Mice, pigs and humans: chimaera research at the ISSCR conference

Novel types of chimaeric pig embryos and chimaeric mice, containing both human and animal cells, have been reported at an international conference.

Scientists developing human-animal chimaeras have presented their findings, which have not yet been peer-reviewed, at the International Society for Stem Cell Research (ISSCR)'s 2025 Annual Meeting in Hong Kong. A team from China reported creating chimaeric embryos, containing both human and pig cells, which developed early-stage, humanised hearts when grown in pigs. Another team, from the USA, reported creating chimaeric mice with human cells in their intestines, liver and brain.

'There were several talks at the meeting that were concerned with prospects of interspecies mixes (chimaeras) – one on obtaining beating hearts with human cells inside pig embryos, and another on mice with human cells in specific organs,' Professor Robin Lovell-Badge, chair of trustees at PET (the Progress Educational Trust) and group leader in stem cell biology and developmental genetics at the Francis Crick Institute, London, who attended the meeting told BioNews.

A team led by Dr Liangxue Lai from the Guangzhou Institutes of Biomedicine and Health, China, previously grew humanised kidneys in pig embryos from human stem cells, which were carried by a surrogate pig for up to 28 days (see BioNews 1206). Now, the team has have reported creating chimaeric embryos that developed early-stage, humanised beating hearts when grown in pigs.

The researchers used single-cell pig embryos lacking two genes required for heart development. The chimaeric embryo was made by transplanting human stem cells into the pig embryo soon after fertilisation, which was then carried by a surrogate pig. The human cells were tagged with a luminescent biomarker so that they could be identified within the embryo.

The team reported that the embryos did not survive beyond 21 days, which they believe was due to the human cells disrupting the function of the pig hearts.

'The research would have a lot further to go to generate anything that could be used for heart transplants, given that only the cardiac muscle was human – other cell types such as blood vessels would be of from the pig embryo, leading to rapid rejection,' explained Professor Lovell-Badge.

The second team, led by Dr Xiling Shen, from the University of Texas in Houston, and Dr Qiang Huang, from the Terasaki Institute for Biomedical Innovation in Los Angeles, California, reported creating mice with organs containing human cells.

They achieved this by using reprogrammed stem cells to grow human organoids of the gut, liver and brain, which they injected directly into the amniotic fluid of mice carrying early-stage embryos. Subsequently, the gut organoids grew in the intestines, the liver organoids in the liver and brain organoids in the cortex region of the brain.

The mice carried the embryos to term and the team reported that the human cells were present and functioning two months after birth, albeit in low concentrations, with the highest concentration in the intestines (where one percent of the cells were human).

'This story is scientifically more of surprise, which makes it more interesting, although any practical use is harder to envisage, certainly without more details. Why cells or organoids placed in the amnion should end up in their cognate tissues is mysterious,' said Professor Lovell-Badge. 'Perhaps this new story presented at the ISSCR meeting suggests the existence of a mechanism to aid normal development by promoting tissue repair, or otherwise help compensate for problems in tissue growth in utero. This would be remarkable'.

However, Professor Lovell-Badge added the caveat that the research will need to be published in a peer-reviewed publication, and other teams will need to replicate the reported observations.