It was described as an "exciting publication" by other researchers.
To create a chimera, human stem cells - the type that can develop into any tissue - are injected into a pig embryo. Image copyright Juan Carlos Izpisua Belmonte Image caption The spherical pig embryo is held in place while a tiny needle is used to inject human cells
The embryo - now a mix of human and pig - is then implanted into a sow for up to one month.
The process appears very inefficient - of the 2,075 embryos implanted only 186 continued to develop up to the 28-day stage.
But crucially there were signs that human cells were functioning - albeit as a tiny fraction of the total tissue - as part of a human-pig chimera.
"This is the first time that human cells are seen growing inside a large animal," Prof Juan Carlos Izpisua Belmonte, from the Salk Institute, told the BBC News website.