OK, this is something we can work with.
It seems what you’re really getting at are two different scenarios:
(1) Embryologic development in an extra uterine (actually extracorporeal) environment, where the placenta is left attached to the fetus, either to (a) complete intrauterine development that was prematurely interrupted, as with premature births, or (b) to begin the process de novo, or very close to, which could, if developed to full capacity, make the intrauterine part of human procreation unnecessary.
(2) Leaving the placenta attached to fully developed humans – born either the old fashioned way involving a uterus, or by germination in an extracorporeal environment – so that said humans may be the lifelong beneficiaries of the placenta’s known and supposed amazing properties.
With (1a), I think you may be onto something, as very briefly described in the linked article. It’s essentially the prospect for a new technology to facilitate the viability of premature infants. The difference is that they are not, as I read it, leaving the placenta attached. Even in the goat experiments they are disconnecting the placenta and catheterizing the umbilical cord. I’d be interested to know what’s on the other end of the catheter. Whatever it is, and whatever it’s doing, the placenta would do that as a matter of innate physiology, so I suppose, all else equal, leaving it attached would make things easier. But disassociating an implanted placenta from a uterus while keeping it all viable would involve its own problems. Here, of course, your original question becomes paramount, namely, would a detached placenta continue to do for the fetus what it does when it’s attached to the uterus. In theory, I don’t see why it couldn’t, provided all the problems of detachment, getting the medium right, and all that, were worked out. Unless you believe in supernatural forces, everything in biology is mechanistic; and anything mechanistic can, in theory, be tweaked and manipulated (but not always controlled to the desired extent).
(1b) is really an extension of (1a), just pushing it further back (earlier) in the course of fetal development. Again, I suppose if all the variables were controlled (and that’s a big if) it should work. Whether we’d want it to is another question. (Isn’t that what happens at the end of 2001: A Space Odyssey?) In either of these scenarios we’re really not talking about extending the life of the placenta beyond a normal gestational period, just having it do its thing in a nonuterine situation.
With (2) you’re really moving on to something else. Before getting into whether the placenta could survive, I’d question some of the underlying assumptions about the supposed benefits of doing so.
(2a) As for “Provision of oxygen 10 times more effectively than lungs” I’m not sure what you mean. Can you explain that further? Fetal hemoglobin has a higher affinity for oxygen than regular hemoglobin. That’s what allows the mother to deliver oxygen across the placenta. But that is not a property of the placenta, but the hemoglobin. Further, the benefit of “greater oxygen provision” is dubious. A healthy person (ex utero and with no retained placenta-in-a-box) will be saturating their hemoglobin at close to 100%. The amount of oxygen going to the brain is more a factor of the hemoglobin content of the blood and the blood flow to the brain rather than “provision ” of oxygen at the source. You can increase the delivery of oxygen slightly by dissolving more into the blood, like when people use an oxygen mask or nasal cannula connected to a tank. But the benefit of that for a healthy person going at normal baseline who doesn’t have an oxygen deficit, like from exercise, is, AFAIK, equivocal.
(2b) “Provision of nutrition from ambient sources” Again, I’m not sure what you mean. Do you mean like harvesting energy from incidental phenomena happening around us, like sunlight, wind, stray EM radiation, tides and things like that? If so, then you have a drastically different understanding of what the placenta is capable of than I do. I’m no placenta expert, by any means, but my understanding is that the placenta is basically a physical interface that allows maternal blood and fetal blood to come in very close proximity without actually mixing, thereby allowing selective bidirectional transfer of various things. Other than synthesize a few hormones and what not, it doesn’t really do anything. It certainly is not a harvester of ambient energy. I suppose a little heat could come across, if you set your attached placenta-in-a-box out in the sun or in a heated box. But you could probably get the same effect by laying in the sun or drinking a nice cup of cocoa. Furthermore, the type of energy you might get from ambient sources is not the type of energy the body runs on. We are heterotrophs, not autotrophs. What the body uses is energy contained in chemical bonds, extracted from molecules that other organisms have made.
(2c) The stem cell thing is interesting. There are, so I hear, a lot of stem cells in the placenta. Amazing things are being done with stem cells generally and undoubtedly more amazing things are to come. So, again, you may be onto something. But keeping your placenta attached for life and carrying it around in a liquid filled box would seem like an inconvenient way to take advantage of anything that might develop in that area. You already have stem cells in your bone marrow, so if at some time it becomes advantageous to kick your stem cells into action and get them to do things in addition to what they are already doing, a better approach might be to develop some way to stimulate the ones that are already there.
(2d) As far as adding a new sensory organ, again, the placenta just isn’t equipped for that. Sensory systems are mediated by neural elements, of which the placenta has none. It’d be like saying you want to use the liver as a sense organ. Sure, it responds to, or is affected by, what it’s exposed to, but I wouldn’t call that perception. It just…doesn’t work that way.
All of this begs the original question: Could you keep a placenta attached to a fully developed human and keep it going, whatever it might be doing, after that human and placenta have left the uterus? As I said before, in theory it seems like, if you really worked at it and solved all the problems that we know about now (like the Wharton’s jelly seizing up) in addition to the problems that would inevitably arise along the way, it might be possible. But unless you can provide some real “benefits” of doing so, I’m not convinced it’d be worth the hassle. (And we haven’t even addressed the liabilities it might entrail [sic].)