper final position in the limb along the proximal-distal (i.e. shoulder-hand) axis. The researchers found that cartilage precursors do not have such positioning abilities, while the Schwann cells do. This indicates that the positional identity is tissue specific (Kragl et al., 2009).

While this research was conducted using salamanders, it may be quite relevant for future regenerative medicine research in humans; it shows that we may not need a pluripotent state for complex tissue regeneration, but instead could use multiple different stem cells with much more restricted fates. However, this does not necessarily make the process more feasible technically, but it may give researchers a more focused direction for future studies. To read other coverage of this ground-breaking work by Tanaka’s group, reported just last month, take a look at (Baker, 2009) or (Johnson, 2009).

References

Baker, M. Regenerating limb tissue may not dedifferentiate. Nat. Rep. Stem Cells. 2009.
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Brockes, J. P. and Kumar, A. Appendage Regeneration in Adult Vertebrates and Implications for Regenerative Medicine. Science. 2005. 310(5756): 1919-1923.
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Johnson, S. L. Memory of Fate and Position, Colorized. Dev. Cell. 2009. 17(1): 5-6.
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Kragl, M., Knapp, D., Nacu, E., Khattak, S., Maden, M., Epperlein, H. H., Tanaka, E. M. Cells keep a memory of their tissue of origin during axolotl limb regeneration. Nature. 2009. 460: 60–65.
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Mescher, A. L. The cellular basis of limb regeneration in urodeles. Int. J. Dev. Biol. 1996. 40: 785-795.
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Original “Salamander Limb Regeneration” image modified from the Wikimedia Commons and redistributed freely as it is under GNU Free Documentation License.

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