It is not so easy to transplant an animal organ into a human. The transplanted organ must match the recipient's age, physique and weight, genetic compatibility is required. Even a human donor is selected very carefully, what to say about a creature of another species.

However, the needs of medical practice dictate their own terms. It would be logical to assume that the organ donor would be the creature closest to humans - a chimpanzee, but transplantologists turned their eyes to ... a pig. People far from science even hurried to question Darwin's theory as a whole in connection with this.

Xenotransplantation: myths and reality

Speculation about the mass transplantation of pig organs to humans is greatly exaggerated. To date, medicine has not gone beyond the transplantation of mechanically functional tissues - heart valves, cartilage and tendons. Tissues before transplantation are treated with special chemicals and ultrasound to destroy antigens and avoid rejection of these tissues by the recipient's body. Even such transplants are very easy to damage during processing, making them unviable, what can we say about more complex formations - the heart, kidney or liver. Therefore, we are not yet talking about the transplantation of whole organs of a pig to a person.

Certain hopes are pinned on the creation of genetically modified pigs. If pig cells are forced to synthesize human glycoproteins on their surface by changing the genome, the human immune system will not perceive such organs as something alien. But this method is still at the stage of laboratory research, it is still far from mass application in medical practice.

Advantages of a pig as a donor

The choice of a pig as a possible organ donor is not at all explained by the genetic proximity of this animal to humans. The most genetically close animal still remains a chimpanzee. But the number of these monkeys in the world is measured in tens of thousands, which is clearly not enough for mass use. Pigs are slaughtered by the millions every year.

As for tissue compatibility, there are animals closer to humans - mice, but they do not fit in size, and pigs in this regard are quite comparable to humans.

People have been breeding pigs for a long time, these animals are well studied. It is unlikely that they will "present" some unknown terrible disease that can be contracted during transplantation. Pigs breed well and grow quickly, and their breeding and maintenance is relatively cheap.

All this makes them prefer pigs to monkeys, the use of which would turn organ transplants - already far from cheap - into a service available only to billionaires.

The achievements of modern genomics sometimes lead to paradoxical conclusions: every animal, including flatworms and blue snails, can claim to be related to Homo sapiens. At least several dozen identical genes can be found in those, and in others, and in the third. But one of our closest relatives, no matter how insulting it sounds to someone, is certainly a pig.
Judge for yourself: in humans and sow, the content of hemoglobin and proteins in the blood, the size of erythrocytes and blood groups are almost the same; a pig, like a person, is omnivorous, their digestion (that is, ours) proceeds similarly. The skin is almost like ours: a pig can even sunbathe. The same features in the structure of teeth, eyes, liver, kidneys. A pig heart weighs 320 g, a human one 300 g, the weight of the lungs is 800 g and 790 g, respectively, the kidneys are 260 and 280 g, the liver is 1600 and 1800 g. Moreover, the diseases of newborn piglets are approximately the same as in infants. According to the Institute of Molecular Biology of the Russian Academy of Sciences, the structure of the molecule of pig and human growth hormone coincides by 70%.
“Scientists from the Israeli Weisman Institute have recently been able to extract a small number of specially selected cells from a human embryo of seven to eight weeks and transplant them into the embryo of a 4-week-old pig,” says Alexander Dubrov, Doctor of Biological Sciences, Professor, Senior Researcher of the Russian Academy of Sciences. – The cells began to develop and formed a fully functioning organ – the kidneys. The close similarity between human and pig cells allows scientists to grow organs from pig tissues that would be suitable for humans. At the same time, such an important problem as rejection of a transplanted organ is eliminated.”
And scientists have found that in many ways, pigs are much more “kindred” to humans than seemingly more similar primates. Their organs are similar to human ones not only in size and physiology, but also in antigenic composition - that is, they are less susceptible to rejection from the human body than others.
It was these facts that prompted researchers to experiment on xenotransplantation - the transplantation of pig organs into seriously ill people. The fact is that there is a catastrophic shortage of human organs for transplantation: in all, even the most developed, countries, there are hundreds of thousands of people on the “waiting list”, many of whom never live to see a life-saving operation.

And their organs in the human body were not rejected?

They were rejected, but in the same way as it happens with transplantation from a human donor. There is no exact data, but, judging by the publications, several such transplants have already been done, some of them successful.

