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Cerebral Organoids Reveal New Angles for Understanding the Human Brain

When you think of researchers growing “mini-brains,” you might picture a mad scientist out of a sci-fi or horror movie. But the reality looks quite different. Cerebral organoids, often colloquially referred to as “mini-brains,” are three-dimensional clusters of cells that mimic basic properties of the developing human brain. These pea-sized grayish-pink spheres are far from developing consciousness, but they can be used to understand the development, structure, and environment of the human brain. They are made from human stem cells, which are “pluripotent,” meaning they have the unique ability to become a variety of cell types. By adding a specific mix of growth-promoting factors, researchers can direct these stem cells to grow into the types of cells they want to study, including those found in the lungs, heart, or brain.

Ever since their creation in 2013, organoids have provided a valuable system to allow researchers to study human cells and development non-invasively. In this way, they help to bridge the gap between studies using animal models and humans. There are currently a variety of ways that researchers use organoids to understand aspects of human brain development, neuropsychiatric disease, and even brain cancers. As the organoid becomes a more popular model system, researchers have sought to improve its ability to mimic the real human brain. But with increased usage of organoids in neurobiology research, ethical questions (Sawai et al, 2019) have arisen. For example, how closely does the organoid resemble the human brain? Will they ever be able to obtain consciousness? And if so, is it ethical to perform experiments on them?

“How closely does the organoid resemble the human brain?”

Currently, cerebral organoids are very far from looking like the adult human brain with thoughts and feelings. While they do have inherent electrical activity similar to a functional brain, they more closely resemble an early developing brain. However, they have distinct structural and functional differences that make them much simpler. For example, the human cerebral cortex, the outermost part of the brain responsible for high-order functions such as awareness, thought, and memory, contains cells organized into distinct functional layers. In contrast, cerebral organoids have a slightly more disordered structure with circular clusters of immature cells that divide to produce mature neurons that diffusely surround those clusters. This is one feature that prevents cerebral organoids from developing more organized activity resembling a true brain’s electrical network. But if organoids only model certain aspects of the human brain, how are they being used in neuroscience today?

Brain Development
One way that organoids have been used is to better understand how the human brain develops. Before organoids, knowledge of the human brain could only be gained from post-mortem human brain donated upon death or brain imaging studies, such as functional magnetic resonance imaging (fMRI) that tracks blood flow in the brain to measure brain activity in human subjects (Shou et al, 2020). However, these studies are unable to understand the beginnings of human brain development that occurs even before birth. Cerebral organoids, however, provide the unique opportunity to address these scientific questions because their development is comparable to that of a fetal brain. They begin as stem cells and progressively mature; organoids grown up to a year are comparable to a fetal brain in the early second trimester. This feature has allowed neuroscientists to probe developmental processes, cell-to-cell interactions, and responses to stressors of the early developing human brain. Researchers have used cerebral organoids to study how oxygen deprivation affects brain development in the prenatal and early postnatal periods (Boisvert et al, 2019). They found that low oxygen prevents appropriate cell maturation and migration during development, but that these effects can potentially be mitigated with an FDA-approved drug called minocycline (Boisvert et al, 2019). Additionally, brain organoids also have electrical activity similar to that of an immature brain, allowing researchers to study how neural circuitry develops. Because of this, organoids are now being used to help understand disorders involving disorganized neural connections, including autism spectrum disorder (ASD), epilepsy, and schizophrenia (Shou et al, 2020).

“Cerebral organoids, however, provide the unique opportunity to address these scientific questions because their development is comparable to that of a fetal brain.”

Neurological Disorders and Neuropsychiatric Diseases
Scientists also use “induced pluripotent stem cells” (iPSCs) to study different types of neurological disorders. These special types of stem cells start as skin or blood cells from adult patient donors. For neuroscientific research, scientists may recruit donors that have normal brains or individuals diagnosed with a psychiatric or neurological disorder, such as schizophrenia or Alzheimer’s disease. Donors’ cells are then “reprogrammed” by adding chemicals to push these cells back toward a pluripotent stem cell state from their “differentiated” or “non-pluripotent” state. These iPSCs can then be used to make different types of mature cells. As these cells come directly from patients, they have the same genetic background as the person that donated them. This is especially useful in cases of diseases believed to be caused by both genetic and environmental factors. For example, organoid studies of autism have investigated the contribution of specific genetic mutations toward the development of autistic traits in children. Cortical organoids derived from individuals with ASD have altered expression of the brain development gene “FOXG1,” resulting in imbalanced excitatory versus inhibitory activity (Shou et al, 2020). This imbalance may contribute to the behavioral differences seen in ASD patients in comparison to neurotypical individuals without neurological disorders.

Cancers Affecting the Brain
Researchers have also collaborated with doctors and hospitals to culture cancer tumors. In this process, patients consent to donating their tumor biopsy or excision. This primary tissue is then cultured to understand the unique cancerous characteristics of the tumor, such as genetic mutations, cell migration and invasion (Drost & Clevers, 2018). Additionally, tumor organoid cultures have been used to screen for potential therapeutic drugs, allowing researchers to find relationships between a patient’s genetics and their drug responses (Liu et al, 2021). This system has the ability to advance personalized medicine without the worries of testing drugs on humans. A previous organoid study of breast cancer tumors found that high expression of certain breast cancer risk genes are associated with greater tumor sensitivity to certain drugs. For example, mutations in a gene called “human epidermal growth factor receptor” (HER) cause aberrant activity that has been linked to cancer growth and spread, and this study found that tumors overexpressing this gene are particularly sensitive to drugs blocking HER signaling pathways (Sachs et al, 2018).

“This system has the ability to advance personalized medicine without the worries of testing drugs on humans.”

Many recent studies are seeking to apply similar techniques to brain cancers. Previous research investigating brain cancers have utilized 2D cell cultures, but more recent 3D cultures better model the complex invasive behaviors and therapeutic resistance of cancer in the real human brain (Ruiz-Garcia et al, 2020). To address this, scientists have generated three-dimensional brain tumor cultures called “spheroids” or “gliomaspheres” (Ruiz-Garcia et al, 2020), and have even begun transplanting tumor cells into cerebral organoids (Mariappan et al, 2020). These transplantation studies have shed light on how tumor cells in glioblastoma, an aggressive form of brain cancer, interact with and change the environment of normal brain tissue (Mariappan et al, 2020). While this technology is still emerging and requires optimization, it provides a promising horizon for the future of modeling human cancers and personalized drug screening.

These applications provide a glimpse into the many ways that cerebral organoids are being used to understand how the brain grows, functions, and how reacts in response to different types of diseases and therapies. It’s a model on the frontier of personalized medicine, allowing researchers to find more effective treatments for each individual’s specific disease and genetic background. As organoid technologies advance and get closer to mimicking aspects of real human brains, these applications will continue to expand. But as ethical questions surrounding organoids arise, it’s important to remember that we are still very far from ever creating a structure capable of consciousness, as organoids will likely never entirely depict the complex structures and functions of the brain. While it’s important to keep these questions in the rearview mirror, they should not prevent the progress of this technology and its incredible ability to help researchers and patients alike to understand brain development and disease.

Cerebral organoids in a dish. (Image courtesy of Alysson Muotri)

Two images above: “Comparison of a developing human brain (first) to a cerebral organoid (second) at the comparable period of development” (Kriegstein Lab / UCSF)


Written by Claudia Nguyen.
Illustrated by Lisa-Ruth Vial.
Edited by Caitlin Goodpaster and Yuki Hebner.


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