In Aztec mythology, the legend of the Five Suns is the creation myth of our world. The
dawning of the Fifth Sun brought about the era in which humans were created, and is the
era in which we currently live.
According to the myth, this Sun was created by the gods, but was not able to move across
the sky. That is, not without the nourishment of sacrifice. The gods themselves were the
first to be sacrificed to the Sun, setting the tradition of human sacrifice that the
Aztecs would later follow.
The god Xolotl, however, did not want to be sacrificed. To evade detection, he changed
himself into other forms- first the maize plant (xolotl), then the agave plant (mexolotl),
and finally, the water monster, the axolotl.
When he was found, his brother, Quetzalcoatl, spared his life, but banished him to live
forever in darkness as an axolotl. Today, this ‘water monster’ is believed to still
hold the spirit of a god.
Axolotls are a type of salamander found only in the area of Lake Xochimilco outside Mexico
City, near the ancient capital of the Aztecs, Tenochtitlan. And beyond their spiritual ties,
axolotls are unlike any other salamander.
They live permanently in the water, and in fact, live permanently in their juvenile form
- never maturing into their adult body, a forever tadpole. And, on top of its age-defying
ability, the axolotl can regenerate almost any part of its body, from missing limbs,
tails, organs, parts of the eye, and even parts of its brain.
For hundreds of years the axolotl has been central to our understanding of how our own
organs develop and function, its unique physiology holding the answers to many of our most pressing
biological questions. And now, we hope to crack the code of its regenerating superpowers,
to one day be able to harness them for ourselves.
But, with only a few hundred kilometers of waterways that remain of its natural habitat,
the axolotl is on the brink of total annihilation in the wild, and the time to unlock these
mysteries is running out.
Why is the axolotl so different from every other vertebrate? What is it, exactly, about
their very unusual physiology that makes them the most important amphibian known to science?
.Axolotls are members of the tiger salamander species complex, and they evolved relatively
recently compared to other salamander species in the region. They grow to be about 23 centimeters
long, and come in a variety of colors. And unlike almost all other salamanders, they
are neotenic, meaning that they keep juvenile characteristics into adulthood. They never
undergo metamorphosis. For them, this means they remain aquatic, never venturing onto
land, they keep their external feathery gills and tail fins, and they lack moveable eyelids.
When they were first shipped from Mexico to prominent French zoologist Auguste Dumeril
in the 1860s, he was certain that the axolotls were the larvae of an unknown type of salamander.
That is until 6 months later, when his specimens, very unexpectedly, reproduced. And because
the adult stage of animals is defined by sexual maturity, these reproducing axolotls had to
be adults. Just how strange the axolotl is was only beginning to be revealed.
Overall, retaining juvenile traits seems like a cute, but weird thing to happen in nature.
At first glance it looks like a whole species that has its growth permanently stunted.
But although axolotls are one of the only salamanders to stay in their juvenile form,
other salamanders from the region can sometimes be found doing the same thing, with some individuals
staying in the water for their whole lives.
At first, naturalists didn’t know what to make of this phenomenon. But looking at where
these salamanders lived started to give a clue as to why this may be selected for in
nature.
All the Mexican lakes where they are found have a few key things in common. First, they
are all large, relatively permanent bodies of water - at least they used to be before
human intervention. Second, the lakes are surrounded by desert, and in many cases are
the only permanent body of water in the region. And third, all of these lakes historically
did not have any large, predatory fish.
The salamanders in these lakes, sometimes, or always, fail to mature into adulthood,
because these lakes are the only suitable habitat in the midst of a harsh and dry terrestrial
environment. Salamanders normally metamorphose to take advantage of both water and land,
but only if the land is somewhat damp. But staying in the water forever is a strategy
that only works with no major predators lurking beneath the surface, along with certain pH,
temperature, and altitude requirements. These Mexican lakes just happen to completely fit
the bill, giving the salamanders a unique ability to thrive there.
For a while, it was thought that the axolotl always fails to undergo metamorphosis, that
its neoteny is a permanent feature of its biology. While this may be true most of the
time, it isn’t true all the time, as French zoologist Dumeril learned in 1865, when he
was given an even bigger surprise from his axolotl specimens.
To his shock some of the individuals in his care transformed into their adult, land-living
form. They looked sort of like tiger salamanders, but different enough to still be considered
their own unique species. Dumeril searched the rest of his life for an explanation of
why these axolotls transformed, but died before he ever got the answer.
But in the early 1900s, the transformation of the axolotl again came to the front of
research interests. During this time, researchers were narrowing in on thyroid gland tissue
as a mechanism for amphibian metamorphosis, and the axolotl was the most suited animal
to clearly demonstrate its effect.
