Once this was done, the researchers then began implanting human cortex organoids into the area where the mouse cortex was no longer developing.
Partial recovery
Just over 85 percent of animals implanted with a human organoid successfully incorporated the transplant tissue. These accounted for 92 percent of the cells found in the cerebral cortex of these animals. So it wasn’t a complete replacement, but the human cells largely occupied the space normally occupied by the cerebral cortex. There they formed all the major types of neurons known to occur in the cortex. So, that’s all good.
The human cells also formed some long-distance connections, as evidenced by the fact that processes from human cells to the spinal cord were detected. And the neurons actually engaged in synchronized spikes of activity, suggesting some degree of coordination.
The not so good thing is a general lack of structure. There is evidence of local organization because certain cell types that form in different layers of a normal cerebral cortex tended to be close together in the cerebral cortex of these animals. However, there was no evidence that these different layers had formed, suggesting a lack of larger organization.
What does this mean for the mice? Overall, it appears that having a disorganized human replacement cortex was better than having no cortex at all, but not as good as having a normally structured cortex.
The researchers released them into an area with video surveillance and then used a machine learning classifier to group similar behavioral patterns. Normal mice and mice whose cerebral cortex was removed showed different behavioral patterns. The mice with a humanized cortex formed a third cluster that was different from the other two. The same was true for body weight: mice without the cortex were much lighter than normal mice, but those with the humanized cortex were in between.
