Need a Hand? Just Grow it Back! How Salamanders Regenerate Limbs (A Guest Post)

By Maranda Cardiel

How cool would it be if you could regenerate your own body parts? Just imagine: you are chopping up some carrots for dinner, but whoops! You accidentally cut off your thumb! No worries, it’ll grow back in a few weeks, good as new and fully functional. No need to take a trip to the hospital and pay all of those annoying medical costs.

That all sounds pretty nifty, but that can’t actually happen, right? Tissue regeneration on that large of a scale is something you can only find in science fiction. …Or so you may think. Nature has actually found a way to regenerate full limbs and other body parts after they have been completely amputated. However, among animals with spines, this unique ability is only found in salamanders. But how does it work, and why can’t we do it too?

A cartoon illustrating examples of the three different methods of tissue regeneration in animals. A.) An adult hydra being cut into two pieces and regenerating into two separate hydras. B.) Part of a human liver being cut off and the remaining liver regenerating via cell division. C.) A salamander’s arm being amputated and undergoing epimorphosis to regenerate an entire new arm.
Source: Maranda Cardiel

There are actually three ways that animals can regenerate tissues. Some animals, such as hydras, can use the tissues they already have to regenerate themselves after being cut in two, resulting in two separate hydras. Mammals, including humans, have the ability to regenerate their livers by having the liver cells divide into more liver cells. This is how liver transplants work – a portion of liver from a live donor will grow into a fully-functioning liver in the recipient. The third method is called epimorphosis, which is the ability to change existing cells of specific types so that they can re-grow as different cell types, and this is what salamanders are able to do.

When the limb of a salamander is cut off, only the outermost layer of skin moves to cover the wound. This single layer forms a special skin cap known as the epithelial cap, and the nerves at the amputation site shrink back from the wound. Then the cells beneath the cap dedifferentiate, losing their specific characteristics so all of the different types of cells become the same and detach from each other.

A cartoon illustrating the process of a salamander regenerating its arm. A.) The limb is amputated. B.) The outermost layer of the skin begins to cover the wound. C.) This single layer of skin creates an epithelial cap and the blastema forms underneath it. D.) The cells of the blastema begin to differentiate into bone, nerves, etc. E.) The cells continue to divide and differentiate until the limb is fully formed. Source: Maranda Cardiel

Now the amputated limb has a mass of indistinguishable cells under the cap, and this mass is called the regeneration blastema. A blastema is simply a clump of cells that is able to grow into an organ or body part. Over the course of several weeks, this blastema divides into more cells and the cells begin to differentiate - or turn into multiple types - again, forming different cell types such as bone, muscle, cartilage, nerves, and skin. Eventually, the salamander will have a brand new limb.

The salamander’s body can even tell what body part it’s supposed to re-grow; if it’s amputated at the wrist it will grow a new hand, and if its entire hind leg is amputated it will grow a new hind leg. And it’s not only limbs that salamanders can regenerate – they can even grow back their tails, retinas, spinal cords, and parts of their hearts and brains!

As you can see, the process of epimorphosis is much more complicated than simply having a single cell type divide a lot. It also requires certain chemicals and patterns of immune signaling to work properly. But why can’t people do this too? One of the reasons is because when our tissues are damaged, all of our skin grows to cover and heal the wound, which forms scars. In salamanders, only the outermost layer of skin does this, which prevents the scarring that would stop tissue regeneration. The salamander’s immune system is also regulated differently than our own, which allows them to regenerate whole body parts.

Unfortunately we are not salamanders, so when you cut off your finger it’s not going to grow back. But researchers are continuing to study salamanders and their astounding regenerative abilities in the hopes of finding a way to apply it to people. Who knows, maybe someday we’ll be able to grow back our own limbs too.


Sources:

Gilbert, Scott F. Developmental Biology 6th Edition. Ncbi.nlm.nih.gov. National Center for Biotechnology Information, 2000.

Godwin, J., Pinto, A., & Rosenthal, N. (2013). Macrophages are required for adult salamander limb regeneration Proceedings of the National Academy of Sciences, 110 (23), 9415-9420 DOI: 10.1073/pnas.1300290110

Let’s Hope She Doesn’t Have Twins! (A Guest Post)

By Eric VanNatta

Of all the oddball bird species in our world, the brown kiwi surly waddles in amongst the flock. Found only in the forests of New Zealand, this small flightless bird belongs to an ancient group of birds called the ratites. Joined by ostriches, emus, cassowaries and rheas, the ratites are all flightless and dressed in shaggy feathers. In addition, the ratites have all been linked to a common ancestor (simply referred to as the ratite) that was isolated after earth’s continents shifted apart some 300 million years ago.

A size comparison of the moa and kiwi.
Drawing by Josef Korenski (around 1901)
at Wikimedia Commons.

