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A common theme in older novels and some more modern publications is that of body parts (potentially from multiple species) being 'swapped' on to new bodies and recombined in (usually grotesque) ways by mad scientists (I prefer the term misunderstood, personally, but that's just me).

An example of this would be Frankenstein's monster: sewn together from multiple corpses and yet seemingly functioning without any real issue (ignore the fact that the monster was mostly made from dead flesh and assume that the body parts in question are relatively fresh). Some depictions of Doctor Moreau also include monstrosities constructed from multiple animal species, crudely reshaped by the whim of man.

In the real world today we often perform transplants of organs, skin and even limbs, but these may require life-long drug treatments to prevent rejection, and they are limited to one species (preferably close relatives) or organs that have been heavily altered using highly complex methods.

Bearing this in mind: What would have to be different about the biology of mammals (for example lower immune response, better sugar regulation, all identical blood types) in order to allow far greater inter-compatibility, and how far can this compatibility be taken?

Preferably any hack with a basic understanding of the cardiovascular, skeletal and nervous systems should be able to cobble together his own pet abomination of nature by sewing the right bits in the right place and adding a few bolts to hold the bones together, but since biology is terrifyingly complex I'm willing to accept that this may not be possible, though I would like to see how far the concept can be taken for my next experiment's sake my own satisfaction.

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    $\begingroup$ Xenotransplantation is an active field of search. See also wikipedia. I would say according to these sources that there are still a lot of problems with the transplantation and the possible transfer of viruses, but we are not so far away from this being reality, especially when looking at pigs+humans, at least for organs. Sadly I have no clue how to make this so easy that everyone in your world can have his own little creature. Not catgirls for now :D $\endgroup$ – Secespitus Feb 9 '17 at 18:01
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    $\begingroup$ Catgirls? Sir, your thinking is too mundane! Ligeranthuma girls, please! $\endgroup$ – Joe Bloggs Feb 9 '17 at 18:09
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    $\begingroup$ Liger-girls? Well, that's certainly an interesting idea. Here is another interesting article: We are breeding transgenic pigs, which produce certain proteins so that the human body won't reject the body parts of pigs (see "Addressing Organ Rejection"). Would something like "Every species you want to use has a bit of genes from the same base-species in their DNA" fit your idea? $\endgroup$ – Secespitus Feb 9 '17 at 18:18
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    $\begingroup$ If it addresses part of the rejection problem then it's definitely worth putting down. I doubt it'll be a one-size-fits-all fix (as John Doe down the street is the same species as me but I'd bet if you chopped off his arm and replaced it with mine he'd still die) but making sure every species has compatible protein structures is a problem I hadn't anticipated. $\endgroup$ – Joe Bloggs Feb 9 '17 at 18:38
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    $\begingroup$ In case this is for a written work, I'd like to mention that the word is "sewing" -- "sowing" what you do to seeds. $\endgroup$ – A C Feb 10 '17 at 2:39
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I can try and point out the obstacles towards your goal. That is, if you remove these obstacles, you would be able to create the chimera you want.

1- Proteins and Tissues

Different mammals have different types of proteins. The difference in tissues is also vast, when you study them at microscopic scale. If this difference is removed and all mammals have the same protein group and same type of tissues, it will be quite helpful in cross-species transplants.

2- Skeletal Structure

In different mammals, the shape, thickness and placement of bones varies. If you want to connect an elephant's leg to a rhino, you must make sure that the bones of the leg are attached properly to the rhino. By properly, I mean that there is not too much stress at any point, so as to break the bone or be painful, and that the final skeletal structure is practically utilizable and feels natural to the rhino.

3- Blood Types

This is also critical. By blood type, I do not just mean A, B, O etc, but the whole biochemical signature of a creature's blood. This not only includes antibodies and antigens but also the thickness (water content) type of blood cells, mineral content and the different types of white blood cells.

4- Immune System

The immune system is quite vigilant at detecting and attacking alien objects through their genetic signatures. If the immune system is tweaked to include genetic information of all mammals and programmed to not attack those cells as pathogens, it would be a big boost toward your goal.

5- Veins, Arteries and Blood Pressure

Different mammals have different structure of the circulatory system. Where would you connect the 5th vein of a tiger's limb during transplant when the host species only has 4 veins in that region? Apart from the structure and placement of blood vessels, it is also important that both the host and the target creature (from which you are taking the organ/limb) work at the same blood pressure. Otherwise, the arteries might burst or the organ/limb might go numb and gradually die, developing gangrene.

6- Nerve Compatibility

You also want to connect the nerve tissues of the transplanted organ to the host. For this, it is imperative that their nerve signals be coded in the exact same fashion and operate at the same electrical signal frequency and amplitude.

7- Recognition by CNS

If you transplant a tail to a human, the human will not be able to willfully use the tail because the human brain does not recognize any tail (the body will, but the brain will not). In order for the target limb/organ to function properly, it must be recognized properly by the host creature's central nervous system.

