August 1989: The cold, scientifically and socially progressive land of Sweden was a long way from the temperate, scientifically and socially conservative land of the USA, yet George Carillo, and his girlfriend Juanita - who had also been paralysed by the same tainted heroin batch - were sent on their way by their Californian doctors, kept warm by the hope of an improvement in their situation. The Lund Hospital in Sweden, partnering with the Lund University and the team in California, were about to perform a potentially life-changing experimental transplant operation on the pair.
Stem cells are well-known today; they serve as a powerful tool in the fight against Leukaemia and other blood disorders, and are usually extracted from the bone-marrow of adult donors. Their discovery has inspired visions of lab-grown organs, produced in bulk, ready for transplantation whenever someone may need them. As of 2024, we haven't come quite that far, but we're making progress. One group began growing a human kidney from stem cells in a pig host only in 2023, the first time any human organ was grown in such a way. Back in 1997, there was also the Vacanti Mouse; a research team lead by the Vacanti brothers grew out cartilage cells in the shape of a human ear on the body of a mouse (using a kind of mould to provide the shape). It wasn't a real ear, but still kinda cool.
We already know stem cells as the basis for powerful regenerative treatment for damaged parts of a human body. The most promising of them are found in a human embryo, often no older than 8 weeks and no bigger than a fingernail. The collection of Neuroblasts — stem cells for neurons — that will form the Substantia Nigra could all fit on the head of a pin. We are talking tiny, like really tiny. Tiny, delicate, fragile. An embryo at this stage is essentially a mish-mash of developmental cells, "Embryonic Stem Cells" (or ESCs) that have only just begun specialising into their target cell types, ready to form into parts of the body and brain.
So many essential medical treatments today rely on donations: blood, spare kidneys, bone-marrow for stem cells, sometimes even antibodies can be donated by living people; those that formally register to donate their organs on their death can — depending on their overall condition — have most of them re-used, saving a number of lives at once. Thankfully, the ability to donate ones own organs has not led to an epidemic of murder for the purpose of obtaining fresh organs, despite being a rather interesting plot device in fiction. People die a lot already, for many reasons. It's an unfortunate fact of life, at least given our current stage of technological and societal development.
Another unfortunate fact of life is that unintended, unwanted, and even dangerous pregnancies do happen, often despite reasonable precautions, and as such, abortion is a necessary medical procedure. In fact, while brain cells cannot normally be donated during life or after death, the one exception is when a pregnancy is terminated between 4 and 8 weeks. The idea of prematurely halting the progress of a human life, no matter how under-developed or how good the reasons for doing so, can evoke the strongest emotions in us (understandably). However, there is no doubt of the necessity of this procedure, despite our deep emotional aversion to it.
So on the one hand, it makes sense: If we must perform the procedure anyway, why not find a way to save more lives at the same time? On the other hand, it is easy to imagine a "slippery slope". Moral panic, no matter how misguided or irrational, is real fear that often comes from a good place.
I don't feel competent enough to attempt to debate the moral complexities here, so instead, perhaps we better return to the science and ask a pertinent question: would foetal ESCs actually make it possible to re-grow the Substantia Nigra in an adult brain?
The answer is: yes.
Welcome to Part 2 of The Brain Factory.
Touching the Sun
Re-growing brain parts from the stem cells of an embryo was something the Swedes were surprisingly good at in the 1990s, and having done a unilateral — meaning only on one hemisphere of the brain, remember most structures come in pairs — "graft" on 4 previous patients with positive results, they were well ahead of the rest of the world in this particular field. Obviously, these partial grafts hadn't been a perfect cure, but there was indeed progress. (A "graft" in this context is just the transplanting of a group of cells not including their original donor's blood supply; they would have to attach to or grow new blood vessels in the receiver)
This time, with George and Juanita, they were going for a full bilateral graft, inserting the new stem cells in both hemispheres; a world-first. Brain surgery is complicated to say the least, even when at its most routine: whether installing a shunt, extracting a cyst, removing a tumour, or severing the Corpus Callosum, every single cut destroys neurons. It is the pinnacle of delicate procedures, and carries a vast array of real risks, both known and unknown.