Originally found throughout the single landmass, future generations of ratites living in the places we now call South America, Africa and New Zealand experienced changing climates and new habitat types. Depending on individual traits, certain birds had better or worse success based upon their ability to survive and reproduce. Most of these species developed longer legs used for running and lost their large wings required for flight, as we can clearly see today in the ostrich and emu. In the islands of New Zealand, the ratite developed into a similar group of species we know as the moas. Similar to the emu and ostrich, the moa was a large bird with powerful legs used for running. Eventually, several million years later, the moa, too, continued to take advantage of different habitats within the island in the absence of mammalian predators. Of the handful of new species that the moa gave rise to, one of them actually began to shrink back down in size; we know it as the brown kiwi. Although moas and kiwis were both exceptional at surviving on the island, humans later drove moas to extinction through over hunting during early island colonization.

However, the unique ancestry of brown kiwis is not the only thing that granted them an oddball award. The size of their eggs and their properties is something that has inspired curiosity in us ever since humans discovered the island several hundred years ago.

Size comparison between an kiwi and its egg.
Photo taken by Hannes Grobe at the Kauri Museum in
New Zealand. Available at Wikimedia Commons.

For a bird that can weigh in between 1.5 and 3.3 kilograms (3-7 pounds), brown kiwis’ eggs are a whopping 0.4 kg (almost a pound)! Due to the tremendous size of the eggs, a female kiwi only has room for one egg at a time. On the extreme end, up to 20% of a female’s mass before laying her egg is from her egg. That’s equivalent to a human carrying a 30-pound baby before birth!

So why the heck do kiwis have such massive eggs? What good could this possibly be? Those are two questions William Calder III set out to ask in his research on kiwi eggs.

Before he started answering questions, he compiled many of the characteristics of kiwis and their eggs. In order to look at these differences, he compared kiwis to other bird species of similar mass (seabirds, chickens, etc.), and he compared kiwi eggs to similar sized eggs from other species (emus).


Size comparison between ostrich,
emu, kiwi, and chicken eggs. Photo
by Zureks at Wikimedia Commons.

To start the laundry list of unique observations, the yolk itself is as large as that of an emu, the equivalent of about 11 chicken yolks. This gives a developing chick plenty of nourishment, and upon hatching it still has excess yolk to last 10 days without the need to forage for food. Incubation time for a kiwi egg takes 75-84 days, which is double the amount of time as comparably sized eggs. This has been hypothesized to be a result of lowered incubation temperature, since kiwis have a lower metabolism and body temperature compared to other birds, but there has been no formal investigation of this. The eggs themselves also appear to have an excess of antifungal and antimicrobial properties to endure the 3-month incubation period.

After comparing these characteristics and taking into account their unique ancestry, Calder supported the scientific understanding that kiwis’ large eggs are simply relics from their past. Generations of the moa likely decreased in size after smaller individuals took advantage of eating small prey and living in forest understory habitats. Although these resources allowed for a change to smaller physical size, there was no reason for their eggs to reduce their size. In fact, fossil collections have shown that kiwi eggs are nearly the same size as those from the 12-kilogram (26 pound) moa. Kiwis historically never had to worry about nest predators entering their burrows since there were none on the islands. Their only predators were large flying birds, so it was advantageous to keep chicks in eggs for a longer amount of time until they were more developed. The large yolk reserve also allows chicks to stay hidden during their first days of exploration, and not have to worry about eating.

Talk about an odd reproductive system and a unique lineage! Who knows, maybe future environments will present opportunities for kiwis to increase their number of offspring. As human development encroaches valuable forest ecosystems, it would be beneficial to increase the odds of the species’ survival. Surely any chance of this will take thousands of years of environmental opportunities, as have the changes from their ancestors, but it wouldn’t be the first time the bird has surprised us!


References: Calder, W. (1979). The Kiwi and Egg Design: Evolution as a Package Deal BioScience, 29 (8), 461-467 DOI: 10.2307/1307538

Infidelity in Nature: a Lion’s Story (A Guest Post)

By Devin Zingsheim

When people think of mating, especially in the case of humans, they often think of one man marrying and mating with a single female. While this provides a nice image of mating, it is not always true. In the case of humans, both males and females may stray from this image and mate with other individuals. For example, a female in a relationship may become attracted to and mate with someone she finds exciting, like a rebel. These exciting individuals are the outsiders because they exist outside the female’s main relationship. But this is only in the human species - Could this observation hold true in another species, like lions?


Photo taken by Devin Zingsheim
at the Wisconsin Wild Cat Sanctuary.

As most people know, lions live in groups of animals, called prides. Prides often consist of between one and three dominant males and several females. Females of one pride typically do not co-mingle with members of other prides. This fact means that typically in a pride, the dominant males do all the mating and father all of the cubs born into their pride. However, as with humans, could there also be exceptions in lions?

