I was surprised this morning to see breathless headlines that a rat limb had been grown in a lab. I have been following the progress of synthetic organ generation, and to date the most advanced techniques I was aware of could only grow very thin organs like skin or bladder sacs or very small organ tissue samples, such as a small patch of liver cells suitable for drug testing but not transplant. I thought we were at least a decade away from growing full, complex organs such as a heart or kidney, and didn't even have an estimate for when we could grow something as complex as a limb (with all its various tissue types that need to connect to each other in just the right places). The ability to grow a limb would represent a quantum leap in technology.
Thus I was not surprised to learn, upon reading the paper, that they had in fact not grown a limb in the lab. Not entirely.
The chief challenge with growing artificial organs today is organizing the stem cells into the correct 3D shape. After all, an organ isn't an undifferentiated mass of cells. It has veins and arteries and functional systems that all need to be in the right place and aligned properly in respect of each other, or the thing doesn't work and quickly dies.
Currently there are three solutions for the above problem. The first one is to use a 3D-printer to "print" the cells into the correct place. This works okay for small tissue samples, but we haven't figured out how to print anything bigger than a couple millimeters. The second solution is to take a donor organ and wash away all its cells, leaving only the scaffolding (or "intercellular matrix") behind. This scaffolding can then be seeded with stem cells from the donee, and the cells (if cared for properly) will grow into the scaffolding like a vine growing up a trellis, forming a new organ. The third solution is a combination of the first two: 3D-print just the scaffolding, and then seed it with stem cells to grow in place.
This second method is how this rat limb was created. A donor limb was necessary, and then the seeded with cells. The advance (and it is a real advance) is that they were able to get all the different necessary tissues to grow nicely - bones, nerves, muscles, skin, etc. This is a good technological advance, but it doesn't free us from the need for organ donors. Alas.
The good news is that the 3D-printing of scaffolding, followed by seeding with stem cells, is coming along nicely. The most recent advance I could find quickly is the growth of this synthetic larynx. It's a promising technology that one day soon should free us from the need for organ donors entirely. But for now, limbs are still at least a decade away I'd guess.