Despite knowing all of that, they also knew there was no other way out. Having already spent years below rock-bottom by now, any hope at all of even a minor improvement to their situation was worth the risk in their eyes. The medications they had to take just to be able to move their own bodies came with near-unbearable side-effects at their current dosages. It just couldn't go on like this.
So to Sweden they went, with hope, courage, and anticipation. George was the first to undergo the operation, spending nearly 11 full hours under general anaesthesia on the first day while the first graft on one side of his brain was applied. The other side was done a week later. There were no complications from the surgery, but also no immediate changes either; the doctors knew it would take time for the cells to proliferate and develop connections, so they wouldn't know whether it had worked for at least a year, maybe two. The longer the delay between the operation and its results, the less certain they could be in establishing causation.
12 months post-operation, a review of George's regular tests showed that he had been making gradual improvements over time in his fine motor control, though it was slow progress. By 24 months, George was able to do most things entirely independently. By the 3rd year, he was making progress towards obtaining a drivers license. His dose of L-Dopa had come down by 2/3rds as well. Juanita's progress was even more spectacular; she was living independently, in her own house, and putting a life back together again.
Clearly, the ESCs were forming into the right types of neurons in the right place. Interestingly, it seemed that at best, only about 20% of the Substantia Nigra had managed to grow back with the new cells. With just that 20%, a remarkable amount of progress is made.
For people with drug-induced Parkinsons, this was an incredibly good result. It returned to them a level of independence that they might never have imagined possible. For those with Idiopathic Parkinsons, the unfortunate fact is that even after replacing the destroyed neurons, it could only turn back the clock for a while; it didn't halt the active disease process. So the new neurons would last a period of time, but would gradually be destroyed by accumulating Lewy bodies. 20% wasn't going to turn back the clock far enough for most.
To obtain enough stem cells to grow back the majority of the damaged cells entirely from foetal tissue was going to be even more of a nightmare — morally, politically, scientifically, ethically — than it already was, as it would require harvesting higher and higher quantities. Wouldn't it be better if we could somehow convince existing adult cells to ignore their pre-programming and form into neurons instead?
Could such a thing even be done?
Alchemy
"We have colonies", said Kazutoshi Takahashi. It was 2006, and two Kyoto University post-docs were observing clusters of cells that they had been working for years to bring about. They had nearly lost all hope, when those fateful words were spoken: "We have colonies."
Takahashi's colleague, Shinya Yamanaka, thought it must be a mistake, so they ran it again. Again, there were colonies.
Prior to specialisation, the earliest Embryonic Stem Cells are considered to be in a "pluripotent" state. Pluripotency simply means there are many possible paths of specialisation that the cell could take; it could become skin cells, hair cells, epithelial cells, neurons, antibodies, or any number of other things. Once they specialise, however, there is no turning back.
Or, so we thought.
In 2006, staring down the microscope at their colonies, Takahashi and Yamanaka were looking at the very first ever induced Pluripotent Stem Cells; that is, cells which were previously just normal adult skin cells, had been reverted several stages into an original unspecialised stem cell. No foetus or embryo needed. This was the real deal. All it took to make this happen was the modifying of 4 developmental genes, and voilà.
The work rightly won the Nobel Prize in 2012.
Upon publishing their results, almost immediately, scientists around the world who had been working with embryonic or foetal stem cells for research began making the surprisingly easy switch to these new induced Pluripotent Stem Cells, or iPSCs. Imagine being able to grow a new replacement organ simply from a few of your own skin cells! This truly was cellular alchemy, divine power in the hands of mortal beings.
Genetic manipulation in humans has had a volatile history. There was the disturbing story of Jesse Gelsinger, an 18-year-old who suffered from a rare genetic liver disease. In 1999, he participated in a clinical trial of a promising genetic therapy. Intending only to test its safety in humans, the dose used would be too small to have any impact on his condition; though if successful, there would be just enough to show up on a later test. Using a modified adenovirus — a common technique in medicine for delivering a therapy directly to cells — to provide a functional copy of a missing gene, it would instruct his cells to produce an enzyme that was missing from his own genome.
Within hours of the injection, Jesse's immune system went thermonuclear, and his organs began to shut down one after the other after the other; 4 days later, he was dead. It was later found that the doctors who ran the trial had failed to adequately disclose the risks.