Martha Lyke of Northeastern Illinois University, Jean Dubach of the Wildlife Genetics Lab at Loyola University Medical Center, and Michael Briggs of the African Predator Conservation Research Organization investigated breeding behavior in lions. These researchers noticed that field observations recorded females of prides residing in the Etosha National Park in Namibia interacting with outsider males. Outsider males can be from other prides or they can be rebels without a pride. This is unique because past studies focusing on lions of the Serengeti revealed that females seldom interact with other prides. These observations led the researchers to three main hypotheses. First, they thought that this mingling with outside males could lead to mating and eventually births. They also thought that females might mate with more than one male, potentially leading to cub siblings with different fathers. Lastly they thought prides with fewer males would be at greater risk for these illegitimate births because the male is not around enough to drive the outsider males away.


Image of Africa with the location of Namibia highlighted.
 “Location Namibia AU Africa” by Alvaro1984 18 –
Own works. Licensed under Public Domain via Commons.

To find the answers to the three questions the researchers had, they set up a study in Etosha National Park. This national park has a program in which every lion that resides in the park is branded for monitoring. At the beginning of the study, the researchers took observations of every lion spotted and its interactions with other lions. Lions that were observed spending a lot of time with one another in a territory were considered a pride. This then allowed the researchers to identify when and if females interacted with outsider males. After identifying prides, the researchers gathered blood and tissue samples for analysis. DNA analysis indicated the parents of any cubs within the prides.

Observations led to the identification of 11 prides containing 102 lions. Prides on average contained 10 individuals and had a roughly 2:1 adult female to male sex ratio. Surprisingly, only 55% of the 164 DNA samples collected came from the 11 identified prides and a whopping 41% of cubs sampled were illegitimate and had outsider males as their fathers. Interestingly, the researchers found that these cubs fathered by non-pride males came from only five of the prides, four of which had an unusually high female to male sex ratio. Additionally, the researchers also found four cases where litters of cubs had different fathers!

These results provide a lot of new and interesting insights into the sexual behavior of lions. Evidence was found to support all three of the researchers’ hypothesis. They found that females do give birth to young whose fathers are not part of the pride and that mixed paternity does occur in this population. They also found that prides with fewer males did have cubs born to males from outside the pride. This might be due to the fact that there are fewer pride males around to protect and drive away outside males. Additionally, with fewer males around, females may want to seek out other males to ensure they get to reproduce. This study has found evidence that there may be a lot more to sexual behavior in lions than meets the eyes. It has shown that, like human females, lionesses may be tempted to run off and mate with that exciting rebel outsider male.


Works cited

Lyke, M., Dubach, J., & Briggs, M. (2013). A molecular analysis of African lion (Panthera leo) mating structure and extra-group paternity in Etosha National Park Molecular Ecology, 22 (10), 2787-2796 DOI: 10.1111/mec.12279

Why Ask for Directions? (A Guest Post)

by Anna Schneider

For the iconic monarch butterfly, the shorter days in fall mean it’s time to pack up and head south to a warmer climate! Just like clockwork, the Eastern population of monarch butterflies makes a 2000 mile journey to their winter paradise roosts in central Mexico. The journey in itself is one of the greatest migrations among all animals.

But here’s the catch: none of these butterflies has made this trip before. Several generations of monarchs have come and gone over the course of a summer, but the generation born in late August and early September are genetically prepared for months of survival without feeding or breeding. But their predecessors didn’t exactly leave them with a map. How do they know where to go? Do they have a map and compass inside their heads? The answer: yes! Well, sort of…

Think about this: if you were lost in the woods and needed to find south, what would you do? Here’s a hint: look up! The sun can be a great resource when you’re lost, and I’m not talking about just asking it for directions. As the Earth rotates on its axis throughout the day, the sun appears to travel overhead. By knowing approximately what time of day it is, you can determine the cardinal directions. Monarchs use specialized cells or organs called photoreceptors that respond to light to establish the position of the sun.

Representation of time compensated sun compass orientation used by monarchs;
Image created by Anna Schneider.

Until recently, it was thought that monarchs simply used the photoreceptors on the top portion of their compound eyes, called the dorsal rim. Past studies have shown that the signals are passed from the photoreceptors on to the “sun compass” region in their brains and the butterflies change direction based on that information. Like most animals, it was assumed that their internal clock was located inside their brains. However, recent research has demonstrated that individuals whose antennae have been painted or removed altogether become disoriented when placed in flight simulators. These monarchs do not adjust for the time of day when trying to fly south. When those same antennae that were removed were placed in a petri dish, they continued to respond to light and showed signs that they continued the pattern of time. This indicates that antennae and the brain are both needed for the monarchs to correctly determine their direction.

