Regeneration: A natural wonder

When we think of regeneration, we all think of planarians, don’t we? In other words,
flatworms are long, brown, and can reproduce by dividing. So, are planarians
the only example of regeneration? Or, as we know, is regeneration just mitosis?
Let’s examine it in a little more detail…

Regeneration, which literally means “renewal”, is the name given to the replacement
of lost tissues by mitosis. This event can happen in two ways: Only the lost tissue
grows back and as a result, there is 1 individual, or both the lost tissue grows back
and the rest of the cut piece grows, so as a result, there are 2 identical individuals.

The situation that results from the repair of a single individual is called repair by regeneration, and the
formation of two individuals is called reproduction by regeneration.

As the level of development of living beings increases, their ability to regenerate
decreases. For example, as we all know, humans, who are highly developed
creatures, do not regenerate in every tissue. If they did, something like a cut arm
spontaneously regenerating would be observed. However, the fact that this is
not observed in arms and legs does not mean that humans have no regenerative
ability at all. Humans cannot reproduce by regeneration, but some different
organ tissues, such as liver and skin tissues, can renew themselves by regeneration.

Let’s start with our most well-known example, planarians. Planarians reproduce by
regeneration. If you cut a planarian in one place, both sides will complete their
missing parts and you will end up with two planarians. This process usually doesn’t
take very long because planarians are very small creatures. Planarians
have neoblast cells that provide the ability to regenerate. These cells are
the name given to all dividing cells of planarians other than their eggs and sperm.
By in situ hybridization, these neoblasts can be located and thus regeneration
in planaria can be studied.

Holger Brandl, HongKee Moon, Miquel Vila-Farré, Shang-Yun Liu, Ian Henry, and Jochen C. Rink, CC BY 4.0, via Wikimedia Commons

Another example is sea stars. Sea stars are invertebrates in the
Asteroidea class, phylum Echinodermata. They digest by protruding their
stomachs, are usually omnivorous or carnivorous, and their main food sources are
mussels, mollusks and small fish. Sea stars can reproduce both sexually and
asexually. Some species have external fertilization, meaning sperm and eggs are
released into the water where fertilization occurs. Some species reproduce by
fission, while others reproduce by regeneration, where two individuals are formed
from the cut parts after they are torn apart. In some cases, regeneration is used
only for repair, such as the regrowth of a cut arm.

Next up is a very cute animal species, axolotls. Axolotls have the ability to regenerate
tissues and organs during embryonic and adult stages. There is a lot of research
on axolotls, which have a very important place in the evolutionary process.
Axolotls, also known as water salamanders, breathe with gills. They have two gills
on the side of their heads and can use dissolved oxygen in the water. Their
natural habitat is the Mexican environment, but they are greatly affected by source
pollution. They prefer to live in cold and fresh water. Although axolotls belong to the
amphibian group, they show a few different characteristics from them. Water
salamanders remain in the larval stage throughout their lives and, unlike other
amphibians, do not undergo metamorphosis. This situation is called “neoteny”
The situation that results from the repair of a single individual is called repair by regeneration, and the
formation of two individuals is called reproduction by regeneration.
As the level of development of living beings increases, their ability to regenerate
decreases. For example, as we all know, humans, who are highly developed
creatures, do not regenerate in every tissue. If they did, something like a cut arm
spontaneously regenerating would be observed. However, the fact that this is
not observed in arms and legs does not mean that humans have no regenerative
ability at all. Humans cannot reproduce by regeneration, but some different
organ tissues, such as liver and skin tissues, can renew themselves by regeneration.
Regeneration, which literally means “renewal”, is the name given to the replacement
of lost tissues by mitosis. This event can happen in two ways: Only the lost tissue
grows back and as a result, there is 1 individual, or both the lost tissue grows back
and the rest of the cut piece grows, so as a result, there are 2 identical individuals.
When we think of regeneration, we all think of planarians, don’t we? In other words,
flatworms are long, brown, and can reproduce by dividing. So, are planarians
the only example of regeneration? Or, as we know, is regeneration just mitosis?
Let’s examine it in a little more detail…
Next up are the other living things, the worms. I’m sure that when we were little, we all got
very excited when we saw a worm and wanted to examine them. Maybe some of
us even took a stick and split the worms in two. In fact, this could be considered
an experiment in regeneration. When worms, especially earthworms, split in two,
they usually grow on both sides. We even notice that even though the worm is split
in two, both sides are still moving. This bilateral growth may not always happen, but worms
usually do not lose their lives when they split.
Crabs are arthropod marine animals in the crustacean class. Their outer shells are made of
calcium and change as they grow. They usually have 8 legs and their eyes are at the ends
of the stalks, giving them a wide field of vision. Crabs do not reproduce by regeneration, but
they can regenerate a lost leg or pincer.
Crabs, especially blue crabs, are also capable of regeneration.
These creatures, which usually live for 10-15 years, have become one of the focuses
of evolutionary biology with their regeneration abilities and unique features.