Is it possible to take genes from a person, transplant them into a pig and raise an individual donor?

Basically, you can. But there is one peculiarity here. If species-specific histocompatibility proteins can be replaced, then individually specific ones (by which each person differs not only from another species - a pig, but also from another person) are incredibly difficult. There are so many of them in the body that for modern science this task is still unbearable. I'm not talking about time anymore, because a patient in need of a transplant cannot wait long. Therefore, now they are preparing original procurement of organs - as for a regular bank of donor organs.

And if in the body of a pig it is simply to grow human organs?

The main objection is the infections present in the body of the pig. The danger is too real that a massive transfer of material containing viruses of a given organism into the human system will lead to their adaptation and the emergence of completely new pathogens that can destroy millions of people. Not so long ago, the Australians conducted a rather disturbing experiment. Mouse pox virus was injected with mouse growth hormone, intending to obtain a means to reduce the population of these rodents. But the result was a pathogen that destroyed all mice without exception, including immunized ones. That is, there was no protection from him, and there was a fear: if he leaves the laboratory, evolves in the body of animals, gets into a person, then a virus will appear that causes instant 100% mortality! The probability that such transfers can lead to very serious consequences is confirmed today and experimentally. If a human organ is grown in the body of a pig, then it will be even worse, because in a foreign body, together with the blood, it will receive a whole set of viruses that partially adapt to a person and can effectively resist his immune system. This is the main argument against the transplantation of pig organs.

In addition, during the transfer of genetic material, a number of non-standard psychological problems arose that have no solution. For example, how many human genes need to be transferred to a pig so that it can be classified as human by species? Usually they say: "it won't be reasonable anyway." But intelligence is not a species attribute of a person. There are seriously ill people who can hardly be called reasonable - nevertheless, they are people. Until recently, this problem was absolutely abstract, since the "merged" nuclei of man and animal were unstable and very quickly disintegrated.

But a few years ago, an unusual experiment was carried out, which no one else tried to repeat. They took a sow, in whose womb piglets developed, and introduced white human blood into the embryos (red does not contain nuclei, which means hereditary information). Pigs were born. Taking their blood, the scientists found cells containing large sections of human and porcine chromosomes. Since there were very few such cells, this did not affect the appearance of the piglets. It was unexpected for scientists that these cells not only appeared, but also turned out to be stable: they remained in the body long after birth (all previous attempts ended with the resulting cell simply disintegrating). Thus, for the first time, a stable combined human-pig genome was obtained! According to rough estimates, it contained up to a third of human material!

I think the authors of the work, after slaughtering piglets, found such cells not only in their blood, but also in other tissues (although there are no such data in the published article). If you take such a cell, clone and grow an animal, then according to its genome, two-thirds of it will be a pig, and one-third will be a human. Naturally, no one would dare to do such a thing even at the level of the first divisions - even just to make sure that the process will proceed in principle. But what is actually being done, judging only by published works, it is impossible to say.

... Scientists are really actively experimenting with pig tissues and cells. So a piglet with a luminous piglet appeared, and a little later - a completely luminous pig. It turned out to be not so difficult to achieve a miracle: it was enough to insert the jellyfish gene encoding the production of the corresponding protein into the pig genome. It is clear that there is no particular benefit from such living creatures, except that it causes positive emotions. It's just that at some stage, scientists encountered a serious problem: when studying a gene, it is often very difficult to trace its work, since it is almost impossible to notice it. So there was a need for special markers that “highlight” the gene without any harm to the cell.

Israeli scientists argue that pig embryos can become a valuable source of donor tissue - but only if they are at a certain stage of development. There are studies showing that nerve cells taken from a pig embryo can put a paralyzed person on his feet (of course, not with all diseases). However, an adult "pig" may well serve medicine. Thus, there are known cases of making contact lenses from pig collagen, using purified cartilage cells from pig ears to grow artificial breasts - instead of those removed during surgery for breast cancer.

