The first human mini-brain with a functional blood-brain barrier has been created

New research by a team led by experts at Cincinnati Children's has created the world's first mini-human brain with a fully functional blood-brain barrier (BBB).

This significant breakthrough, published in the journal Cell Stem Cell, promises to accelerate understanding and improve treatments for a wide range of brain diseases, including stroke, cerebrovascular disease, brain cancer, Alzheimer's disease, Huntington's disease, Parkinson's disease and other neurodegenerative conditions.

“The lack of an authentic human BBB model has been a major obstacle in the study of neurological diseases,” said lead study author Dr. Ziyuan Guo.

"Our breakthrough involves the generation of human BBB organoids from human pluripotent stem cells, mimicking human neurovascular development to create an accurate representation of the barrier in growing, functioning brain tissue. This is an important advance because the animal models we currently use do not accurately reflect brain development human and the functionality of the BBB."

What is the blood-brain barrier?

Unlike the rest of our body, the blood vessels in the brain have an extra layer of tightly packed cells that sharply limit the size of molecules that can pass from the bloodstream into the central nervous system (CNS).

A properly functioning barrier supports brain health by preventing harmful substances from entering while allowing vital nutrients to reach the brain. However, this same barrier also prevents many potentially beneficial drugs from reaching the brain. In addition, several neurological disorders are caused or worsened when the BBB does not form properly or begins to break down.

Significant differences between the human and animal brains have meant that many promising new drugs developed using animal models later fail to perform as expected in human trials.

"Now, through stem cell bioengineering, we have developed an innovative platform based on human stem cells that allows us to study the complex mechanisms that govern BBB function and dysfunction. This provides unprecedented opportunities for discovery of new drugs and therapeutic interventions," says Guo.

Overcoming a long-standing problem

Research teams around the world are racing to develop brain organoids—tiny, growing 3D structures that mimic the early stages of brain formation. Unlike cells grown in a flat laboratory dish, the cells of organoids are interconnected. They self-organize into spherical shapes and “communicate” with each other, just as human cells do during embryonic development.

Cincinnati Children's has been a leader in the development of other types of organoids, including the world's first functional intestinal, stomach and esophageal organoids. But until now, no research center has been able to create a brain organoid containing a special barrier layer found in the blood vessels of the human brain.

We call them new models "BBB assembloids"

The research team called their new model "BBB assembloids." Their name reflects the achievement that made this breakthrough possible. These assembleloids combine two different types of organoids: brain organoids, which replicate human brain tissue, and blood vessel organoids, which mimic vascular structures.

The combination process began with brain organoids with a diameter of 3-4 millimeters and blood vessel organoids with a diameter of about 1 millimeter. Over the course of about a month, these separate structures fused into a single sphere just over 4 millimeters in diameter (about 1/8 inch, or about the size of a sesame seed).

Image description: The process of fusing two types of organoids to create a human brain organoid that includes the blood-brain barrier. Credit: Cincinnati Children's and Cell Stem Cell.

These integrated organoids recapitulate many of the complex neurovascular interactions observed in the human brain, but they are not complete models of the brain. For example, the tissue does not contain immune cells and has no connections to the rest of the body's nervous system.

Cincinnati Children's research teams have made other advances in fusing and layering organoids from different cell types to create more complex "next generation organoids." These advances have helped inform new work on creating brain organoids.

It is important to note that BBB assembloids can be grown using neurotypical human stem cells or stem cells from people with certain brain diseases, thus reflecting gene variants and other conditions that can lead to dysfunction of the blood-brain barrier.

Initial proof of concept

To demonstrate the potential utility of the new assembloids, the research team used a patient-derived stem cell line to create assembloids that accurately recapitulated key features of a rare brain condition called cerebral cavernous malformation.

This genetic disorder, characterized by disruption of the integrity of the blood-brain barrier, results in the formation of clusters of abnormal blood vessels in the brain, which often resemble raspberries in appearance. The disorder significantly increases the risk of stroke.

“Our model accurately reproduced the disease phenotype, providing new insights into the molecular and cellular pathology of cerebrovascular diseases,” says Guo.

Potential applications

The co-authors see many potential applications for the BBB assembleloids:

• Personalized drug screening: Patient-derived BBB assembloids can serve as avatars to tailor therapies to patients based on their unique genetic and molecular profiles.
• Disease Modeling: A number of neurovascular disorders, including rare and genetically complex conditions, lack good model systems for research. Success in creating BBB assemblies could speed up the development of human brain tissue models for more conditions.
• High-throughput drug discovery: Scaling up assemblyloid production may allow more accurate and rapid analysis of whether potential brain drugs can effectively cross the BBB.
• Environmental Toxin Testing: Often based on animal model systems, BBB assembleloids can help evaluate the toxic effects of environmental pollutants, pharmaceuticals, and other chemical compounds.
• Development of immunotherapies: By exploring the role of the BBB in neuroinflammatory and neurodegenerative diseases, new assembleloids may support the delivery of immune therapies to the brain.
• Bioengineering and Biomaterials Research: Biomedical engineers and materials scientists can take advantage of the laboratory model of the BBB to test new biomaterials, drug delivery vehicles, and tissue engineering strategies.

“Overall, BBB assembleloids represent a revolutionary technology with broad implications for neuroscience, drug discovery and personalized medicine,” says Guo.