Diagram of features on the head of a monarch butterfly; Image created by Anna Schneider.

Now, estimating which way is South might be fine and dandy on a bright sunny day, but what happens when it’s cloudy? Not a problem for these super-insects! In another recent study, researchers tethered monarchs to flight simulators and altered the magnetic field conditions to see what would happen. When the magnetic field was reversed so magnetic North was in the opposite direction, the butterflies altered their bearings and flew exactly opposite as well. This suggests that monarchs could have some sort of way to detect the earth’s magnetic field, called magnetoreception, which could enhance the photoreception capabilities.

Many of the mechanisms behind the migration of these incredible creatures are yet to be discovered, but much progress has been made in the past decade. So next time you see a monarch butterfly, take a second look. There is more than meets the eye.

Sources:

Gegear, R., Foley, L., Casselman, A., & Reppert, S. (2010). Animal cryptochromes mediate magnetoreception by an unconventional photochemical mechanism Nature, 463 (7282), 804-807 DOI: 10.1038/nature08719

Guerra, P., Gegear, R., & Reppert, S. (2014). A magnetic compass aids monarch butterfly migration Nature Communications, 5 DOI: 10.1038/ncomms5164

Merlin, C., Gegear, R., & Reppert, S. (2009). Antennal Circadian Clocks Coordinate Sun Compass Orientation in Migratory Monarch Butterflies Science, 325 (5948), 1700-1704 DOI: 10.1126/science.1176221

Steven M. Reppert. The Reppert Lab: Migration. University of Massachusetts Medical School: Department of Neurobiology.

A True Underdog…or Undermouse (A Guest Post)

By Spencer Henkel

People love a good underdog story, and nowhere is that image more embodied than in the rodents that live in deserts. In the desert there are two main problems that animals must face: it is way too hot and way too dry. You would think that rodents, the smallest of mammals, would not have much difficulty surviving in this kind of habitat. You might think that they would need far less food and water than their larger neighbors like reptiles and birds. Unfortunately, this is not the case; in fact, rodents’ small size actually makes life harder for them in such harsh conditions. Rodents gain and lose body heat faster through surface exchange with their environment, their highly active lifestyle requires a lot of food and a high metabolism, which generates a lot of extra heat that must be dispersed, and the distance they can travel to find food and water is extremely limited. Desert rodents must find ways to deal with all these issues, a tremendous feat for such tiny creatures.


Photo of a Golden Spiny Mouse (Acomys russatus) in Israel
by Mickey Samuni-Blank at Wikimedia Commons.

The most pressing concern of any animal that lives in the desert is making sure its body has enough water to carry it through the day. Needless to say, water can be hard to come by in such arid lands, and what water is present is usually found in seeds, tubers, and other plant material. Rodents will find and take in this water, but they face another problem: the contents of their diet are very salty. The rodents must now find a way to get rid of this excess salt while still holding onto a fair amount of water, for they cannot afford to simply excrete a steady stream of urine like we can. They must call upon a chemical from their brain, vasopressin, to help them out with this process. Vasopressin is an antidiuretic hormone, what I like to call an “anti-makes-you-pee”. It is made in the hypothalamus part of the brain, and when called upon it exits the pituitary gland and travels by blood to the kidneys. Once there, vasopressin causes the tiny blood vessels in the kidneys to clench up, slowing the flow of blood and increasing the time water has to be reabsorbed before urine is produced. When Nature eventually does call, the rodents will have made a small amount of urine that rids them of a whole lot of salt.

Now the rodents must turn to the other issue at hand: keeping cool. Water plays an active role in cooling an animal’s body by evaporation through sweating, panting, urinating, and defecating. Unfortunately, as with the salt in their diet, rodents can’t afford to lose all that water if they want their insides to keep functioning. So instead, rodents will lower their metabolisms. This reduces the amount of heat generated inside the body, so their core temperatures will decrease. A lower metabolism will also reduce the amount of water the rodents need to cool themselves down. However, if this process keeps up, the animal could die of hypothermia, ironically. So to keep that from happening, these rodents increase the amount of heat generated by their brown fat, masses of fat found primarily in animals that hibernate. This tissue will keep the animal’s core body temperature stable even when their metabolism slows way down.

In spite of their size, rodents actually have a rather tough time surviving in the desert. Yet they have found efficient ways of dealing with such extreme challenges. They can conserve enough water to live while still filtering out a great deal of salt, and they can slow down their own heat production while maintaining stable body temperatures. It is indeed quite a feat when the smallest of mammals succeeds in living in one of the harshest places on earth!

Sources Cited

SCHWIMMER, H., & HAIM, A. (2009). Physiological adaptations of small mammals to desert ecosystems Integrative Zoology, 4 (4), 357-366 DOI: 10.1111/j.1749-4877.2009.00176.x