The tree · Replacing lost cells and parts
A human organ is grown inside an animal and transplanted into a person
Between a pig's organ and an organ grown in a dish lies a third idea: grow a human organ inside a pig, from a person's own cells, so that it is the right size, has its own blood supply, and is not rejected. The trick is to breed pig embryos that cannot make the organ and add human stem cells to fill the gap. In 2023 a Guangzhou group reported pig embryos carrying early kidneys made mostly of human cells, grown in sows for 28 days; the kidneys were the size of a fingertip and not working, and the embryos were not carried to term. No animal has been born with a working human organ, so the leaf is at idea only, and it is marked as a long shot because the two species' cells do not yet cooperate well enough to build one.
How it could happen
Breed pig embryos lacking the gene for one organ; add human stem cells tuned to survive among pig cells; carry the embryos to term with the human cells building that organ; keep human cells out of the pig's brain and germ line; show the organ works in the grown animal; transplant it into the person whose cells it came from; then a trial and an approval.
Where today's science would have to give way
Human and pig cells grow at different speeds and answer different signals, so human cells have so far made up only a small share of the organ past the embryo stage; a working organ needs them to win that competition throughout, and nothing yet does that. The ethics of human cells reaching a pig's brain or sperm adds a second barrier that is not technical.
What would count
An animal is born carrying an organ (kidney, liver, pancreas, heart or lung) made mostly of human cells, and that organ does its job in the animal, in a peer-reviewed report; that is the animal rung. The first-human rung is such an organ transplanted into a person and working for at least thirty days. For the clinic rung: a regulator approves organs grown this way for use outside a trial.
Who settles it: Peer-reviewed reports; the transplant centre's report and the regulator's record for the later rungs.
Where it stands
Idea only. Not yet shown to work in any living animal, so there is no record to point at. Next rung more than 20 years away, if ever.
When it reaches the clinic: the project's estimate
This is the one thing on this page that is a judgment rather than a record. The project's estimate of the year it reaches the clinic is 2060, no earlier than 2040 and no later than not this century; the model draws from that range every run, with the likely year the most probable draw. Set 16 October 2026, a first pass awaiting an outside review.
| Rung | No earlier than | Most likely | No later than | |
|---|---|---|---|---|
| Shown in animals | 2028 | 2035 | 2070 | |
| First human | 2034 | 2045 | 2100 | |
| In clinical use | 2040 | 2060 | not this century | feeds the year at the top |
Why. No animal has been born with a working mostly-human organ; the likely year for that is 2035, for a first person about a decade later, and for an approval around 2060, each step allowing for the species barrier and the ethics review to take longer than the biology. The earliest assumes the cell-competition problem is solved within a few years; the latest says it may not be solved this century.
Changes to this record
None since it was written on 16 October 2026. Every change to the stage or the estimate lands here with its date, the old value, the new one and the reason, and a change to the estimate never happens because a number looked wrong: it needs a dated reason and a source.
The last readings
- 17 October 2026 Quiet
Nothing in the window. The searches returned only the 2023 Guangzhou humanised-kidney embryos (Cell Stem Cell, 2023-09-07), the 2017 Wu and Izpisua Belmonte chimera paper, and a 2025-06 report of beating part-human hearts in pig embryos carried for 21 days, all of which the record already knows or which fall below the animal rung (no animal born with a working mostly-human organ).
What the story so far says about it
No promise, self-experiment or logged paper names this one yet.
Notes
The pig-organ leaf (a gene-edited pig's own organ) and the lab-grown-organ leaf are the two nearer routes; this one would be the better organ if it could be built, and the tree keeps it so that a reader can see how far it is.
Not yet verified
These facts in the record were written from memory or from a secondary source and have not yet been checked against the paper, the regulator's record or the registry. Nothing here is presented as settled until it is cleared, by checking it and either deleting the line or correcting the record.
- That human cells made up less than one in a thousand of the cells in the 2017 human-pig chimeric embryos (Wu, Izpisua Belmonte and colleagues, Cell) is from press coverage (under 0.001 per cent); the paper's own figure was not found in its text.