Scientists from the University of Pittsburgh have created a pig that produces heart-healthy substances called omega-3 fatty acids. Canadian and American scientists are proposing to transplant insulin-producing cells from milk piglets, they say, this will help solve the problem of type 1 diabetes. And Ukrainian scientists from the Ternopil Medical Academy. I. Gorbachevsky proposed their own method of using xenografts from pig skin - for the treatment of burns. Dried and specially preserved pig skin is applied to the wound for two or three days. During this time, the body has time to adapt to the injury, and after that it is already possible to close the wound with thin flaps of the patient's skin taken from other parts of the body.

The progenitor of domestic pigs is a wild boar, belonging to the genus of artiodactyl non-ruminants. Currently, these farm animals are bred in many countries of the world. But they are most popular in Europe, Russia and the states of East Asia.

Appearance of a pig

From their ancestors, wild boars, domestic pigs do not differ too much. The only thing is that piglets are not usually covered with such thick wool. The anatomy of a pig and a wild boar is almost identical.

Distinctive features of domestic piglets are:

• compact body;
• legs with hooves;
• bristled hairline.

An elongated muzzle ending in a heel that serves when searching for food to loosen the soil is, of course, also one of the main characteristic features of a pig. In the photo below you can see how convenient it is for piglets to use this organ of theirs even when kept at home. It is a cartilaginous movable disc.

The shape of the pig's head can, among other things, determine its appearance. In representatives of meat breeds, it is somewhat elongated. In greasy piglets, this part of the body has a more rounded shape.

Pig Anatomy: Musculoskeletal System

Piglets belong to the class of mammals. The skeleton of these animals is represented by about 200 bones. In this case, the following varieties are distinguished:

• long tubular;
• short;
• long curved;
• lamellar.

The pig skeleton itself consists of several sections:

• skulls;
• body and tail;
• limbs.

The muscular system of the pig is represented by smooth muscles and skeletal muscles. The bones in the body of these animals connect the collagen fibers that form the joints. In total, pigs have several unpaired and about 200-250 paired muscles.

Digestive and excretory system

Piglets are almost omnivorous animals. And the digestive system of pigs is developed, of course, very well. Its main departments are:

• oral cavity;
• pharynx and esophagus;
• single chamber stomach;
• large and small intestine;
• rectum;
• anus.

For filtering blood and neutralizing harmful substances in pigs, as in any other mammals, the liver is responsible. The stomach in these animals is located in the left hypochondrium, and the pancreas - in the right.

genitourinary system

One of the absolute advantages of pigs as farm animals is their high fertility. The reproductive system of boars is represented by the following organs:

• scrotum and testis;
• duct and spermatic cord;
• urogenital canal;
• penis;
• a special skin fold covering the penis - the prepuce.

The reproductive system of the female pig is represented by the following organs:

• ovaries;
• fallopian tubes;
• uterus and vagina;
• external organs.

The sexual cycle in a pig can last from 18 to 21 days. These animals bear cubs for 110-118 days. One sow can have up to 20 babies. This is even more than that of rabbits famous for their fertility.

The genitourinary system of the pig is also represented by:

• paired kidneys;
• ureters;
• bladder;
• urethra.

In males, the urethra, among other things, conducts sexual products. In pigs, it opens into the vestibule of the vagina.

Nervous system

Pigs are highly developed animals. It is believed that they are similar in intelligence to dogs. These animals, for example, can be easily taught to carry out various kinds of commands. Like dogs, pigs are able to return from afar to the places where they once lived.

The nervous system of these animals is represented by:

• brain and spinal cord with ganglia;
• nerves.

The brain of these animals has two hemispheres with convolutions and is covered with a bark. Its mass in pigs ranges from 95-145 g. The length of the spinal cord in these animals can be 119-139 cm.

The cardiovascular system

Like other mammals, the central organ of blood circulation in pigs is the heart. It has a conical shape and is divided into right and left halves by a longitudinal partition. Rhythmically contracting, the pig's heart drives blood throughout its body. Each half of the animal heart, in turn, is divided by transverse valves into a ventricle and an atrium.

The blood of pigs consists of plasma and erythrocytes, platelets and leukocytes floating in it. From the heart through the animal body it flows through the arteries, but returns to it - through the veins. Also, the circulatory system of the pig is represented by capillaries, through the walls of which oxygen enters the tissues.

All kinds of foreign particles and microorganisms are neutralized in the body of these animals in the lymph nodes.

