(Edit note: I somehow disappeared this whilst correcting an image, so if it’s still problematic, drop me a note!)
One of the major medical advances of the last century was that of organ transplantation: replacing diseased organs with healthy ones from donors (usually the recently dead, but there are exceptions: you can donate one kidney, or parts of your liver, for example). It is a process that has become ever more successful, with improvements in surgery and drugs that suppress the immune system, preventing it from destroying the donated organ. However, this has created a demand for donor organs that is not being met: about 100,000 people worldwide are waiting for donor organs, and many thousands die before they receive one.
You’ve probably all heard those horror stories about people waking up in a bathtub to discover missing a kidney which has now been sold on the black market, although you’re not likely to have it happen to you. More realistically, the high demand for organs means ethically dubious practices such as harvesting organs from executed prisoners without consent, and people in poverty selling a kidney to get desperately needed cash. Science fiction has explored this area too, often erring on the dark side. Remember those human patients lined up in giant freezers in the 1978 film Coma? Creepy. And playing to a deep-seated fear of our bodies – our selves – just being commodities in somebody else’s supermarket. Perhaps, thinking about it, a very capitalist fear, or maybe just an extension of the bodysnatching meme – not stealing your entire body, but some rather essential bits of it, killing you in the process.
Coma movie pic from IMDB: © 1978 Bruce McBroom – Image courtesy mptvimages.com
Much of the death toll could be eliminated if more people volunteered to donate organs, which is why many countries are now switching to an “opt-out” versus an “opt-in” system of organ donation, but one idealistic goal of medical research is to be able to generate transplantable human organs in vitro, as in, in a dish (or probably a rather complicated incubator). How close are we to getting there? Recent scientific advancements would suggest that we’re closer than you might think.
There are two rapidly advancing fields that show great promise in the generation of organs for transplantation. One is organoids, the other 3D printing.
These have arisen as a result of research in my own field of developmental biology, which studies how single eggs give rise to embryos, and in turn how these give rise to mature organisms, plus its spin-off field of stem cell biology. Most of the cells in your body are differentiated; that is, they have become a specialised type of cell with a specific job, like a liver or nerve cell. A stem cell is simply a type of cell that is able to give rise to many other types of cells. Stem cells that can potentially give rise to many different cell types are called pluripotent: usually you will only find these naturally in embryos. The more specialised a cell becomes, with whole batteries of genes turned on and off (e.g. a liver cell will have genes for liver function turned on, but brain, skin, muscle etc. specific genes turned off), the harder it is to revert it to a less differentiated state. This is no longer impossible. Major breakthroughs were the derivation of the first human embryonic stem cells in 1998 and the reprogramming of human somatic (body, as opposed to embryonic) cells to pluripotency in 2007. Protocols have since been developed that can direct human PSCs (pluripotent stem cells) to generate a huge variety of different cell types. It should be noted however that this has relied heavily on the knowledge of morphogens, signalling molecules that pattern the embryo, which came out of more classical developmental biology studies. We study those frogs and fruit flies for a reason.
Morphogenesis, the process by which the complex structures of the body are formed, was long postulated to arise from the relative position of cells to each other and the secreted morphogens they are exposed to, but it’s becoming evident that there is also a degree of self-organisation: put the right mix of cells and chemicals together, and they will spontaneously assemble into a biologically recognisable structure, particularly if they are allowed to develop in a three-dimensional space, like a fluid or porous gel, as opposed to being confined to a layer of cells on a plate. The emergence of a self-organising but recognisable tissue from human PSCs came less than five years ago, in the form of a patterned optic cup, a little mini-organ, or organoid, as they are known. Following this pioneering study, human PSC-derived organoids of a number of tissues have now been reported, including the small intestine, cerebral cortex (part of the brain), stomach, lung, liver and kidney. It’s not just embryonic stem cells either: organoids can also be generated from adult stem cells, because there are epithelial layers lining the surfaces and cavities of many organs, and these contain epithelial stem cells. So far, so good: but can you get real, functional organs from these? Well, not yet.
