Growing Brains in a Dish: Organoids Present Great Promise in Modeling Neural Tissue
Benjamin Kheyfets
Illustrations by: Max Freedman
If you saw miniature brains, livers, and pancreases suspended in jars of viscous broth, would you think you were in a mad scientist’s lair? While this sounds like an image lifted directly from a low budget sci-fi film, this scene is actually more plausible than you might expect. Artificially grown tissues resembling human organs are becoming increasingly popular in neuroscience research [1]. These fascinating structures, known as organoids, are incredibly similar in both structure and function to their corresponding organs, even down to the microscopic level. This striking degree of similarity makes organoids great candidates for novel research in medicine. Organoids of neural tissue, in particular, allow the use of innovative neuro-therapeutic techniques to study the human nervous system [2]. But how exactly do these seemingly magical structures work?
What Are Organoids?
The term organoid refers to tissues that have been derived and grown from stem cells [1]. Stem cells can specialize into any cell subtype; theoretically, a stem cell can turn into a blood cell, immune cell, or neuron depending on the environment in which it’s grown. Remarkably, it takes just a few stem cells to grow any type of organ tissue. And, while this tissue may be small in size, it appears nearly identical to its corresponding organ when magnified [2]. For instance, tissue can form into specific regions of a developing brain and give rise to certain types of neurons, allowing us to study the brain in great detail [3, 4]. In this way, organoids are powerful tools that broaden the scope of what neuroscientists can study.
Because the human brain is shielded by the skull, neuroscience has historically been stymied by our inability to microscopically study a living brain. Currently, we can either study brains post-mortem, or use large-scale imaging technology like the MRI to analyze the brains of living humans. While scans like the MRI provide information about brain tissue, they fail to provide information at the cellular level. The invention of neural organoids, however, offers a unique and up-close look at living brain cells; neural organoids can act as a proxy for the brain by accurately mimicking its structure and development. Depending on the environment in which the organoid is grown, the tissues that form can capture the vast array of neural cell types, including those from specialized brain regions [5, 6]. Neural organoids can even grow into complex structures like our cerebral cortex, the outermost region of our brain associated with language, memory, and reasoning [7]. Amazingly, organoids can accurately model the development of brain tissue. Similar to sowing a seed and watching it grow, when we place stem cells into a dish to develop, organoids will form and organize themselves into the brain tissue they are modeled after. This pattern of growth, similar to that of naturally grown brain tissue, offers us a means to study and analyze human neurodevelopment like never before [8].
Step Aside, Rodents: Organoids are Here to Stay
Generally, when scientists want to test something in humans, they first use animal models to get some sense of what results may occur. Organoids are particularly useful as a complement to animal models and other popular testing techniques in neuroscience. Typically, researchers use rodents — like rats, mice, or guinea pigs — as animal models in research due to their low cost of care, quick breeding time, and high degree of similarity to humans on an anatomical and physiological level [9]. However, using these animals still comes with disadvantages. Testing anything on an animal, whether it be a genetic change or a newly developed drug, introduces new variables that often complicate the study’s results.
One such complication arises from the constant cross-talk that occurs between the brain and other physiological systems. For example, the immune system can influence the nervous system’s development by regulating whether synaptic connections are kept or eliminated; this, in turn, influences how our brain develops into adulthood [10]. Certain genes associated with immunity have even been identified as risk factors for Alzheimer’s — a surprising example of this physiological cross-talk [11]. Unfortunately, interactions between body systems are inherent to any animal model and cannot be controlled for or eliminated, making it difficult to interpret results in isolation. Further complications arise from the potentially limited transferability of animal model research to humans. For instance, rodents can not model human diseases perfectly, nor can they always replicate symptoms of a disease [13]. Thus, even though animals like rodents generally make for useful models, they can’t be used to study everything [13]. In some cases, uncommon, non-model species are used in specific fields of research; for example, the mouse lemur and African turquoise fish are ideal species to study aging in vertebrates [13]. However, while all of these animal models are greatly useful for specific research, the results of these studies still may not apply directly to humans. In fact, most drugs tested in these studies don’t make it to human trials; of those that do, most fail the clinical trials necessary to gain medical approval [14].