8- Skin Compatibility

This should be self explanatory. In case you are transplanting a whole limb, you would also want to make sure that the skin type of the host and target species are compatible with each other so that the skin graft does not end up as a failure.

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    $\begingroup$ Great points. As for 2: Who cares about the comfort of the chimera? ADD MORE STEEL PINS!!! $\endgroup$ – Joe Bloggs Feb 9 '17 at 19:03
  • $\begingroup$ Oh you sadist! Just wait while I send my army of Moreau Island freaks at you!! :@ $\endgroup$ – Youstay Igo Feb 9 '17 at 19:19
  • $\begingroup$ For 1, could not agree less. Mammal protein and tissues are similar. $\endgroup$ – Mołot Feb 9 '17 at 20:42
  • $\begingroup$ @JoeBloggs I prefer glue, but to each their own I suppose. $\endgroup$ – Jake Feb 9 '17 at 23:10
  • $\begingroup$ @Mołot: Yes, they are similar indeed. But they are not the same. You can actually identify all mammals through their blood samples alone, by mapping their blood proteins alone. $\endgroup$ – Youstay Igo Feb 10 '17 at 20:58
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Basically you need to shut down inmune system in order to perform any kind of transplant. Most tissues in mammals are quite similar, to the point that laboratory rats are good experimental subjects for many things that later will be used in humans. For example, the growing of a human ear inside a rat:

the growing of a ear inside a rat

Another problem would be the ramification of new nerves inside the new tissue. So maybe you need to relax the constraint of nerve growing in order to get a limb useful after few time. Also is present the problem of body temperature, this constraint demands that you must use organs of animals that require the same temperature. In the case of limbs it doesn't matter, but a liver needs a precise temperature. Some hormones might not be the same, either by structure or concentration.

Summary, most differences are not quite difficult to solve, but the inmune system will always reject foreign tissue, but if you relax the inmune system your Frankenstein will probably die quite fast due to any infection.

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    $\begingroup$ Hadn't considered internal organ temperature regulation. Works nicely though as replacing limbs should be easier than replacing internal organs, and the addition of organs outside the body (Perhaps inside a scrotum of sorts for temperature regulation) is a wonderfully awful touch. $\endgroup$ – Joe Bloggs Feb 10 '17 at 7:35
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Nothing must change

...it's just likely to be fatal.

Human (and mammal) immune systems are fairy good at detecting foreign stuff and attacking it. This is a huge problem in biomedical devices, from pacemakers to artificial hips and everything in between (see e.g. here). That's all part of your general immune response. I've even heard (plausible) speculation that most food allergies are versions of this, where some protein makes it through your intestines unexpectedly intact (like through an ulcer), and so your immune system attacks it thereafter. True or not, immune systems are basically xenophobic assholes, and smash up everything even near a foreigner they don't recognize. The extreme version of this is "anaphylaxis," and it's bad news. That's the "stung by a bee, swelled up and died" response.

With foreign tissue (think heart transplant), everything can be doubly bad as each part reacts against the other. That's bad times.

To fix that, most transplantees take immuno-suppressors. Forever. That's sort of a balancing act though, because your immune system also does a few things which you don't want suppressed (keeps you from getting sick, etc). Essentially, transplantees have to medically induce a mild form of AIDS, because that's the better option.

Separately, you need to think about how to connect nerves. This is wildly species dependent: Rats recover from anything. Humans never reconnect broken nerves. Except sometimes they do. Why? Nobel prize for anyone who can tell you (literally, probably. Understanding what makes nerves mend would be a HUGE deal).

So, underall, what do you actually need? Nothing, you might just get lucky. No really, that's a real possibility! For more certainty though, you would want a way to deal with immune response and a way to regrow nerves. For nerves, you might try "neuro-prosthetic operant conditioning" (stick stimulators on either side of the break to bridge the gap; eventually they'll grow together). Sprinkle some pluripotent stem cells in there, because that's poorly understood magic anyway. For the immune stuff, I would either select an AIDS-derived viral infection as part of every transplant, or use some technobabble with an Immunoglobulin-G (IgG)-infused albumin bath, or use the same words with some sort of genetic re-write on the implant (still techno-babble).

You'll also want to read up on Serge Voronoff. N.b. how many of his patients survived, and even reported improvement.

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    $\begingroup$ Reading on Voronoff led me to Sertoli cells led me... down the rabbit hole and into the territory of biological specialties. Dammit man I'm a physicist not a doctor! $\endgroup$ – Joe Bloggs Feb 10 '17 at 7:33
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Given all the biological obstacles, I think the simplest (but requiring weird/advanced science) solution would be some kind of 'bio firewall' connecting the different parts. It could somehow block immune system and transfer/translate nervous system signals and nutrients needed.

As I said, it would take either weird science or tech far advanced from what we have now. I could see it as a logical plug and play technology for this purpose, though.

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