By Abby S. Van Voorhees, MD, June 03, 2013
In this month’s Acta Eruditorum column, Physician Editor Abby S. Van Voorhees, MD, talks with Ashley W. Seifert, PhD, about his recent Nature article, “Skin shedding and tissue regeneration in African spiny mice.”
Dr. Van Voorhees: Let’s start by defining autotomy. Since most of us are far from our biology days, please remind us what it means. In what part of the body is it generally seen? Which animals have been known to demonstrate this ability?
Dr. Seifert: Autotomy is the self-inflicted or externally induced loss of an appendage without death or drastic decline in fitness. Basically it means if an animal is attacked it can release a body part, usually an appendage. This is usually a self-defense mechanism to avoid predation. The classic example would be something like a gecko, which can autotomize its tail in response to a predator attacking — if a bird or rodent tries to grab hold of the gecko by the tail, the gecko’s tail will break off. The gecko moves in one direction, the tail is left wriggling, and that distracts the predator, allowing the gecko to escape. Autotomy in vertebrates is usually associated with geckos and skinks. It’s also seen in some invertebrates, including crustaceans like lobsters and crabs that lose their claws, and in other vertebrates, including several species of lizards and salamanders as well as several species of rodents that can autotomize their tails and, as we show in our paper, sometimes even their skin.
Dr. Van Voorhees: What is the difference between “true autotomy” and “false autotomy”?
Dr. Seifert: Autotomy is quite rare in rodents; it’s been documented in 34 species. Of those documented cases in mammals, you see two versions, true and false autotomy. In one case, an animal will grab on to the tail and the entire tail sheath will slide off the tail and leave behind the vertebrae and connective tissue associated with it; later on the animal usually chews off the tail. That’s referred to as false autotomy because the tail itself isn’t breaking at some sort of break plane. True autotomy refers to those few rodent species where when something grabs the tail or bites it, it actually breaks along pre-defined break plans in between the vertebrae. [pagebreak]
Dr. Van Voorhees: What prompted you to look at these two species of African spiny mice?
Dr. Seifert: There are really two reasons. Firstly, when I began my post-doctoral work in regeneration I was working with species of salamanders that are well-known for their powers of limb regeneration and I had just become interested in skin and wound repair. So I started developing a model in the axolotl [a Mexican salamander] for full-thickness excisional wounds. Right after I began that work, I had the opportunity to accompany my wife, who is an ecologist, to Kenya for her field work, and before we left one of her colleagues who knew I was interested in regeneration told me about these species of rodents over there that, when mammalogists trapped them, released portions of their skin or shed it, as they described it to me. I laughed when I heard that because it sounded like a terrible adaptation for a mammal in Africa to lose part of its skin — a behavior like that could result in all sorts of problems: infection, water loss, blood loss, etc.
I figured I would spend some time trying to trap these animals and first document whether or not they tore their skin easily or lost pieces of it. And that’s exactly what I experienced trapping some of these spiny mice in the wild; they’re very difficult to handle, especially when they’re stressed, and I saw everything from small tears to very large tears — in our paper we show an animal that lost 50 percent of its back skin. Once I figured out that it was actually happening, I thought it would make an interesting project to investigate how they repaired these kind of injuries. You might predict an animal with this kind of adaptation to have some compensation with respect to its ability to heal wounds. [pagebreak]
Dr. Van Voorhees: Tell us about your study. What were you able to document about these mice?
Dr. Seifert: First, from a behavioral standpoint we were able to document that the mice have skin that readily tears and that this can cause pieces to come completely off their bodies, either when captured and they’re stressed or from holding them together in a cage if they fight. And those are usually deep wounds, full thickness; the skin may be removed or there may be just a little tear which will then heal. Secondly, we made some mechanical measurements and what we found is that their skin is incredibly weak so it tears under very low amounts of tension. That was a way for us to mechanistically show, compared to lab mouse skin, how much weaker it was. We found that it took 70 times more energy to break a piece of lab mouse skin compared to the skin of these animals.
Dr. Van Voorhees: In addition to the mechanical properties, were you able to identify structural properties in the mouse skin that allowed for this tearing?
Dr. Seifert: We compared the histological and cellular structure of their skin to lab mouse skin. We found that it was pretty similar in terms of the basic organization of the skin. But the percentage of the dermis that was occupied by adnexa, which are the hair follicles and glands, was greater in the spiny mice, mainly because they have these large spiny hairs. We hypothesized that this might have something to do with the structural weakness. We also found that their skin was rather oily; we’re not sure exactly how, or if, this contributes to healing, but it seems to be associated with weaker skin in other rodents. [pagebreak]
Dr. Van Voorhees: Once injured, what allows these tears to heal? Is it a form of regeneration or fibrosis? Does wound contracture play a role? Do these wounds heal quickly? What proof were you able to identify for this process?
