The progress in tissue engineering in just the past two decades has been like the construction industry moving from simple lean-to structures to homes with plumbing, heating and cooling systems. We are not yet ready to build a high-rise—think of a beating functioning heart—but we are making major strides toward that goal.
One of the founders of the field, Wake Forest’s Dr. Tony Atala, led off this morning plenary session at the annual meeting of the International Society for Stem Cell Research. He started trying to build simple organs in 1990. His talk nicely mapped his progress through four levels of complexity of structure.
The first level, accomplished by a few teams, was our largest organ, skin, which is relatively simple because it is flat. Next, came simple hollow organs like blood vessels and the urethra that carries urine from the bladder. He followed that with more complex hollow organs, first the bladder and more recently the vagina. Last up were complex solid organs: the heart and the penis. He expects to begin clinical trials with the latter soon, which is eagerly anticipated by our military dealing with the aftermath IED explosive injuries from the wars in Iraq and Afghanistan.
He noted that researchers in the field quickly learned that just throwing cells on scaffolds and hoping they knew what to do was not enough in most cases. They need to grow blood vessels so they can get nourishment and communicate with their surroundings and they often have to make multiple cell types. His own work here benefited from a bit of geographic serendipity. His lab at the time was on the same floor as Judah Folkman’s at Harvard affiliated Children’s Hospital. Folkman is the father of the field of angiogenesis, the art of growing blood vessels.
Atala showed slides comparing injecting cells where you need new muscle, to cells plus scaffold, and finally to the two combined with a vessel growth factor. The three-way combo far outperformed the others. He published his first study using this technique for a hollow simple organ, the urethra, in 2011. At that point his patients had been living with the functional new organ for six years. They work and last.
Researchers almost always place a cell-scaffold complex in a soup of nutrients and growth factors called a bioreactor before implanting it. But at the time of implant, the organ is not mature. Atala said the body acts like a “finishing bioreactor” to fill out and strengthen the organ, which becomes fully mature around six months after implant. He showed images of this in-body growth in his first patients who had been born without a complete vagina and were given a fully functioning organ. He just published that study two months ago, eight years after the implants in order to make sure they stayed functional over time.
He then showed his animal model work creating a penis in rabbits. Being a highly vascular organ it required much more structure. He used a donor organ that had all its cells chemically washed away to leave just the intracellular scaffold. This structure helped guide the blood vessel growth and the rabbits succeeded in mating and having offspring.
His lab has begun early stage work for both liver and heart. They have created miniature livers about the size of a half dollar that are able to produce the appropriate proteins and metabolize drugs. They have used a 3-D printer to build two chambers of a heart that are able to beat in a dish, but their structure has not been stable. So, he noted much more work lies ahead for complex organs.
The second speaker, Jason Burdick from the University of Pennsylvania, concentrated on making better scaffolds for the stem cells, which can have three enhanced properties:
- they can be instructive, they can tell cells what to do;
- they can be dynamic, they can react to their environment and the cells around them;
- they can lead to heterogeneity, they can provide varied instructions so you get the different cell types that you need for a complex tissue.
He discussed two examples, the first was growing better cartilage (as he joked, for injured World Cup soccer players). One problem with early gels used as scaffold was they held the cells individually apart from each other limiting their ability to communicate with each other. This cell-to-cell cross talk is key to tissue maturation. He showed how you could chemically alter the gel to enhance this communication. He also showed how you could implant the gels with microspheres loaded with growth factors to deliver instructions to the cells.
Burdick’s second example focused on minimizing injury after an induced heart attack in rodents. But instead of loading the gel with cells, they loaded it with microspheres that release chemicals that summons the stem cells waiting quietly in reservoirs in all of us. They saw sustained release of the chemicals for 21 days and significant improvement in heart function.
But he closed with a fun twist. The first heart experiment used a strict time-release formulation. He said it would be much better if the chemicals were released at the points the heart needs it the most. So, he is working on a system that releases the chemical based on the levels of an enzyme the heart makes when it is injured. He is hoping this right-amount-at-the-right-time formula will be even better.
We have a short video of the highlights of a workshop we held on tissue engineering that you can watch to get a better feel for where the field is going.