Stroke is the second leading cause of death and main cause of disability worldwide, but with few effective therapies. Although stem cell-based therapy has been proposed as an exciting regenerative medicine strategy for brain injury, there are limitations. The recent generation of three-dimensional cerebral organoids (COs) from human pluripotent stem cells overcomes the limitations of stem cell-based transplantation therapy to a certain extent and shows advantages in diverse cell types, rich cell source, considerable cell number, and controllable degree of cell differentiation and certain tissue volume (up to 4 mm in diameter) . Moreover, the COs show neural connectivity and brain functionality in vitro that recapitulates in vivo features of brain development and maturation , providing possibility for directly repairing and replacing damaged brain tissue after stroke.
Prof Dr Jürgen Knoblich, Scientific Director and Senior Scientist at the Institute for Molecular Biotechnology, Vienna, Austria, said:
“The work is characterised by its methodological progress, as the organoids were implanted in rat brains. These are larger compared to mouse brains and one can transplant larger amounts of tissue. In addition, the organoids were transplanted very early, that is, when the rats were only a few days old. The advantage here is that the brain is still developing, and the transplant can therefore ‘co-evolve’.
“In addition, the researchers show that the human neurons, when activated, interfere with the rats’ behaviour. The human cells functionally connect to the rat brain. This is the reason why the work is so outstanding.
“The human brain is home to some of the most horrific diseases and so far, we don’t understand it very well. A lot of brain experiments are done on animals like mice or rats, but really, they should be done on primates (as primate brains are more similar to human brains; editor’s note). This is very controversial. Organoid models from human stem cells are promising and resolve this conflict.
“Using brain organoids, you can gain some insights because the neurons form connections. The problem with the organoids so far, though, is that they don’t have blood vessels. When they are transplanted, they become vascularised, that is, they have blood vessels growing through them. The transplanted organoids now make it possible to study network properties of human nerve cells in a different way. This could have an impact on research into neurological diseases such as epilepsy or autism.
“Until now, experiments on the brain have only been carried out on animals, but their brain functions are often different from those of humans. Animal experiments are necessary, but they only provide part of a mosaic. For the complete picture, you have to study humans. For that, organoids from human stem cells are needed because they are less ethically controversial than animal experiments.”
Dr Agnieszka Rybak-Wolf, Head of the Organoids Technology Platform, Max Delbrück Center for Molecular Medicine (MDC), Berlin, said:
“The authors transplanted human cortical organoids into newborn rat brains in order to stimulate neuronal maturation and to promote the integration of human neurons into rat sensory and motivation-related circuits. Such cortical neurons showed more complex anatomical and functional properties, extended axons through the rat brain and – what is important and novel – transplanted organoids receive sensory-related inputs and their optogenetic activation (activated by light; editor’s note) could drive rat behaviour during reward-seeking.
“Human-rodent chimeras (an organism consisting of cells of different genetic origins; editor’s note) – although raising some ethical debate about mixing human and animal brain tissue – are well accepted experiments to demonstrate functionality of human in vitro brain cells within in vivo circuits. The authors’ idea is not completely novel. There have been already several studies published in the last years using a similar approach. Just to mention a few examples: Wang et al. demonstrated that transplanted cerebral organoids improves neurological motor function after brain injury . A study by Bao et al. suggested that cerebral organoid transplanted in lesion sites can serve as potential therapeutic approach for traumatic brain injury by reversing deficits in spatial learning and memory . Kitahara et al. optimized the time point and the conditions for organoids transplantation into mouse and monkey brain  and Daviaud et al. grafted cerebral organoids into mouse brains to achieve organoids vascularization . The ethical concerns of such models have been also previously discussed  .
“Although brain organoids form a relatively complex brain tissue like structure, they still lack brain immune cells, vasculature and the circuit connectivity found in vivo. Therefore, they often fail when it comes to model complex human brain diseases related to circuits formation such as autism or schizophrenia. Engrafting of human brain organoids into highly vascularized immunodeficient rodents’ brains (the immune system of the animals used in the study is not fully developed as they lack the thymus and thus functional T cells, thus preventing rejection of the transplanted organoids; editor’s note) gives a unique opportunity to incorporate missing components into the model and to fully form neural circuit in human in vitro brain models. The area of chimeric research models is quickly evolving, motivated by the potential application of such models to for example grow human compatible organs for transplantation. However, when it comes to the brain, it always raises several ethical concerns, such as: ‘Can we create human-like cognition in animals by such transplantations?’