Features of the structure of the skin of pigs

The thickness of the skin of piglets can vary between 1.5-3 mm. In purebred pigs, this figure may even be equal to only 0.6-1 mm. At the same time, the subcutaneous layer in piglets contains a very large amount of fat and can reach a huge thickness.

Mature males have a shield on the sides of the shoulder girdle and chest, consisting of compacted bundles with fatty pads. This formation protects wild boars during fights during the period of sexual hunting.

Rigid bristle hairs on the skin of pigs alternate with soft ones. The density of the hairline in piglets of different breeds may vary. In most cases, bare piglets are, of course, bred on farms. But there are also breeds whose representatives are covered with thick hair, about the same as wild boars.

Analyzers, organs of hearing and vision

The circulatory system of the pig is thus very well developed. The same applies to other organs of piglets. For example, the sense of smell of pigs is simply excellent.

The organ responsible for the perception of smells in these animals is located in the nasal passage and consists of:

• olfactory epithelium;
• receptor cells;
• nerve endings.

The sense of touch in pigs is carried out by receptors of the musculoskeletal system, mucous membranes and skin. The organs of taste in these animals are papillae located in the oral mucosa. The eyeballs in pigs are connected to the brain by the optic nerve.

The ears of these animals consist of the following sections:

• cochlear part;
• pathways;
• brain centers.

Similarities and differences between pigs and humans

Humans, as everyone knows, belong to the class of primates and are descended from apes. Purely outwardly, a person, of course, most of all resembles this particular animal. The same applies to the structure of internal organs. However, in terms of physiology and anatomy, a person is quite close to a pig.

For example, like humans, piglets are omnivores. It is believed that they were once tamed precisely because of this. Wild boars willingly ate the remains of human food. The only difference between humans and pigs in this regard is that the latter have fewer bitter taste receptors in their mouths. Piglet perceives sweet and bitter in a slightly different way than a person.

As you know, the structure of the pig's heart is not much different from the human heart. Doctors even try to use piglets in this regard as donors for both humans and monkeys. The heart of piglets weighs 320 g, in humans - 300 g.

Very similar to human and pig skin. These animals, like people, can even sunbathe. Similar in structure to humans and pigs as well:

• eyes;
• liver;
• kidneys;
• teeth.

The yellow press sometimes even flashes information that sometimes sows in the United States and China are used to carry human embryos.

What do scientists think

People have been raising piglets for a long time. And the anatomy of pigs is studied, of course, just fine. However, there is no clear answer to the question of why piglets and primates are so similar, unfortunately. In this regard, there are only a few untested hypotheses. For example, some scientists believe that the pig itself once descended from a primate.

There is even confirmation of this incredible hypothesis. On the island of Madagascar, researchers have found fossils of lemurs with a long muzzle with a snout. Like pigs, these animals once tore the ground with their noses in search of food. At the same time, instead of hooves, they had a five-fingered hand, like a person's. Yes, and in the embryos of modern pigs, oddly enough, there is the laying of a five-fingered hand and muzzle, like a primate.

Ancient legends are also a kind of confirmation that piglets were once primates. For example, in one of the legends of the inhabitants of the island of Bot, it is stated that in ancient times the hero Kat made people and pigs according to the same pattern. Later, however, the piglets wanted to have their own differences and began to walk on four legs.

Diseases of humans and pigs

Scientists have noticed that the similarity between humans and pigs is not limited to the anatomical structure of the organs. Almost the same in primates and piglets and diseases. For example, in pigs, as in humans, Alzheimer's disease can be diagnosed in old age. Piglets are also very often obese. Can be observed in these animals and Parkinson's disease. The pig in the photo below suffers from just such a disease.

transgenic animals

The heart and other organs in piglets and humans are similar. However, they are not identical. Experiments on the transplantation of pig organs in humans have ended, unfortunately, in failures due to tissue rejection. To solve this problem, scientists began to breed special transgenic pigs. In order to get such piglets, two human genes are introduced into the embryo and one pig gene is turned off.

Many scientists believe that experiments to breed transgenic pigs in the future may actually help solve the problem of tissue rejection during organ transplants. By the way, there is already evidence for this. For example, in 2011, Russian surgeons successfully transplanted a heart valve from a transgenic pig into a patient.

similarity at the genetic level

The anatomy and physiology of pigs is such that, according to some scientists, they are an accurate biological model of a person. According to the structure of DNA, monkeys are, of course, closest to humans. For example, the differences in human and chimpanzee genes are only 1-2%.