Since organoids are highly controllable and customizable, their use can help address many of the issues posed by animal testing. In an organoid model, a change is made to brain tissue alone, allowing us to visualize how cells respond to stimuli without interference from or modulation via other bodily processes [15]. In this way, using human brain organoids to model real human brains makes scientific results more translatable to real world applications. It’s important to note, however, that organoids can never entirely replace animal testing. Some fields of neuroscience, such as neuropsychology and behavioral studies, require animal subjects with working brains. Animal models are necessary to study how the brain will respond in the context of existing in a living body, as we are not brains with bodies in tow. Further, myriad similarities between mice, rats, and humans make animals valid candidates for research subjects. Still, organoid research is incredibly useful for filling the knowledge gaps left by animal testing.
Organoids In Action
Beyond being a potential alternative to animal models, the applications of organoid research extend into several fields of neuroscience and other scientific disciplines. Multidisciplinary fields such as neuroendocrinology — which connects the nervous system and our hormonal system — and neuropsychiatry — which combines neuroscience, behavior, and social psychology — can be effectively studied using organoids. Neural organoids can even be used to determine the genetic mutations associated with neurodevelopmental disorders like microcephaly, a condition which causes a child to be born with an abnormally small head [15, 16, 17]. For example, since neural organoids develop similarly to human brains via the development of distinct lobes and structures, they can also successfully model the progression of microcephaly. Replicating the progression of microcephaly using organoids revealed the disorder’s cause: defective neuron development [18]. Identifying the genetic mutations of debilitating conditions such as microcephaly is essential to understanding and developing treatments.
Organoids can advance our understanding of neuropsychiatric disorders, such as Alzheimer's and schizophrenia. Many neuropsychiatric disorders are difficult to study because individuals affected tend to have a variety of genetic profiles, lifestyles, and symptoms [20]. However, organoids allow each of these factors to be isolated with ease. In one instance, collagen-producing cells from schizophrenic patients were collected and manipulated to become brain organoids [21]. These organoids were then used to identify twenty five potential genes involved in the onset of schizophrenia, deepening our understanding of the role genetics might play in the disorder’s onset [21]. Important findings like these typically result from the manipulation of gene activity in neural organoids. Genes can be altered to be more or less active, and then observed to determine what changes follow; for example, this method has allowed us to determine the precise proteins that are involved in the neurodegeneration associated with Alzheimer’s disease [22]. Identifying these proteins is critical for pharmaceutical manufacturers to develop drugs that slow neurodegeneration and disease progression. While there are currently animal models that allow us to study the onset of Alzheimer’s before it becomes clinically diagnosable, there are still major differences in brain development and structure that limit the translatability of these models [23, 24]. Because Alzheimer’s is unique to humans, and experimenting on living human brains is not an ethical option, organoids can help us to develop treatments by providing us with functioning brain tissue to test [22].
However, the benefits of organoid research are not just limited to neurodegenerative disorders; recently, organoids have helped us understand the mechanisms underlying some infectious diseases. One example of organoids’ potential in this regard concerns an infectious disease known as the Zika virus, which causes microcephaly in developing fetuses. You may have heard of the Zika virus, since it garnered public attention after an outbreak in 2016. Using organoids, different genes related to the virus’s onset were isolated by accurately recreating a fetus’s developing brain tissue [25]. As a result, an existing drug was discovered to block the spread of the Zika infection, helping us understand the virus while simultaneously improving patient care [26]. Recently, neural organoids were used to study SARS-CoV-2, the virus behind the COVID-19 pandemic. When exposed to SARS-CoV-2, neural organoid models demonstrated that the virus could enter brain organoids and alter the distribution of certain proteins within neurons, culminating in neuronal death [27]. Using an organoid model, COVID-19, which is mainly regarded as a respiratory disease, was shown to adversely affect the central nervous system, too [27]. These are just a couple examples of neural organoids’ broad potential in revolutionizing infectious disease research.
However, in spite of the immense progress in this research field, organoid technology is still in its infancy [28]. A significant hurdle encountered in organoid research is that live animal brains do not operate in a vacuum; rather, they respond to inputs from the rest of the body. As isolated pieces of tissue, organoids currently cannot model this interaction between body systems. Fortunately, this problem can be addressed by combining multiple organoids into a singular larger structure, called an “assembloid” [15]. Assembloids are particularly useful for studying the brain’s development because they can model interactions between different regions of the brain and parts of the central nervous system, such as the spine. Assembloid research can also explore how neurons organize themselves on a larger scale during development, and the formation of long range connections between them. In fact, this technique has already been used to study the interactions between different types of cells during early brain development or in response to an injury [15, 29].
