Scientists transplanted a human brain organoid into newborn rats. An article about this was published in Nature. Experiments on humans, including their brains, can be difficult, potentially harmful, and severely restricted by law. In this regard, scientists are growing brain organoids – models that are a collection of interconnected nerve cells. They tried to transplant it into animals to test the organoid’s performance, but were unsuccessful.

Sergio Pashka of Stanford University and his colleagues have succeeded in using a mouse brain organoid. They transplanted the brain organoid into newborn mice, not adults as their colleagues had done. At this time, the brain was still fully developed, allowing the organoid to take root and become embedded in its structure. Additionally, the mice lacked a thymus, meaning their adaptive immunity could not attack the transplant.

Not only did the brain organoids take root in the mice’s brains: the scientists noted a nine-fold increase in transplant volume three months after surgery. Most appeared in vessels and cells of microglia – macrophages of the nervous system.

The cells of the brain organoid implanted in the rodent’s body began to differ from those grown in vitro. They express more genes related to the maturation of neurons, and therefore scientists have succeeded in the development of an organoid in a living organism.

In addition, the implanted tissue was unexpectedly attached to the mice’s body. For example, the cells of the organoid were activated when the mouse was touched by the whiskers. The authors hope that their method of growing organoids will make it possible to fully investigate the functioning and development of the human brain.