When axolotls were fed thyroid tissue from livestock, the axolotls would metamorphose,
losing their external gills, shedding their larval skin, and venturing onto land. Basically,
thyroid tissue is a real life moonstone for the axolotl.
In other instances, axolotls can transform when forced to breathe air, but this is stressful
on the animal and takes much longer. However it may be why some in captivity have unexpectedly
metamorphosed.
The discovery of transforming axolotls played a central role in the road to our understanding
of thyroid hormones like thyroxine, which was first isolated in 1915 and synthesized
in 1926. This marked the beginning of what would become the axolotl’s immense contribution
to medical research.
All organisms have some level of regenerative abilities. For some animals, regeneration
is just a part of life. Flatworms and hydras can regenerate their whole bodies, even from
a tiny piece of their original selves. Starfish, octopuses, or even crabs can lose an arm,
and grow it back without much of a problem. But these animals all share something in common
- they are all invertebrates.
In vertebrates like us, regeneration is pretty much limited to regrowing skin or scar tissue
over a wound. Except for our liver, we can’t regrow our organs, nor our limbs. In fact,
almost no vertebrate can regenerate much beyond what we can.
That is, except for the axolotl, and the other members of the salamander order. The only
other vertebrates that can regrow limbs are frogs, but frogs lose the ability once they
fully turn into their adult form. The axolotl retains its regeneration ability throughout
its entire life.
The way an axolotl regrows a limb begins with the limb’s surviving cells. Once the limb
has been severed or lost, a blood clot rapidly stops the bleeding at the cut, and a layer
of cells covers the area of amputation. This part is not so different from the way we would
heal.
Then, over the next few days, the cells underneath the epidermis begin to rapidly divide, forming
a cone-shaped structure known as a blastema, also called the regeneration bud.
The blastema is what holds the key to regeneration. It’s where regular bone, cartilage, or muscle
cells de-differentiate, meaning they lose their identity, and transform back into stem
cells. Slowly, they start reforming bone, skin, and veins, much like stem cells would
do when the animal is first developing in its egg. As the cells in the blastema continue
to divide, nerves and blood vessels connect it to the rest of the body. The limb eventually
grows back completely, looking like an exact copy of the limb that was lost.
And while total limb regeneration is impressive enough, axolotls are also capable of something
that seems like it should be downright impossible: regrowing parts of their brain.
In mammals like us, and in fact most vertebrates, neurons typically only grow in the embryonic
stage. Any neurogenesis that happens in adulthood is quite limited. If the brain is damaged
in adulthood, it usually can’t regenerate new neurons, blood vessels or tissue.
Yet somehow, doing this is no problem for the axolotl. By de-differentiating stem cells,
they can add new neurons throughout their life and can regenerate the spinal cord and
parts of the brain after an injury.
Salamanders are the only four-legged vertebrates that can transform regular cells back into
stem cells, allowing them to re-develop into new tissue. This may seem like a superpower,
but the axolotl doesn’t actually have special regeneration genes to do this. They are simply
regulating their genes differently from other animals. This fact gives scientists hope that
we may have this ability still locked within us - an ability that, if unlocked, could greatly
benefit people suffering from certain organ failures, amputation of a limb, or serious
burns.
While we still don’t know the exact mechanisms that allow the axolotl to de-differentiate
cells, some of the key components are starting to be unveiled, many of them resembling the
cells and molecules that are already at work in our bodies.
One way to learn what molecules and cells are necessary for an axolotl to regenerate,
is to look for molecular tweaks that cause them to lose their regenerative ability.
For example, macrophages are known to be critical during inflammation in response to an injury
in many animals, and have previously been connected to regeneration. And so researchers
found that injecting a drug that eliminates the macrophages in an axolotl’s limb before
amputation causes it to form scar tissue instead of regenerate. This shows that macrophages,
a type of cell we readily have, are an important component for regeneration.
Researchers similarly found that a protein called transforming growth factor-β is key
in axolotl regeneration - a molecule that is also essential in preventing scar tissue
in injured human embryos during the first trimester.
And, on top of this, researchers have also found that both mice and humans can regenerate
a partially amputated finger or toe - though humans lose this ability as they get older.
Researchers believe this is due to a type of stem cell that lies beneath the fingernail,
that normally helps the fingernails to perpetually grow. They now think that these cells could
also be a remnant from a time when our regeneration abilities were stronger.
The regrowth of the mice digits studied depends on two certain types of proteins called Wnt
and FgF2 proteins and certain signalling pathways - proteins and pathways that all seem to be
the same as those in salamanders during regeneration.