But pigs in terms of DNA structure are quite close to humans. The similarity between human and pig DNA is, of course, not so great. However, scientists have found that in humans and piglets, some types of proteins are very similar in composition. That is why piglets were once actively used to obtain insulin.

Recently, in the scientific world, such a topic as growing human organs inside piglets has caused a lot of controversy. Purely theoretically, carrying out such procedures is nothing impossible. After all, the human and pig genomes are indeed somewhat similar.

To obtain organs, human stem cells can simply be placed in a sow's egg. As a result, a hybrid will develop, from which in the future not a full-fledged organism will grow, but only one organ. It can be, for example, the heart or spleen.

Of course, organs grown inside pigs could save the lives of many people. However, many scientists are opposed to this method. Firstly, conducting such experiments, of course, is inhumane in relation to the pigs themselves. Secondly, it is believed that the cultivation of human organs in pigs could lead to the emergence of new genetically modified pathogens that could kill millions of people.

pig man genome

The blood of pigs is biologically 70% identical to human blood. This made possible a very interesting experiment. The scientists took a pregnant sow and injected the embryos with white human blood containing hereditary information. The animal's pregnancy ended in a successful birth.

In the blood of newly born piglets, researchers subsequently found cells containing large sections of both human and porcine chromosomes. This, of course, became a real sensation in the scientific world. Among other things, such cells in the body of piglets were also resistant. That is, they persisted for a long time after birth. Simply put, for the first time, scientists have obtained a stable human-pig genome. Of course, there were few such cells in the body of the test pigs, and the animals were in no way similar to humans. However, the resulting genome contained more than a third of human material.

Other research scientists

Be that as it may, the anatomy of pigs is well studied, and the idea of ​​using these animals as donors looks quite attractive. Most scientists at the same time believe that there is nothing impossible in this. Researchers in this regard already have quite serious developments. For example, scientists managed to find out that nerve cells taken from the body of pigs are capable of putting paralyzed people on their feet.

Very high-quality contact lenses are already made from porcine collagen today. Cartilage cells from piglets' ears are used to grow artificial breasts. Scientists have also created a pig that produces omega-3 fatty acids that are useful for the human heart.

Not certainly in that way! This statement was relevant even several decades ago. Then it was believed that the closest relative of man is anthropoid. This was confirmed by the so-called scale of ingenuity among animals. According to this scale, great apes were the closest. However, a number of experiments and experiments carried out at the present time, all move away from close relationship with man.

According to the theory of evolution, Homo sapiens is an underdeveloped ape that has one less chromosome than, for example, a chimpanzee, but has a similar structure of the skull and forelimbs. Currently, Charles Darwin's theory of the origin of man from apes is not confirmed, which allows the world's scientific minds to look for more and more new "relatives" of man.

Human resemblance to a dolphin

Researchers who studied the brain found that the encephalogram of bottlenose dolphins brings them closer to humans. The fact is that the brain of this species of dolphins is as similar as possible to the human one. The gray matter in these animals is slightly larger than in humans, and also contains more convolutions. According to the research of the Swiss professor A. Portman, the mental features of the dolphin took an honorable second place after the man (the third place among elephants, and the fourth place among monkeys).

What unites a person with pigs?

The anatomical structure of pigs allows us to call them the closest relatives of humans. The fact is that the embryo of this mammalian animal has a bookmark of a five-fingered limb and a muzzle that is very reminiscent of a human face. Piglet on a pig's muzzle and hooves on legs develop immediately before childbirth. In addition, already born pigs have the maximum physiology with humans. That is why pig organs (liver, kidneys, heart, spleen) are used in surgery for human transplantation.

Similarities between humans and rats

These rodents also amazingly copy a person at the anatomical level, but not as much as pigs. Rats have the same blood composition and tissue structure as humans. Curiously, these rodents are the only animals in the world that (like humans) have abstract thinking. Rats can make simple inferences, which allows them to be so tenacious. In addition, if a rat is enlarged to human size, and then the skeleton is straightened, one can see that the joints of humans and rats have the same anatomical structure, and the bones have an equal number of fragments.