In general, although there are many creatures capable of regeneration, these are only a
few of the most popular. In fact, the ability to regenerate is found in most creatures, even
at the tissue level. Now that we know which creatures it is seen in, we can now look at
the advantages of regeneration in the evolutionary process and why creatures acquired
this feature. We can list the advantages that regeneration provides to animals as
follows:

1) Survival and Recovery from Injuries

As we know, lost limbs or tissues can be renewed with regeneration, so the dysfunction and
disadvantage within the species that comes with the loss of these limbs are
eliminated. During the evolutionary process, creatures that do not
have the ability to regenerate within the species have been easy victims of natural
selection, and therefore the number of creatures with the ability to regenerate has increased.
Apart from this, we can see that in regenerating organisms, in general, there is
not much death in cases of division or fragmentation. For example, if a human is
cut in half, there is almost no chance of survival, but if you cut a planarian
in half, we can say that the chance of death is very low.

2) Increasing Reproductive Success

Asexual reproduction has almost always been much faster than sexual reproduction. In asexual reproduction, more
offspring are produced in a shorter time, and although genetic diversity is low, this
is a very advantageous situation in terms of increasing the numbers of
organisms that have been successful in natural selection.
Reproduction by regeneration is actually a type of asexual reproduction. In fact,
we can say that its basis is mitosis. Like the mitosis divisions we undergo for
repair when we are injured, these creatures also undergo many mitosis divisions,
but as a result, two individuals are formed. Reproduction by regeneration allows
creatures to reproduce quickly, and also turns the lost limb into an
advantage rather than a disadvantage. We can say that it creates a copy, an analogy such as mitosis, the size of an organ can be used.

3) Adaptability

In order for living things to survive, they must adapt to their environment and “adapt”. Living
things that cannot adapt to their environment are eliminated by natural selection, and the number
of living things that adapt to their environment increases. Regeneration is a great advantage,
especially for underdeveloped living things. As we said at the beginning, as the level of
development of living things increases, their regeneration abilities decrease. Regeneration is a
great advantage, especially for living things with a low level of development, because it
allows them to reproduce quickly and minimize their damage levels. In this way, they succeed in
natural selection and gradually increase their numbers.

4) Eliminate the Risk of Permanent Damage

When a living being loses a limb, it is at a great disadvantage for that being and the probability of it
succumbing to natural selection increases greatly. The living being is probably unable to adapt
to its environment and faces death. However, the risk of losing a limb is almost non-existent for
living beings that have the ability to regenerate, which puts them in an advantageous position
compared to other living beings.

5) Energy and Resource Efficiency

When regenerating, organisms repair and grow existing parts instead of creating a completely
new organism from scratch. Creating a living thing requires energy. Creating a living thing from
scratch requires more energy than regenerating. This means that organisms that reproduce by
regeneration need less energy to reproduce, and in the long run, they live longer and healthier
lives.

Ruben Undheim from Trondheim, Norway, CC BY-SA 2.0, via Wikimedia Commons

We have learned about regeneration in animals and the advantages of regeneration. Now let’s examine
regeneration in humans. Humans do not reproduce by regeneration and their regeneration abilities are
limited compared to other living things. However, this does not mean that there is no regeneration at the
tissue level in humans. We can list the regeneration abilities in humans as follows:

• Skin

Human skin is the part with the best regeneration ability in
humans. What we are talking about as regeneration here is the cells multiplying and repairing,
that is, at the tissue level. In fact, we can say that mitosis is occurring. For example, you fall to the
ground and your knee is injured, you see a scab forming a few days later and your knee
returns to its original state a few days later. In this way, the skin is renewed and
repaired, it is “regenerated”.

• Bones

Bones can be damaged in severe injuries. Maybe you or someone you know has experienced
situations such as a bone fracture. In this case, the bone takes a little longer to heal because the
bone is hard and difficult to break, so it will take some time to repair. Regeneration in bones occurs
with the formation of cartilage and then the reconstruction of bone tissue.

• Liver

The liver is also an organ that regenerates a lot. Liver cells, namely hepatocytes, multiply and repair. The
liver is a very important and critical organ for our life functions, so having this feature gives
humans an evolutionary advantage.

• Muscles

Muscles do not have the ability to regenerate like skin. It is a little harder to regenerate and takes a little longer.

So, understanding regeneration is actually a huge step towards understanding evolution. As a result, we can observe regeneration in most living things. In some, it is limited to the tissue level, while in others it can even be used for reproduction. Although it seems simple, regeneration is a very complicated process and is actually a very important part of understanding evolution. Even in creatures that are more primitive in terms of development, such as hydras, worms, and starfish, there are features that serve their own advantages. In fact, every living thing has gone through these processes to adapt to its environment.