All of this suggests that we may partially retain the same regeneration abilities as
the axolotl, abilities that could, one day, be reawakened. For this reason, the axolotl
is one of the most incredible and important research animals in science, along with becoming
a beloved pet in freshwater aquariums worldwide. And yet, they are facing almost complete annihilation
in the wild. The conservation story of the axolotl is a unique one, and if we are not
careful, we risk losing one of the most incredible creatures on earth.
The wild axolotl is teetering on the brink of extinction.
Their population has exponentially declined in the last few decades. In a 1998 survey,
it was reported that around 6,000 individuals lived per square kilometer in their habitat.
In 2015, just 35 individuals were found per square kilometer. They were predicted to go
extinct in the wild in 2020, though have just barely escaped this fate. No one knows exactly
how many are left now, but the numbers are dire.
Today, the axolotl is found naturally in only one place: Lake Xochimilco.
But the modern day version of this lake is more like a series of small canals. It’s
a phantom of what it used to be, representing only 2 percent of the original massive wetland
system. Centuries ago, this area contained several great lakes, which were home to many
Mesoamerican cultures, including the Teotihuacanos, the Toltecs, and later the Aztecs. At the
time of Spanish conquest in 1519 the size of these lakes would have covered much of
modern day Mexico city.
Due to its naturally shallow waters and its freshwater springs in the south, Lake Xochimilco
was the center of chinampa agriculture in the centuries before the Spanish arrived.
These artificial islands created an extensive network of canals. The ancient cities here,
in a way, looked a lot like the Venice of today. Here, the axolotl thrived alongside
humans.
But over time, the population of the area grew and grew, and eventually, the lakes were
all drained with the exception of Lake Xochimilco, to prevent flooding and to allow urban expansion.
What remains of Lake Xochimilco today is highly polluted, full of dangerous algae blooms and
invasive, large fish which both outcompete and eat the axolotls. It’s no surprise that
the axolotl is almost gone there.
And yet, in labs around the world, the axolotl is abundant. They have even been called the
"white mice of amphibians" since they are ideal for research and are easy to rear in
the lab. They are the most distributed amphibian in the world - so you may ask the question,
if the axolotl is thriving in aquariums and labs, what’s the issue?
Unfortunately, the lab colonies are not staying healthy. These captive populations are becoming
inbred, lacking the genetic diversity that wards off disease. To help mitigate this,
scientists have crossbred axolotls in captivity with the similar species, the tiger salamander.
This has left many lab specimens with a host of foreign genes, meaning they aren’t quite
the same as wild axolotls anymore.
One proposed solution to the decline of wild axolotls is to simply reintroduce some of
the captive population into their natural habitat. But until the lake is cleaned up,
putting them back would be a death sentence. And despite the efforts of some scientists
and conservationists, Lake Xochimilco becoming clean and healthy any time soon seems unlikely.
Instead of reintroducing them into their original habitat, some scientists have a different
idea - to give the axolotls a new home.
One suitable site has been identified near Mexico’s National Autonomous University.
A freshwater lake has been born out of an abandoned quarry, and researchers are now
using it as an ecological research site. Dozens of axolotls have been released here, and are
tracked using radio transmitters. [14] Eventually these research subjects are expected to breed
here, and a new type of a semi-wild population may be born. Perhaps with the introduction
of wild caught axolotls along with lab reared ones, genetic diversity could improve. And
even though it’s a different ecosystem than the swampy canals they evolved in, places
like this may be the best option for the long term survival of these magnificent creatures.
Mexico is one of the most biodiverse countries in the world. It sits with Brazil, Colombia,
and Indonesia at the very top of the biodiversity list, with thousands of unique, endemic species.
It is estimated that more than 10% of all of the world’s species live in Mexico. For
this reason, it is considered to be a megadiverse country. The axolotl is far from the only
notable, native species that Mexico has to offer. The Mexican prairie dog, burrowing
owls, the Mexican wolf, the rarest subspecies of wolf in North America, and the American
bison - the largest land animal in the Americas - are all amazing creatures that live in the
grasslands of Mexico. Grasslands, an ecosystem that may be easy to overlook, are in fact
teeming with biodiversity. The number of different types of grasses there is comparable to the
number of different types of trees in a tropical rainforest. I personally didn’t know much
about this incredible ecosystem, before watching My Wild Backyard on CuriosityStream. The ​​Janos
Biosphere Reserve episode grabbed my attention, especially since I live in Texas, and this
ecosystem is truly in my backyard. Other episodes explore the unexpected wildlife of New York
City, or the stunning national parks of Argentina. It follows local ecologists and scientists
as they show us the hidden wonders of wildlife that exist all around us.
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