Most of the organoids are more like embryonic organs or proto-organs: simple gut tubes, for example, or liver buds, and on a very small scale; they are certainly not fully mature organs, but it’s a big step forward. Many questions still need to be answered: are the organoids truly mini-organs, or something rather artificial that has arisen by an abnormal patterning process? Are the genes expressed the same? The connections between the cells? Moreover, there is the time taken to mature a functioning organ: could we sustain an artificial environment long enough to make something as complex as a functioning kidney? And how would you get a blood vessel network forming correctly in a scaled-up version, or innervation by the nervous system? Much of this remains to be worked out, but huge strides have been made already, so it’s not beyond the realms of possibility.
(2) 3D bioprinting.
The other development I want to talk about is the use of 3D bioprinting to make implantable biological structures. I know, “3D bioprinting”, it sounds so Star Trek, doesn’t it? Quite literally, printing out biological structures in three dimensions, which is pretty damn cool.
3D bioprinting has already been used for the generation and transplantation of several tissues, including cartilage, bone, vascular grafts, tracheal splints, heart tissue and multilayered skin structures. In this process, layer-by-layer precise positioning of living cells, biological materials, and biochemicals is used to fabricate 3D structures via spatial control over the printing of the biological components. There are several approaches to 3D bioprinting, all with various pros and cons. It’s a complicated field, and I don’t want to particularly go into the mechanics here, because that’s several posts all on its own. Even without the technical detail, however, it’s not hard to imagine some of the potential issues: you need a detailed, microscopic understanding of the structure you are trying to assemble, a complicated and non-homogenous mix of chemicals and cells, a way of printing that won’t damage the biological components, substrates to print on that can be safely implanted in a human body, a way of keeping the cells alive, etc. Printing an entire human organ seems pretty fanciful, almost as fanciful as printing an entire human! However, you may not necessarily need to: you can take advantage of some of the self-assembly features that are also seen in organoids, or you could print, for example, mini tissue blocks of the smallest functional unit of an organ, like the nephron of a kidney, but this approach obviously won’t work for everything.
Until recently, 3D printed biological matter has been firmly on the tissue scale of things as opposed to the organ, and on the small side, at that. One issue is that the hydrogels used as biomaterials to support the cells aren’t strong enough to make larger structures. Another major problem has been the inclusion of a blood vessel network, essential for the supply of oxygen and nutrients, and removal of waste (over very small distances, as in less than 200 micrometres in engineered tissues, diffusion is sufficient). 3D printed constructs also don’t generally form connections with the local vasculature after implantation, and so tend to die. It’s no coincidence that the big successes have been with cartilage, a non-vascularised structure. However, only a couple of years ago, researchers managed to find a way to print capillaries, tiny blood vessels, that were functional, a development significant enough to make the national news. Actually, what they technically did was print a microchannel structure for a capillary network, lined with endothelial cells, which then went on to make capillaries themselves.
Now, only this year, researchers have developed an integrated tissue-organ printer (ITOP) capable of making human-scale tissues of any shape. They use a PCL matrix to impart mechanical strength, and overcome the diffusion problem by again incorporating microchannels in their constructs: implanted experimental muscle managed to integrate sufficiently that it began to be innervated by nerves and develop a vasculature. It’s preliminary, but very promising. An example is illustrated below:
It should also be noted that both organoids and 3D printed tissues have a whole variety of other potential applications (as all the best science does). Organoids are hugely useful simply for studying normal human development and disease: already, enterprising research groups are growing organoids from cells harvested from patients with diseases like cystic fibrosis to test different drugs, and even using techniques like CRISPR to introduce genetic mutations known to cause disease in humans, making faulty organoids as models to study the disease process. 3D printed tissues are already providing “organs on a chip” to screen drugs for toxicity. We are looking, potentially, at the replacement of the vast majority of laboratory animals for testing, and whole new avenues to study disease mechanisms and treatments.
With both these solutions, it remains an open question as to exactly how complex any artificially generated structures actually need to be: of course, organs have a complicated supporting structure of blood vessels, lymphatics and nerves, that connect them to the rest of the body, but it’s also true that many tissues are able to regenerate and remodel to a degree after disease or injury. Any artificial constructs may be able to take advantage of this by recruiting developmental signalling pathways and local stem cells to promote, for example, the growth of a blood vessel network in an implanted tissue.
The interesting question perhaps is not whether we will someday be able to create a perfect artificial organ, but just how perfect it needs to be.
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