It’s important to note though that, like organoids, assembloids are still a very new development in scientific history. Current assembloid research mainly seeks to develop more accurate models of the interplay between our brain and the rest of our body [10, 30]. Furthermore, since assembloids are simplified models of physiological systems, they cannot account for the influences of environmental and psychological factors like childhood nurturing, emotion, and memory [15]. Therefore, while organoid and assembloid models allow researchers to explore otherwise unobservable brain processes, their novelty in the world of science and lack of connection to other bodily systems still leaves us with doubts surrounding their potential for real-world applications.
Growing Pains: Addressing Ethical Concerns in Organoid Research
Organoids are revolutionary new tools in neuroscience and biomedicine; their development has even led to a new field of research known as personalized medicine. Personalized medicine uses organoids developed from a patient’s cells to create custom medicines best suited to their needs. A cancer sample, for example, can be collected from the patient, grown into an organoid, and then tested to determine which treatment is most effective against the malignant cells characteristic of cancer [31, 32]. While this may seem like a miraculous development, this branch of organoid research also raises some complicated questions.
One major ethical concern brought about by organoid research is the issue of ownership and consent. Since these novel treatments could generate millions in profit, there is a clear incentive for research institutions, companies, and patients to claim ownership of organoids and the products developed from them. Because organoids carry the genetic information of the patient from whom the cells were first taken, patients may seek the right to own organoids developed using their cells. At the same time, an organoid’s successful growth and development requires the knowledge and resources of research institutions and large corporations. A high profile case of a similar dispute is that of Henrietta Lacks, whose cervical cancer cells were removed without her consent [33]. These cells, called HeLa cells, were found to have an incredible ability to survive and reproduce as they were essentially immortal. HeLa cells were widely shared in biomedical research communities and corporations made immense profits through their contributions to cancer research, immunology, and even COVID-19 vaccine development [33]. However, Lacks’s family never received any financial compensation for such use, and fair ownership of the cells is still being fought over in courts. To avoid disputes like the one Lacks’s family is facing, there is a clear need for regulations that ensure fair ownership is taken into consideration when researching, developing, and marketing treatments derived from organoids. In sum, while organoids may originate from patient tissue, outside institutions and corporations are an integral component of successful organoid research. This nuanced relationship must be discussed when discussing ownership and consent.
When it comes to brain organoids in particular, morality is a greatly controversial issue [34]. While brain organoids are undoubtedly simpler than real brains, researchers continue to make them more complex, so that a wider range of conditions can be studied. In fact, a recent study demonstrated that brain organoids exhibit patterns of electrical activity similar to the brains of premature infants [35]. This form of brain activity is a hallmark of our conscious brains, raising the question of whether brain organoid development should be allowed to reach stages of advanced development. Some neuroethicists even feel that some experiments — such as adding live, functioning human neurons into the brains of living mice — should not be performed at all. Advanced organoid development calls our understanding of consciousness into question; do these structures deserve status as conscious beings, and the inherent protections that entails [34]? As organoid development continues to advance, scientists and lawmakers alike are forced to contend with these complexities. With this in mind, the US National Academies of Sciences, Engineering, and Medicine recently launched a study to determine the potential legal and ethical issues associated with brain organoids. Even so, there are currently no regulations in the United States that ban the creation of a conscious organoid. This is complicated by the fact that neuroscientists don’t yet have any established way to measure consciousness. Until a standard to measure consciousness is developed, it is unclear how to tell if an experiment crosses a line [34].
Research involving brain organoids is a double edged sword. Studying human brain organoids may be critical to understanding and developing treatments for conditions that uniquely affect humans. At the same time, brain organoid research poses moral uncertainties that warrant proper regulation [34]. However, halting all research due to these uncertainties would undoubtedly close doors in research targeting potential cures or treatments for a wide range of disorders and diseases. When used properly, responsibly, and ethically, brain organoids have the potential to improve lives and develop remarkable medical treatments and products.