X-SCID (sometimes called SCID-X1), a genetic disease, is usually fatal within 2 years of birth; a genetic defect essentially prevents the formation of a functioning immune system, making even the mildest pathogens deadly. David Vetter was one of the most prominent sufferers of the condition. Commonly known as the "Bubble Boy", he had to spend the entirety of his short life in perfectly sterile environments. Today, nearly every child born with SCID is treated with a transplant of stem cells taken from the bone marrow of a matching donor. So long as a matching donor is found, chances are often good of achieving a full or near cure. For cases where no suitable match can be found, researchers have been trying to find a Plan B.
In 2000, a French team pulled off what seemed to be a stunning achievement: they used a retrovirus to re-insert the missing gene in two baby boys born with X-SCID, apparently with complete success. For the next few years, more groups received this treatment, also to apparent success; but then those first 2 boys developed cancer. Then a 3rd, a 4th, and many more followed. All further trials of the treatment were halted.
Cancer typically arises when a mutation happens to a specific gene, or sometimes a specific combination, resulting in that cell multiplying continuously. Such genes are called "Oncogenes". Further investigation into what went wrong in the X-SCID treatments discovered an oncogene, hiding directly adjacent to the inserted gene, was being activated by accident, and in some cases, the inserted gene itself was acting as an oncogene.
Despite so much success with genetic therapy in animal models, no matter how much work was done prior to human trials, somehow, when we began to work with humans, everything just fell apart.
In the case of stem cells, however, it wasn't exactly the same thing. Although we were modifying the genes to revert the normal somatic cells from our own bodies into pluripotent stem cells, the goal was to use them to re-grow parts of the body; the rest of our cells would keep their original genome as-is, untouched. The real worry was if the new cells came with any accidental defect on an oncogene. It was critical to be absolutely sure these cells did not become prone to a never-ending proliferation. Any time we mess around with the genetic code of a cell, it carries some risk of this problem. The medicine itself must not become a disease.
Excitement and fear are two sides of the same coin, and activate mostly the same brain structures. Exciting opportunities can also be frightening in their potential for failure. At all times, we must be cautious and meticulous, never rushing things no matter how great the pressure to push ahead. At the same time, we cannot allow fear alone to deny us the right to pursue an end to human suffering. The line we must walk is so fine, we may think that keeping to our principles makes the task impossible. Without those principles, however, the result is too often unnecessarily deadly.
So, although exciting, it would take a long time and a lot of research and many studies before such powerful technology could make a real clinical impact.
Hubris
For years after the Lund grafting operations, limited numbers of foetal ESC grafts were still being done, and long-term studies observing the progress of patients began to trickle out in journals. There was a lot to be positive about: many of the transplant recipients found their symptoms receding, both in Idiopathic Parkinsons patients and in the Frozen Addicts patients. The level of independence and physical self-control that was returning to many of the recipients was unheard-of.
Then, just as we thought we could reach out and touch the Sun, came the fall. Some of the recipients developed severe dyskinesias, a few of which required further surgical intervention. Then it was discovered that the disease process was still active, eating away at the new neurons as well. Some later died of unrelated causes, and on being autopsied, agglomerations of α-synuclein - lewy bodies - had begun developing in the transplanted neural grafts.
It was by no means a failure, but it was far from a success. A loud and growing backlash against the experimental transplant therapy was causing many scientists to abandon it entirely, and return to pursuing other more traditional methods of treatment.
So we fell back to earth, with wings that were once glorious and broad now charred and shrivelled; we had come so far, to discover only our hubris awaiting us.
This was not the beginning of the end, however; it was merely the end of the beginning.
This has been Part 2 of The Brain Factory, a history of Neurogenesis. Part 3 has no specific deadline, but I hope to have it out in the near future. It will take some time to work through the clinical trials being done using iPSCs for various things. It's been wonderful to hear from a few folks about my last post. I'm hoping we can conclude the series in the next part. Depends on how many words I get to. I'd love to get an idea of the number of studies and research papers I've had to read to get to this point, it must be in the hundreds by now.
I have so many more posts still to come on so many more topics. I wasn't totally happy with it, but after way too long trying to work out how to put this together, I realised it was time to just get it out there.
Stay tuned, and if you liked this one, please do share it around; it would mean the world to me. Thank you so much for reading. Until next time.