Chimeric brain organoid created using stem cells from five people

Stem cell lines from multiple human donors have been combined to create 3D brain organoids for the first time.

A brain organoid, often referred to as a 'brain in a dish', is a simplified model of the brain grown from human pluripotent stem cells or fetal tissue (see BioNews 1223). By combining stem cells from multiple human donors, researchers from Harvard University and the Broad Institute of MIT and Harvard, both in Massachusetts, have created what they're calling 'chimeroids'. The authors hope that these chimeric brain organoids could provide new insights into how individual genetic differences can affect susceptibility to neurodevelopmental diseases and neurotoxins.

'What if one day we could use chimeroids as avatars to predict individual responses to new therapeutics before testing these in a trial? I like to imagine that future,' Professor Paola Arlotta, senior author of the study from Harvard University, told Nature News.

Previously, 2D brain organoids have been made using 44 different human donor stem cell lines by other researchers at the Broad Institute of MIT and Harvard. To generate 3D chimeroids, Professor Arlotta and colleagues had to overcome a key technical challenge: different stem cell lines grow at different rates, leading one cell line to outcompete the others.

In a study published in Nature, the researchers reported a new technique to overcome this challenge. They grew individual organoids from five separate donors, then broke these down into cells and pooled them to form composite chimeroids. After three months, the chimeroids measured three to five millimetres across and contained all the cell types found in early fetal cortical tissue. In humans, the cerebral cortex is the brain's outer layer of tissue and plays a vital role in attention, awareness and consciousness, memory and perception.

The use of neurotoxins during pregnancy, such as alcohol or valproic acid (a medication for epilepsy), can significantly impact fetal brain development, leading to lifelong health consequences. When researchers applied alcohol to the chimeroids, they discovered that stem cells from one donor were more severely affected than others. Similarly, different stem cell lines exhibited varying degrees of response when exposed to valproic acid, highlighting the influence of individual genetic differences in determining the severity of the effects of neurotoxins.

'It's a really powerful technology, and a powerful approach,' Dr Tomasz Nowakowski, a neurobiologist from the University of California, San Francisco, who was not involved in the study, told Nature News. 'Many groups are likely to embrace this method… It's a technical tour de force.'

While the chimeroids in this study were created by combining stem cells from up to five individual donors, the researchers aim to increase this number to hundreds or even thousands in future studies.

Currently, the process requires multiple highly trained researchers working simultaneously over many weeks to ensure that each organoid develops at a similar rate before they are combined. Therefore, significant advancements in automation and scalability will be necessary to reach their goal.