However, something like a revolution in medical science has indeed taken place. At the end of January, the scientific journal Cell published an article by molecular biologist Juan Carlos Ispisua Belmonte, who runs a laboratory at the California Salk Institute (USA), and 38 of his co-authors. The article tells how scientists managed to create viable embryos, consisting of a mixture of pig and human cells.

Who are they

If these creatures were allowed to be born (and biologists did not do so, not least for ethical reasons), they could not be formally assigned to any biological species. Such organisms are called chimeras. Chimeras, which we know from medieval miniatures, have eagle wings attached to the body of a lion, and a snake sting to goat hooves. Who remembers a mouse with a human auricle on its back - the result of a high-profile experiment 20 years ago, will easily admit that biologists can expect something different from that. But in this sense, the new creatures from the Belmonte laboratory hardly had a chance to surprise anyone: after birth, they would look like the most ordinary pigs. It's just that some of the cells in their body - about one thousandth of a percent - would contain pure human DNA. And in this way, the piglets would compare favorably with the eared mouse of 1997, which was more of an experiment in plastic surgery and did not have a single human cell.

According to recent estimates, there are 30-40 trillion cells in humans, and about the same number in a pig. Is a thousandth of a percent of such an astronomical figure a lot or a little? It only takes one cell to conceive a child. Therefore, in theory, a chimera pig could become a parent to a human baby.

Donor without motorcycle

Doctors see pigs not as potential relatives, but as potential donors for transplanting their organs into humans. In the US alone, 27,000 kidneys, lungs, hearts, and intestines are transplanted per year. And in all 27 thousand cases, surgeons deal with the organs of living or dead people. But who in their right mind would dare to ask to be transplanted into the place of their own failing heart taken from a pig, when the procedure with the usual, human, is debugged and works perfectly? Those who will not reach the turn for a transplant: 118 thousand people are recorded in the United States on the so-called waiting list. According to statistics, about 22 of them will die today (and the same number tomorrow, and the same number next Sunday) without waiting for their transplant.

There are too few human donors - and it's not even that volunteers are a rarity. (Unlike the United States, in Russia, according to the law, anyone who has not explicitly forbidden the removal of their organs is considered a potential donor. The law does not require the consent of relatives to be asked.) Only three people out of a thousand, New Scientist cites British data, die in circumstances that make their organs are suitable for transplantation. The numbers obviously vary from country to country - they depend both on how quickly an ambulance arrives at the scene of an accident or shooting, as a result of which the most promising donors appear, and on how many transplant centers are nearby, where the organs will be able to dispose of correctly. Finally, you need to find and prepare for the operation a patient from the “waiting list” in a few more hours - there are much stricter compatibility rules here than for blood transfusions with its four different groups.

The cells that are least susceptible to rejection are our own. What if we use animals as incubators for kidneys and pancreas grown from human cells (and ideally from the cells of exactly the patient to whom the organ will be transplanted)? The same problem with rejection prevents us from solving the problem head-on: for the ready-made immune system of an adult pig, human cells are no less foreign than pig cells are for us.

So, you need to act differently.

Cut and glue

Imagine that in front of your eyes two people were cut in half at the same time - say, by a combat laser from a bad science fiction movie. Then they connected half of one with half of the other, and the glued halves would then live a lifetime as if nothing had happened. The option is even more paradoxical: they took two thin ones, pressed them against each other - and got one fat man. If both people have not yet turned four days from the moment of conception, nothing is impossible here. At this stage, the future organism is a ball of identical cells. “You remove the outer protective layer from inanimate matter and physically connect the embryos,” Virginia Papaioannu, professor at Columbia University (USA), explained in an interview how scientists since the 1960s have been producing chimera mice with a full set of genes of two individuals at the same time. Having come into contact, two embryos simply form a new larger ball - almost like soap bubbles that met in the air. The ball of cells does not yet have an immune system that could prevent this - as well as all other systems: they will develop much later.

A more subtle intervention is to add someone else's biomaterial to the embryo when its cells are already divided into different varieties. At the blastocyst stage, the embryo - both in mice and in humans - is a hollow ball with a small portion of cells locked inside. Only this inner portion will become the future lungs, liver, kidneys, brain, skin and other parts of the adult organism, and the entire outer portion will turn into a placenta that will not survive childbirth. Biologists prefer to introduce foreign cells at this stage.

