Human neurons merge well with rat's brain to control senses

Organoids are clumps of lab-grown neurons in the human brain that can be transplanted into rodent brains. Recent research demonstrates that human organoids can integrate with developing rat brains. In a study published in the journal Cell Stem Cell on February 2, 2023, researchers discovered that human brain organoids could integrate with a rat’s brain. 

However, the possibility of human organoids integrating functionally with the visual systems of the injured adult brain is yet to be known. Researchers are working to explore more possibilities for using human organoids in treating neurological illnesses.

Focused on transplanting brain cells and tissue

H. Issac Chen, a senior author, physician, and Asst. Professor of Neurosurgery at the University of Pennsylvania says they focused on transplanting brain cells and tissue transplanting. They also looked at the structure of individual neurons to gain a deep understanding of how the transplanted human organoids integrate with rodent brains. 

The researchers cultivated neurons derived from human stem cells in their lab for around 80 days before they grafted them into the rodent brains that had sustained injuries to their visual cortex. The grafted organoids integrated with the rodent brain within three months. They became vascularized and grew in number and size, sending out neural projections and forming synapses with the rat’s neurons. 

The research team used fluorescently tagged viruses to detect and trace the physical connection between human organoids and brain cells of the host rodent. Chen said they injected a viral tracer into the rodent’s eye, by which they traced the neuronal connections from the rat’s retina. The viral tracer went all the way to the organoid. 

Neural tissues have the potential to repair brain injuries

The study aimed to evaluate the activity of individual neurons within an organoid. To do so, they utilized electrode probes to measure the responses of the neurons to various stimuli. The team then exposed the organoid to flashing lights and alternating white and black bars.

Researchers found in the study that a large number of neurons within the organoid responded to specific orientations of light, which showed that these neurons had not only integrated with the visual system but also adopted specific functions of the visual cortex. This integration occurred within three months, which was unexpected for the team. Previous studies have shown that even nine or ten months after transplanting human neurons into a rodent, they are still not fully mature.

According to the lead researcher, Chen, neural tissues have the potential to repair brain injuries, and this study is a solid first step toward that. The goal is to better control and speed up this integration process.

This finding provides crucial evidence that organoids can potentially rebuild areas of the injured brain. However, the rules that govern organoid integration with the brain and how organoids can be used in other areas of the cortex are still to be understood. Researchers need to further explore these aspects and understand how to better control and speed up this integration process.

The study published in the journal Cell Stem Cell can be found here.