Being a foolish child, I caught a lizard by the tail and, wanting to show off my hunting skills, decided to show it to my mother. The lizard that day was not ready to expand its social circle, and therefore dropped its tail and disappeared into the grass. And here I am standing in the middle of the garden with a wriggling tail without its owner in my hands, not understanding what had happened. This striking childhood event is a prime example of adaptive behavior and reparative regeneration. A person also has regeneration, but not reparative, but physiological (outer layer of the skin, nails, hair, etc.). But the salamanders are considered the masters of regeneration, capable of regrowing any lost limb. However, they are far from the only owners of this unique gift. For example, hydras, a genus of freshwater sessile coelenterates,are able to almost completely recover even after cutting into parts. A group of scientists from the University of Arizona (USA) found that the Mississippi alligator also has reparative regeneration. How is the recovery process in alligators, how different is their regeneration from other organisms, and are there any differences between the original limb and the regenerated one? We will find answers to these questions in the report of scientists. Go.
Basis of research
Regeneration is inherently a fairly common ability. However, its strength and functionality differ between species, families and genera of living organisms. For example, non-avian reptiles are the only group that regenerates complex multi-tissue structures (for example, the tail), while mammals and birds show a very limited ability to regenerate in adulthood (embryonic regeneration was not considered in this work).
Unlike vertebrates with regeneration, most mammals experience a long process of healing and repair of damaged tissues after injury. This process leads to the formation of scar tissue, which has reduced functionality and sensitivity and an increased risk of infection. In other words, the process of wound healing on the human body cannot be called regeneration.
To understand the mechanism of regeneration, one must first understand its essence, i.e. why is it needed. At the moment, the main reason for the appearance of regeneration in some animals is the influence of predators. Among many species of bony fish, tailless tadpoles, marine amphibians and non-avian reptiles, sublethal predation is widespread, when prey has a certain chance of avoiding death, although it receives serious damage. As a result, some species of salamanders and lizards have developed the ability to autotomy * their tail as a tactic to avoid predators.
Autotomy * - the ability of an animal to throw off any limb of its own accord in case of danger or in view of some external stimulus.That being said, dropping the tail is not the most extreme example of autotomy. Eagle mice ( Acomys ) literally go out of their way in case of danger. These amazing rodents are capable of completely regenerating skin (including hair follicles, dermis, etc.) and even ears (i.e. cartilage tissue).
For more information on the autotomy of needlepoint mice, see the work of scientists at the University of Kentucky.
Whether or not to regenerate is an extremely important issue for many species in the course of evolution, as this skill has both advantages and disadvantages. On the one hand, there is a chance to avoid death in the teeth of a predator. On the other hand, loss of a limb due to autotomy can greatly affect mobility, energy conservation, and even sexual selection during the breeding season. Therefore, the species must determine for itself what is more important to it in such a situation.
The loss or initial absence of regeneration in many mammals is associated with the development of a specialized immune system, increased regulation of the cell cycle, or the development of endothermia. Processes such as decreased response to injury, the formation of a specialized epidermis in the wound area, remodeling of the extracellular matrix, reinnervation and reactivation of conservative developmental pathways are common for vertebrates capable of regeneration. This indicates the presence of a common core of the regenerative capacity of different species. On the other hand, the regenerative capacity of vertebrate appendages varies greatly and can be considered as a spectrum with different activation mechanisms, functional characteristics, and the degree of regeneration.
Image №1: distribution of regenerative abilities in different animals.
For example, fish from the genus Danio , axolotls and tadpoles of Xenopus frogs are able to restore structures that are almost identical to the original appendages. Adult Xenopus frogs show non-identical regeneration. And the vast majority of mammals do not replace the lost limbs at all.
The physical characteristics of the individual also affect the regenerative ability of the animal: body size, age, stage of the life cycle, etc. There is very little data on these aspects. So far, it is known that increasing body size can slow down the healing of newt limbs, and different stages of the life cycle affect regeneration in Xenopus frogs .
Let's return to our rams, more precisely to reptiles. Given their species diversity, representatives of this class live in different parts of the planet, which is why there are a number of differences in physiological, behavioral and morphological characteristics.
If we consider the tail, then it is quite obvious that this limb is of great importance for movement, energy conservation and sexual selection. The habitat of this or that species in the course of evolution strongly influenced the structure of the tail and, therefore, its regenerative abilities.
Reptiles are represented by three existing infra-classes: Neodiapsida , which includes turtles; Archosauromorpha (crocodiles and birds); Lepidosauromorpha , i.e. scaly (lizards, snakes).
Growing a new limb for a salamander is not difficult, but it takes a long time.
Lizards, as we know, are able to shed their tail and grow a new one that is anatomically different from the original. The main endoskeleton of the regenerated lizard tail consists of an unsegmented cartilaginous tube that encloses the ependymus and central descending axons. During tail regeneration, no new neurons are formed; instead, regrowth axons originate from neurons located in the spinal cord and dorsal root ganglia, which originate in the tail stub. In addition, the regenerated skeletal muscle forms longitudinal fibers that are radially located around the cartilaginous tube and have no organization.