REFERENCES
de Souza, N. (2018). Organoids. Nature Methods, 15(1), 23–23. doi:10.1038/nmeth.4576
Eisenstein, M. (2018). Organoids: The body builders. Nat Methods 15, 19–22. doi:10.1038/nmeth.4538
Corrò, C., Novellasdemunt, L., & Li, V. (2020). A brief history of organoids. American Journal of Physiology. Cell physiology, 319(1), C151–C165. doi:10.1152/ajpcell.00120.2020
Russell, J. J., Theriot, J. A., Sood, P., Marshall, W. F., Landweber, L. F., Fritz-Laylin, L., Polka, J. K., Oliferenko, S., Gerbich, T., Gladfelter, A., Umen, J., Bezanilla, M., Lancaster, M. A., He, S., Gibson, M. C., Goldstein, B., Tanaka, E. M., Hu, C. K., & Brunet, A. (2017). Non-model model organisms. BMC biology, 15(1), 55. doi:10.1186/s12915-017-0391-5
Lin, V., Hu, J., Zolekar, A., Yan, L. J., & Wang, Y. C. (2020). Urine sample-derived cerebral organoids suitable for studying neurodevelopment and pharmacological responses. Frontiers in Cell and Developmental Biology, 8, 304. doi:10.3389/fcell.2020.00304
Velasco, S., Kedaigle, A. J., Simmons, S. K., Nash, A., Rocha, M., Quadrato, G., Paulsen, B., Nguyen, L., Adiconis, X., Regev, A., Levin, J. Z., & Arlotta, P. (2019). Individual brain organoids reproducibly form cell diversity of the human cerebral cortex. Nature, 570(7762), 523–527. doi:10.1038/s41586-019-1289-x
Lancaster, M. A., Renner, M., Martin, C. A., Wenzel, D., Bicknell, L. S., Hurles, M. E., Homfray, T., Penninger, J. M., Jackson, A. P., & Knoblich, J. A. (2013). Cerebral organoids model human brain development and microcephaly. Nature, 501(7467), 373–379. doi:10.1038/nature12517
Benito-Kwiecinski, S., & Lancaster, M. A. (2020). Brain organoids: Human neurodevelopment in a dish. Cold Spring Harbor perspectives in biology, 12(8), a035709. doi:10.1101/cshperspect.a035709
Bryda E. C. (2013). The mighty mouse: The impact of rodents on advances in biomedical research. Missouri Medicine, 110(3), 207–211.
Dantzer R. (2018). Neuroimmune Interactions: From the brain to the immune system and vice versa. Physiological Reviews, 98(1), 477–504. doi:10.1152/physrev.00039.2016
Taams L. S. (2019). Neuroimmune interactions: How the nervous and immune systems influence each other. Clinical and Experimental Immunology, 197(3), 276–277. doi:10.1111/cei.13355
Gabriel, E., Ramani, A., Altinisik, N., & Gopalakrishnan, J. (2020). Human brain organoids to decode mechanisms of microcephaly. Frontiers in Cellular Neuroscience, 14, 115. doi:10.3389/fncel.2020.00115
Pifferi, F., Epelbaum, J., & Aujard, F. (2019). Strengths and weaknesses of the gray mouse lemur (Microcebus murinus) as a model for the behavioral and psychological symptoms and neuropsychiatric symptoms of dementia. Frontiers in Pharmacology, 10, 1291. doi:10.3389/fphar.2019.01291
Drummond, E., Wisniewski, T. (2017). Alzheimer’s disease: Experimental models and reality. Acta Neuropathol 133, 155–175. doi:10.1007/s00401-016-1662-x
Paşca, S. P. (2019). Assembling human brain organoids. Science, 363(6423), 126–127. doi:10.1126/science.aau5729
Nawathe, A., Doherty, J., & Pandya, P. (2018). Fetal microcephaly. BMJ (Clinical research ed.), 361, k2232. doi:10.1136/bmj.k2232
Trujillo, C. A., & Muotri, A. R. (2018). Brain organoids and the study of neurodevelopment. Trends in Molecular Medicine, 24(12), 982–990. doi:10.1016/j.molmed.2018.09.005
Lancaster, M. A., Renner, M., Martin, C. A., Wenzel, D., Bicknell, L. S., Hurles, M. E., Homfray, T., Penninger, J. M., Jackson, A. P., & Knoblich, J. A. (2013). Cerebral organoids model human brain development and microcephaly. Nature, 501(7467), 373–379. doi:10.1038/nature12517
Wang, L., Li, Z., Sievert, D., Smith, D., Mendes, M. I., Chen, D. Y., Stanley, V., Ghosh, S., Wang, Y., Kara, M., Aslanger, A. D., Rosti, R. O., Houlden, H., Salomons, G. S., & Gleeson, J. G. (2020). Loss of NARS1 impairs progenitor proliferation in cortical brain organoids and leads to microcephaly. Nature Communications, 11(1), 4038. doi:10.1038/s41467-020-17454-4