“As we cannot conduct research on human adult or fetal brains for obvious reasons, human brain organoids are a major advance in the study of the human brain. Developing physiological conditions that reflect the ‘real human brain’ is one of the main aims in the field. Therefore, we need to carefully find a compromise between the gains and the risks when it comes to such chimeric models.”
 Wang SN et al. (2020): Cerebral Organoids Repair Ischemic Stroke Brain Injury. Translational Stroke Research. DOI: 10.1007/s12975-019-00773-0.
 Bao Z et al. (2021): Human Cerebral Organoid Implantation Alleviated the Neurological Deficits of Traumatic Brain Injury in Mice. Oxidative Medicine and Cellular Longevity. DOI: 10.1155/2021/6338722.
 Kitahara T et al. (2020): Axonal extensions along corticospinal tracts from transplanted human cerebral organoids. Stem Cell Reports. DOI: 10.1016/j.stemcr.2020.06.016.
 Daviaud N et al. (2018): Vascularization and Engraftment of Transplanted Human Cerebral Organoids in Mouse Cortex. ENeuro. DOI: 10.1523/ENEURO.0219-18.2018.
 Powell K (03.08.2022): Hybrid brains: the ethics of transplanting human neurons into animals. Nature. DOI: 10.1038/d41586-022-02073-4.
 Chen HI et al. (2019): Transplantation of Human Brain Organoids: Revisiting the Science and Ethics of Brain Chimeras. Cell Stem Cell. DOI: 10.1016/j.stem.2019.09.002.
Dr András Lakatos, Neuroscientist and Neurologist at the University of Cambridge, (Group Leader in Neurobiology, Centre for Brain Repair, Department of Clinical Neurosciences, University of Cambridge & Wellcome Trust-MRC Cambridge Stem Cell Institute), said:
“This work has increased our confidence in that human organoids, complex tissues grown in a laboratory dish from stem cells, have the potential to revolutionise brain research. Although it has been pretty clear that organoids can provide a great advantage for studying how the human brain works and what might go wrong in disease, the extent of their maturity required for such analyses has been questionable. One way to prove that cells in brain organoids are mature enough is to show that they do whatever they are supposed to be doing in the brain, and that is to form the right connections that can control behaviour. Sergiu Pasca’s team did just that and did it quite convincingly.
“The choice of implanting human organoids into rat brains to allow such observations is, of course, not without ethical considerations. There are ongoing discussions on the topic to address the arising concerns and, equally, to avoid obstacles to discovery. Nevertheless, this paper in Nature is a significant leap and a great example of why such research should be continued.”
Prof Tara Spires-Jones, UK Dementia Research Institute at The University of Edinburgh &
Deputy Director, Centre for Discovery Brain Sciences, University of Edinburgh, said:
“This paper from Pasca and team from Stanford University shows that clumps of human brain cells derived from stem cells (called organoids) implanted into newborn rat brains can mature in the rat brain and integrate into the rat’s neuronal circuits. Implanting the organoids in rat brain provided a blood supply and brain environment that let the human neurons mature better than they do in culture dishes. These neurons also made connections with other neurons in the rat brain and when activated, they could influence the behaviour of the rats. Researchers implanted organoids from stem cells of people with Timothy syndrome, a rare genetic disease that causes autism spectrum disorders as well as heart defects. The neurons from Timothy syndrome organoids had abnormal development, illustrating that this new type of experiment may be useful for finding treatments for human neurodevelopmental disorders. However, these human grafts did not replicate all of the important features of human developing brain and some of the experiments analysed only a handful of neurons from 3-4 rats per group so more work will need to be done to be sure this system is a robust model for brain development and neurodevelopmental disorders.
“This research has the potential to advance what we know about human brain development and neurodevelopmental disorders, but there is more work to be done to be sure this type of graft is a robust method. I also agree with the experts Drs Camp and Treutlein who wrote a commentary accompanying the paper who point out that these experiments pose several ethical questions that should be considered moving forward including whether these rats will have more human-like thinking and consciousness due to the implants.”
Comments collected by the German SMC