Snakes do not replace the tail lost due to injury, but Tuatars (a genus of reptilesSphenodon , found exclusively in New Zealand) grow cartilaginous endoskeleton (as do lizards), but skeletal muscle is minimal or absent. Most of the regrown tail of the tuatara consists of dense connective tissue that resembles fibrous tissue.
From these examples, it becomes apparent that tail regeneration is not unique to any species, and the process of restoring a lost limb is different.
Keeping an alligator at home is not a good idea, and here's why.
It is quite obvious that there have been suggestions that modern crocodiles (alligators, caimans, crocodiles and gharials) also possess regenerative abilities. Research in this area was carried out rather superficially, therefore, until now there was no full description of the process. Therefore, in this work, scientists decided to consider in detail how exactly crocodiles in the face of the Mississippi alligator are able to grow a lost tail.
Research results
Using anatomical and histological data, the scientists conducted a comparative analysis of the tissues of the original and regrown tail segments located near the junction of the Mississippi alligator.
Image # 2
The original tail segments were covered with non-overlapping rectangular scales and dorsal plates arranged in transverse rows ( 2a - 2c ). The dorsal scales were patchy and darker than the ventral scales ( 2a - 2f ). Among the samples analyzed, only sample A01 showed paired dorsal shields, indicating that A01 suffered more proximal (closer to the center) injury ( 2d ).
Radiographs showed that each proximal caudal vertebra corresponded to one row of scales and had elongated spines (dorsal processes that are part of the crest) and papophysis (processes of the vertebra on the ventral side, i.e. below) ( 2g - 2i ).
The caudal vertebra, located immediately proximal to the proposed site of injury, lacks these dorsal processes and has bony fissures, indicating the result of its remodeling ( 2g - 2i ).
As a control sample, the tail of a young female was analyzed, on which there was no damage. The axial skeleton of alligators consists of 65 vertebrae, of which 38–41 are caudal (abbreviated as Cavertebrae). In sample A00, a total of 40 caudal vertebrae were identified.
The caudal vertebrae 1-14 have transverse processes, which is consistent with previous anatomical studies. The vertebral column was elongated along the entire length of the tail, and the vertebral processes gradually narrowed towards the distal end. In addition, each caudal vertebra corresponded to one scale segment with paired dorsal scutes terminating in segment 18.
Since only A01 exhibits paired dorsal scutes in the original tail segment ( 2d), scientists have suggested that this individual has lost about half of the posterior (so to speak, rear) tail. Samples A02 and A03 showed only single dorsal scutes in the original tail segment. This indicates that the tail has been truncated distal to segment 18 ( 2e and 2f ). Counting the rows of scales made it possible to clarify the position of the injury in A02 and A03: truncation was near vertebrae 24 and 20, respectively.
Given that the vertebrae of crocodiles do not have any specialized areas for autotomy, the loss of a part of the tail caused either an injury or a birth defect, and not throwing away, like in lizards.
Image # 3
At the next stage of the study, attention was paid to the anatomy and histology of the skeletal muscles of the original tail.
Around the spinal column there is a large volume of muscles, divided in half by a thick horizontal septum into separate epaxial (located on the dorsal side of the axis) and hypaxial (located on the ventral side of the axis) sections ( 3a and 3b ).
The epaxial muscles consisted of M. longissimus and M. transversospinalis , which were separated by an intermuscular dorsal septum ( septum intermusculare dorsi ). While M. longissimus occupied most of the epaxial area, M. transversospinalis was relatively thin.
The hypaxial region consists exclusively of M. ilio-ischiocaudalis muscles( 3a and 3b ), which are usually complemented by M. caudofemoralis in the proximal region of the tail. It should be noted that the absence of M. caudofemoralis was expected in the studied samples , since the damage to the tail was distal to the location of the transverse processes and M. caudofemoralis .
Hematoxylin and eosin (H&E) staining of proximal muscle transverse sections revealed homogeneous muscle fiber bundles surrounded by a basement membrane ( 3c and 3d ).
Immunohistochemical study (IHC) showed that the muscle contains mainly type II * fibers ( 3e - 3h ).
II* — , , .. .
Image №4
Understanding what a healthy alligator's tail consists of, the scientists started analyzing the regrown tail and comparing it with the original.
Crocodiles can grow (regenerate) their tail, but not other limbs. The average length of regrown tails is 15.7 ± 7.3 cm; about 6-18% of the total body length. At the same time, it is quite simple to determine the regrown segments by their external morphology.
The scales of the regrown tail differ in color and pattern relative to the original tail. Small black scales were evenly distributed around the circumference of the regenerated tail, which did not have dorsal plates ( 4a - 4d). These scales firmly adhered to the underlying tissue. A more detailed examination of the skin of the regrown tail showed the presence of all typical layers of the epidermis and dermis ( 4i ).
The X-ray showed that there was no bone in the regrown segment of the tail, but the presence of a rod-like structure was found ( 4e - 4g ). In one sample (A04), this structure was present in the regrown tail, which protruded from the dorsal surface of the original tail ( 4h ). This situation is most likely due to an injury that did not lead to complete amputation of the tail.