Quadrato, G., Brown, J., & Arlotta, P. (2016). The promises and challenges of human brain organoids as models of neuropsychiatric disease. Nature Medicine, 22(11), 1220–1228. doi:10.1038/nm.4214
Kathuria, A., Lopez-Lengowski, K., Jagtap, S. S., McPhie, D., Perlis, R. H., Cohen, B. M., & Karmacharya, R. (2020). Transcriptomic landscape and functional characterization of induced pluripotent stem cell-derived cerebral organoids in schizophrenia. JAMA Psychiatry, 77(7), 745–754. doi:10.1001/jamapsychiatry.2020.0196
Zhao, J., Fu, Y., Yamazaki, Y., Ren, Y., Davis, M. D., Liu, C. C., Lu, W., Wang, X., Chen, K., Cherukuri, Y., Jia, L., Martens, Y. A., Job, L., Shue, F., Nguyen, T. T., Younkin, S. G., Graff-Radford, N. R., Wszolek, Z. K., Brafman, D. A., Asmann, Y. W., … Bu, G. (2020). APOE4 exacerbates synapse loss and neurodegeneration in Alzheimer's disease patient iPSC-derived cerebral organoids. Nature Communications, 11(1), 5540. doi:10.1038/s41467-020-19264-0
Götz, J., Bodea, LG. & Goedert, M. (2018). Rodent models for Alzheimer disease. Nat Rev Neurosci 19, 583–598). doi:10.1038/s41583-018-0054-8
Do Carmo, S., Cuello, A.C. (2013). Modeling Alzheimer’s disease in transgenic rats. Mol Neurodegeneration 8, 37. doi:10.1186/1750-1326-8-37
Dang, J., Tiwari, S. K., Lichinchi, G., Qin, Y., Patil, V. S., Eroshkin, A. M., & Rana, T. M. (2016). Zika virus depletes neural progenitors in human cerebral organoids through activation of the innate immune receptor TLR3. Cell Stem Cell, 19(2), 258–265. doi:10.1016/j.stem.2016.04.014
Mesci, P., Macia, A., LaRock, C., N., Tejwani, L., Fernandes I., R., Suarez, N., A., Zanotto, P., M., de A., Beltrão-Braga P., C., B., Nizet, V., Muotri A., R. (2018) Modeling neuro-immune interactions during zika virus infection. Human Molecular Genetics, 27(1), 41-52. doi:10.1093/hmg/ddx382
Ramani, A., Müller, L., Ostermann, P. N., Gabriel, E., Abida‐Islam, P., Müller‐Schiffmann, A., Mariappan, A., Goureau, O., Gruell, H., Walker, A., Andrée, M., Hauka, S., Houwaart, T., Dilthey, A., Wohlgemuth, K., Omran, H., Klein, F., Wieczorek, D., Adams, O., … Gopalakrishnan, J. (2020). Sars‐cov‐2 targets neurons of 3d human brain organoids. The EMBO Journal, 39(20). doi:10.15252/embj.2020106230
Paşca, S. P. (2019). Assembling human brain organoids. Science, 363(6423), 126–127. doi:10.1126/science.aau5729
Marton, R. M., & Pașca, S. P. (2020). Organoid and assembloid technologies for investigating cellular crosstalk in human brain development and disease. Trends in Cell Biology, 30(2), 133–143. doi:10.1016/j.tcb.2019.11.004
Ao, Z., Cai, H., Wu, Z., Ott, J., Wang, H., Mackie, K., & Guo, F. (2021). Controllable fusion of human brain organoids using acoustofluidics. Lab on a chip, 21(4), 688–699. doi:10.1039/d0lc01141j
Takahashi T. (2019). Organoids for drug discovery and personalized medicine. Annual Review of Pharmacology and Toxicology, 59, 447–462. doi:10.1146/annurev-pharmtox-010818-021108
Perkhofer, L., Frappart, P. O., Müller, M., & Kleger, A. (2018). Importance of organoids for personalized medicine. Personalized Medicine, 15(6), 461–465. doi:10.2217/pme-2018-0071
Nature. (2020). Henrietta Lacks: Science must right a historical wrong. Nature, 585(7823), 7. doi:10.1038/d41586-020-02494-z
Reardon, S. (2020). Can lab-grown brains become conscious? Nature, 586(7831), 658–661. doi:10.1038/d41586-020-02986-y
Trujillo, C. A., Gao, R., Negraes, P. D., Gu, J., Buchanan, J., Preissl, S., Wang, A., Wu, W., Haddad, G. G., Chaim, I. A., Domissy, A., Vandenberghe, M., Devor, A., Yeo, G. W., Voytek, B., & Muotri, A. R. (2019). Complex oscillatory waves emerging from cortical organoids model early human brain network development. Cell Stem Cell, 25(4), 558–569.e7. doi:10.1016/j.stem.2019.08.002