Dr. Seifert: On the healing and regeneration front, we found that when we made large circular wounds on their backs, 1.5 cm in diameter, first they would contract those wounds to a very high degree, which is common in loose-skinned mammals, so about 95 percent of the original wound area would contract. But then in the center of that wound we found that they were capable of regenerating hair follicles, all throughout the wound area, and when we looked at the dermis during repair it looked as if the fibrotic response was muted. We observed production of collagen 3 and a much slower onset of collagen type 1 production, which is sort of the opposite of what you see in a normal scarring wound where you see aggressive production of type 1 collagen that’s deposited along with collagen 3. Lastly, building on some work that was done in the ’50s, we turned to the ear, where contraction doesn’t occur, and made 4 mm hole punches through their ears. That was extremely exciting because we found that when we removed contraction from the equation, they were able to regenerate almost all of the missing tissue, which includes not only the skin and hair follicles, but also the cartilage which runs down the center.
Dr. Van Voorhees: So you’re saying that there’s a component of both regeneration and of more traditional fibrosis and contraction occurring in most places on these mice, maybe in different percentages in different locations?
Dr. Seifert: Yes, and that hair follicles, glands, and cartilage are regenerating. From a clinical perspective those are some of the most exciting aspects of the finding. I think the general assumption is that to regenerate these structures you have to go about employing different, unique molecular pathways or that the process might be completely different in some way. Of course, that may very well be true in some situations. However, what we’ve showed is that all the same wound repair processes are occurring — inflammation, fibrosis, contraction, etc., but either the timing or the relative composition of what’s being produced differs. And when fibrosis is controlled in some way it’s almost as if you liberate a regenerative capacity that’s already there, it just isn’t occurring because the fibrotic response is so strong that scarring ablates any structure that was there and prevents its regeneration. [pagebreak]
Dr. Van Voorhees: What are possible implications of this work for human wounds?
Dr. Seifert: I think this type of basic science helps us understand the mechanisms that are responsible for how wounds either heal with a scar or regenerate. I think first and foremost it will provide us comparative information about how scarring and regeneration in an adult mammalian model of wound healing are different and the relevant aspects of the processes involved, the timing, etc. It may be that as we continue to investigate we can test some of the outstanding hypotheses about why regeneration doesn’t occur in mammals, such as the role of the immune system, the proposed tradeoff between adaptive and innate immunity with adaptive immunity favoring scarring. We’ll be able to test that in these animals. Also the role of inflammation, how either pro-inflammatory or anti-inflammatory molecules might be signaling and controlling fibrosis. Lastly, I think we’ll be able to take a look at the composition of the extracellular matrix and what role the environment within the wound bed is playing that either stimulates regeneration or stimulates scarring. It will serve as a model to test hypotheses and understand basic mechanisms of mammalian regeneration.
Dr. Van Voorhees: It will also be very interesting to look at the role that having the spiny hairs plays in the dermis for wounds. Certainly there are differing wound properties for humans as well in hair-bearing vs. non hair-bearing locations.
Dr. Seifert: That’s exactly correct. I’ve had several people when I’ve given talks about this ask me about the role of those hair follicles, and you can see in the paper how much bigger they are. It’s possible that stem cells of those hairs may have a larger population which contribute not only to re-epithelialization but to the creation of new follicles, and that’s something I suspect researchers will be interested in following up. [pagebreak]
Dr. Van Voorhees: How does this connect to what you’ve seen in salamanders? Where might it lead?
Dr. Seifert: We use salamanders as a model for skin regeneration as well. (See “Skin Regeneration in Adult Axolotls: A Blueprint for Scar-Free Healing in Vertebrates.” PLoS ONE 7(4): e32875. doi:10.1371/journal.pone.0032875.) One of the things the wound healing field lacks is an adult model of skin regeneration. In the early ’90s a popular basic science model was the fetal model for scar-free healing. People would do in-utero surgeries on all sorts of mammals, sheep, mice, rats, etc., and showed that the fetal animals had the capacity to regenerate skin after wounding compared to the adults, which would scar. The problem is that the animals were fetal — so we don’t have an adult model for what skin regeneration would look like.
I started using the axolotl and adopted a wound healing model that people used in mammals, these full-thickness excisional wounds, and showed that these salamanders, over the course of 80 days or so, were capable of regenerating all the different layers of the skin, including the muscle beneath. I was excited about that because these animals are tetrapods, they live on land the same way that mammals do; their physiology might be different, but we were able to compare similar wounds and the composition and timing of the extracellular matrix and production. I’m excited about using this model as well because I think that, in conjunction with some of the spiny mouse work, it will give us insight into common pathways and events that are used during skin regeneration and allow us to ask questions about how fibrosis is regulated. Using this knowledge, we may be able to develop new therapies or products to treat human wounds; at least, that is my hope.