A study published in Nature details the transplantation of human brain-derived organoids into mice. In this work, the mice had their cortical neurons genetically removed, allowing human stem cell-derived organoids to grow into the emptied cerebral cortex area.
The study demonstrates that these human-cell-based nerve cells integrated into the mouse brain and exhibited functional activity. This method differs from previous work by creating a developmental niche, rather than transplanting into an already established cortex, by largely suppressing the development of the mouse cortex while preserving subcortical structures.
Experts note that the integration was well documented at cellular and anatomical levels. Fibres from the graft extended into subcortical structures, mouse neurons formed connections with the graft, and mouse interneurons were found within the human tissue. However, the graft remained immature, with network activity similar to developing neural tissue rather than a mature cortex.
The significance of this work is primarily in the technique, showing that a mouse's entire cerebrum can be replaced by human organoids. While the human cells integrated, they developed at a slower, species-specific rate, corresponding to an embryo in the second trimester of pregnancy by the end of the experiments.