Image No. 5
This sample also lacked the distal end of the tail and a regenerated small segment was present. MRI confirmed the presence of an unsegmented, hollow, rod-like structure ( 5a ) with holes distributed along the length of the tail ( 5b - 5e ).
Scientists note that similar holes in the regenerated tail were previously found in the green lizard ( Lacerta viridis ), and they serve as channels for the regrowth of blood vessels and axons. In the alligator's tail, the rod-like structure was located ventrally ( 5c - 5f ).
Image No. 6
Histological examination of the structure of the endoskeleton confirmed that it is composed of cartilage rather than bone. Trichrome tissue staining revealed an avascular collagen-rich extracellular matrix (ECM) that was sparsely populated with large round chondrocytes embedded in the lacunae * ( 6a and black arrows at 6b ).
Gaps * - gaps between tissue sites.Chondrocytes closer to the interface between cartilage and surrounding connective tissue were smaller and denser (white arrow at 6b ).
By means of IHC using COL2A1 collagen, it was possible to identify the region ( 6c and 6d ) separating the cartilage from the overlying connective tissue. No similar formations were found in the control samples ( 6e and 6f ).
Image # 7
Dissection showed that skeletal muscle was absent in the regrown tail segment, and immunostaining with an antibody that recognizes a muscle-specific cell marker confirmed this ( 7a and 7b versus 3e and 3f).
Histological examination revealed excess cutaneous collagen, as well as a dense network of uneven fibrous connective tissue that was sparsely populated with mononuclear cells ( 7c and 7d ). The intertwined fiber network was stained red with Picrosirius Red. It follows from this that it is based on collagen ( 7e and 7f ) .
It was also found that larger fibers belong to type I collagen, and smaller ones to type III collagen ( 7g and 7h ). A similar pattern was observed in all samples. The only difference was found in sample A03 ( 7i ) where large pockets of adipocytes (adipose tissue) were present.
The regrown tail had a lot of axons and blood vessels of various sizes. Nerve bundles, represented by axons in a sheath of connective tissue, were often in close proximity to each other ( 7i and 7j ). Given the lack of skeletal muscle, it can be assumed that these nerve processes are involved in sensory perception rather than motor skills.
The identification of the blood vessels was carried out due to their distinctive features, such as the presence of a lumen lined with endothelial cells and sometimes smooth muscle ( 7k ). In addition, erythrocytes were found in the lumens of larger vessels, which in reptiles have an elliptical shape with a centrally located nucleus ( 7l ).
For a more detailed acquaintance with the nuances of the study, I recommend that you look into the report of scientists and additional materials to it.
Epilogue
Regeneration is an amazing, but controversial ability that is not inherent in all living organisms on Earth. At first glance, regeneration has common functional roots; however, upon closer examination, it becomes obvious that there are different mechanisms of its work.
In this work, scientists examined the regenerative abilities of Mississippi alligators. Like lizards, these formidable predators are capable of growing their own tail. However, the loss of this limb does not occur of its own accord (i.e., it is not an example of an autotomy), but is the result of an injury. The very process of restoring the lost tail takes many months, and the result is very different from the original in appearance and structural composition. No skeletal muscle was found in the samples of regrown tails. Instead, collagen predominated, forming cartilage tissue. Nevertheless, the blood vessels and nerve endings were sufficiently developed.
Why do alligators need regeneration? There are many answers to this question, choose any, as they say. First, alligators are not born with three-meter death machines. Consequently, at a young age, they can become victims of the attack of large predators and lose part of the tail, which they still need in adulthood. Secondly, crocodiles are distinguished by their bloody competition. When two males fight for a female or for territory, they are ready to tear each other apart, just to get what they want. Broken limbs and bitten off tails are common injuries during a fight. In the first case, the glory of evolution, alligator bones grow together quickly enough. In the second, regeneration enters the scene.
Of course, the regenerative abilities of alligators are far from ideal. However, the very fact that this species has such a unique skill raises the question - why did the birds lose it? In other words, when, in the course of evolution, some species decided to abandon regeneration completely, while others retained it in some form? The answers to these questions remain to be found.
At the moment, any research on regeneration leads one way or another to medicine. People, unfortunately (and maybe fortunately), do not have such a strong regeneration as, say, lizards or hydras. Yes, our body is, to one degree or another, renewed over time at the cellular level, but this is just physiological regeneration. If we could gain reparative regeneration, this would radically change not only medicine, but also the very life of a person as a species.
However, do not forget that everything in nature happens for a reason. Evolution is, albeit a confusing, complicated and sometimes incomprehensible, but still a verified process, during which everything happens for some reason. Nature resembles a house of cards, where a specific place is allocated to each species. If one of them changes abruptly, the fragile balance may be upset and the house may collapse.
A person is often associated with progress, the main engine of which is science, which often generates many ethical debates. What is good for us and what is good for the planet, what is right and what is profitable - these questions constantly pop up when the scientific community is replenished with another incredible research or discovery. Whatever the answer to any of these questions, the power of the human intellect cannot be denied. The main thing is that it is directed in the right direction.
Friday off-top:
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Thanks for your attention, stay curious and have a great weekend guys! :)
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