This is not to say that this scenario in its purest form opens up exciting opportunities for transplantologists. The need for donor organs usually arises later - when a person has already passed the age of the fetus. How to cross it with another embryo? Take those cells of an adult organism that have not acquired a clear mission (like brain or liver cells) and have not lost the ability to turn into anything, which is characteristic of embryonic cells. They are called stem cells, but they are rare in the body. In 2012, the Nobel Prize in Medicine was awarded to the Japanese scientist Shinya Yamanaka for inventing a way to turn ordinary body cells into stem cells - to forget one's background and “fall into childhood”. The full name is induced (because they were forced to change) pluripotent (that is, "capable of anything" - of any transformation) stem cells. Researchers of chimeras also use them.

Is it possible to combine embryos of different species in this way - for example, rats and mice? This is what Toshihiro Kobayashi's team at the University of Tokyo first did with stem cells in 2010—and the American team, which published their results seven years later, perfected the method. How can you be sure you actually bred a chimera? Take as a basis the embryos doomed to death with specially damaged DNA. Using the newly invented CRISPR-Cas9 "gene scalpel", a DNA spot-editing technique, the scientists knocked out the genes responsible for the growth of the pancreas or the heart. With such a defect, there is no chance of survival (and even being born alive). But then rat stem cells were introduced into the embryo. And if a chimera mouse was still born, scientists could be sure that a rat heart was beating inside it.

But the most surprising result concerned the gallbladder. Rats don't have it, but mice do. But the chimeras, in which the mouse genes responsible for this organ were disabled, were still born with a working gallbladder - from rat cells. The mouse cells somehow told the rat cells the right context, and they, succumbing to the influence, formed an organ impossible in a rat.

Closer to pigs than to rats

It was not possible to cross a pig and a rat in this way - because these organisms are too different from each other. Different lengths of pregnancy and different organ sizes suggest that cells are programmed to divide at different rates. Finally, can the chimera's tiny rat heart pump blood through a giant pig's liver?

But with people there is no such difficulty: we are much closer to pigs - primarily in terms of the size of the organs. Therefore, pigs (and mini-pigs as a separate option) have always been the No. 1 candidate for xenotransplantation. In parallel with growing human cells in a pig body, biologists are considering other possibilities - for example, simply take and hide from human immunity those proteins on the surface of pig cells that cause the most acute reaction. Such research has been going on for a long time, so the pig as a candidate for organ transplant is not new.

A new experiment has shown that there is a possibility, and it is not at all speculative - and not even an incredible accident. 2075 embryos were transplanted into pigs, and 186 of them reached sufficient, according to scientists, maturity. Human cells were labeled with a special label in DNA that causes them to produce a fluorescent protein - and 17 mature, healthy embryos glowed confidently in ultraviolet light, proving to scientists that they were definitely chimeras.

From this moment to the organs in a living incubator - years, the researchers say. And it's not just that the proportion of human cells in the chimera's body is too small. To see how they grow and what happens to cells in an adult organism would be difficult for scientists anyway.

We are much closer to pigs - primarily in terms of the size of the organs. Therefore, pigs have always been the No. 1 candidate for xenotransplantation.

Mouse and rat chimeras, bred earlier, lived a full-fledged mouse life at two years. There is no reason to think that human and pig chimeras would have serious health problems that prevent them from reaching maturity. They were prevented from being born not by biological problems, but by ethical ones. And so serious that the Salk Institute team was forced to conduct research with private money, because the rules of the US National Institutes of Health - analogous to the US Department of Health, which funds most of the biomedical research in the country - prohibit spending money on any experiments with the introduction of human stem cells into animal embryos.

What is unethical about the birth of a pig with a human spleen? Our uncertainty about the results of such an experiment. The proportions of cells in an adult embryo are not the same as in the embryo. And if pig cells prevail by a million to one, it's not as scary as if human ones take over. And a creature will be born that looks more like a man than a pig, with a human brain, but with deformities caused by the circumstances of the experiment. In order for doctors to be able to save people, it seems that, among other things, a more precise definition of a person is needed - and a more accurate answer to the question of where people come from.