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 led to the creation of the world's first human mini-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 studying 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 human brain development and BBB functionality."

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 keeps the brain healthy 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 entering the brain. In addition, several neurological disorders are caused or worsened when the BBB does not form correctly 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 live up to expectations when tested in humans.

"Now, through stem cell bioengineering, we have developed an innovative human stem cell-based platform that allows us to study the complex mechanisms that govern BBB function and dysfunction. This provides unprecedented opportunities for drug discovery 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 lab dish, organoid cells are connected to each other. They self-organize into spherical shapes and “talk” to each other, just as human cells do during embryonic development.

Cincinnati Children's has been a leader in developing other types of organoids, including the world's first functional intestinal, gastric, and esophageal organoids. But until now, no research center had succeeded in creating a brain organoid that contains the 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 assembloids combine two different types of organoids: brain organoids, which replicate human brain tissue, and blood vessel organoids, which mimic vascular structures.

The combining process began with brain organoids measuring 3-4 millimeters in diameter and blood vessel organoids measuring about 1 millimeter in diameter. Over the course of about a month, these separate structures merged into a single sphere measuring just over 4 millimeters in diameter (about 1/8 of an 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 recreate many of the complex neurovascular interactions seen 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.

Importantly, BBB assemblies 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 may lead to impaired blood-brain barrier function.

Initial proof of concept

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

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

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

Potential applications

The co-authors see a variety of potential applications for BBB assemblies:

• Personalized drug screening: Patient-derived BBB assemblies can serve as avatars to tailor therapy to patients based on their unique genetic and molecular profiles.
• Disease modeling: For a number of neurovascular disorders, including rare and genetically complex conditions, good model systems for research are lacking. Success in creating BBB assemblies could accelerate the development of human brain tissue models for a wider range of conditions.
• High-throughput drug discovery: Scaling up assembloid production could 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 assemblies can help assess the toxic effects of environmental pollutants, pharmaceuticals, and other chemical compounds.
• Immunotherapy Development: By exploring the role of the BBB in neuroinflammatory and neurodegenerative diseases, new assemblies may support delivery of immune therapies to the brain.
• Bioengineering and Biomaterials Research: Biomedical engineers and materials scientists can take advantage of the availability of a laboratory BBB model to test new biomaterials, drug delivery vehicles, and tissue engineering strategies.

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