Introduction
The rapid development of stem cell technology has opened up unprecedented avenues for studying human neurodevelopment. One of such avenue is the study of brain organoids, or “mini-brains”. These are three-dimensional, stem-cell derived suspension cultures, capable of self-assembling into organized forms with features resembling the human brain.
While considerable progress has been made for in vitro models of organoid development for other systems—namely the intestine, pituitary and retina—three-dimensional culture modelling of the brain had for long remained out of reach, until a breakthrough study in 2013. In this study, led by postdoctoral student Madeline Lancaster, researchers developed innovative new methods to generate “cerebral” organoids, inspired by past work in the field with a focus on improving conditions for growth and higher-level development of cells. ‘Organoids’, in this sense, refer to stem-cell-derived, three-dimensional cultures that self organize to some extent and include multiple cell types and features of a particular organ. These developing tissues were placed in a rotational bioreactor. Within a few weeks, they yielded organoids containing anatomical brain structures resembling those of a 9-week-old human foetus. In the years since, developments in the field of stem cell research has allowed for other teams of researchers to give cerebral organoids increased degrees of structural complexity: from transplanting small organoids into mice to expose them to a greater supply of blood vessels, to making several organoids that mimic various parts of the brain and combining them for more complex cytoarchitecture. This provides immense potential for the study of human foetal brain development, neurodevelopmental disorders and degenerative diseases.
However, it remains unclear precisely what cell types arise in these brain ‘organoids’, how much individual organoids vary, and whether mature neuronal networks can form and function in organoids. Many limitations and hurdles lie in the way of growth for this novel field, and even further, ethical questions await on the question of sentience and autonomy.
Technical Advances and Methodology
To make an organoid in 2013, Lancaster’s team began with an embryoid body, floating aggregates of cells that resemble embryos. These could be obtained either from natural, embryonic stem cells (from the inner cell mass of a blastocyst) or from induced pluripotent cells, which were made from adult cells (typically skin cells) that would have been treated with four crucial biochemical factors which caused them to be reprogrammed to forego their original function and behave like embryonic cells. (See: Fig. 1) These embryoid bodies were differentiated into neural tissue and then transferred into three-dimensional gel matrix droplets. Once these ‘aggregates’ had reached a certain size, they were placed in a rotational bioreactor where they were spun to enhance to flow of nutrients into the medium without being shaped by the constraint of a vessel such as a Petri dish.
With minimal external interference, this approach produced cerebral organoids possessing human pluripotent stem cells with the most freedom in regards to self-organisation and construction, exhibiting a variety of cell lineage identities, ranging from the forebrain, midbrain and hindbrain, to the retina, choroid plexus and mesoderm.
This is known as the ‘unguided approach’ for the production of cerebral organoids. Although cell-type diversity offers a unique opportunity to model interactions between different regions of the brain, the high degree of variability and unpredictability present significant challenge for reproducibility and systemic studies.
On the other hand, in the ‘guided’ or ‘directed’ method for generating brain organoids, small molecules and growth factors are applied to developing organoids throughout the differentiation process to instruct human pluripotent stem cells to form cells and tissues resembling certain regions. These directed organoid cultures are sometimes capable of generating mixtures of cell types with relatively consistent proportions with less variation. However, they typically contain relatively small neuroepithelial structures and their architecture is often not well-defined. Nevertheless, the guided method remains the most common one for generating brain organoids today.
There is also the avenue of advanced techniques that allow for greater complexity. This includes used organoid technologies, in which pluripotent stem cells are differentiated into region-specific organoids separately and then fused together, forming an end result with multiple distinct regional identities in a controlled manner. An example would be fused dorsal and ventral forebrain organoids, together forming an ‘assembloid’. These structures reveal the manner in which migrating interneurons connect and form microunits.
The choice between guided and unguided methodologies will be dependent on the focus of the investigation. Where unguided organoids are suitable for exploring cell-type diversity during whole-brain development, brain region-specific organoids better mimic brain cytoarchitecture with less heterogeneity, and assembloids allow for the investigation of interactions between different brain regions.
With there being many routes to obtaining organoids that can then proceed to act as ‘models’, the logical next step in their development is their capability to, in fact, model the brain and study it, and what new avenues of treatment and application this can lead to.
Potential Application
As the organoids contain striking architectures strongly reminiscent of the developing human cerebral cortex (evolutionarily the most complex tissue), they display great potential for the effective modelling of neurodevelopmental brain disorders. As it would in the native brain, the cortical areas segregate into different layers, with radial glial cells dividing and giving birth to neurons in the innermost and subventricular zones, from which the quantity of neurons to develop the larger cerebral cortex is generated.
This process presents fascinating opportunities for the study and treatment of microcephaly in particular. Microcephaly is a developmental conditions in which the brain of young infants remains undersized, producing a small head and debilitation. Replicating the condition is not suitable for mice models, as they lack the developmental stages for an enlarged cerebral cortex possessed by primates such as humans. Naturally, this means the disease would be impossible to show in a mouse model, as they do not have the developmental stage in which microcephaly is expressed in the first place. In this instance, brain organoids provide the most ideal model for study.
Other studies involving brain organoids have been able to provide glimpses into the cellular and molecular mechanisms involved in brain development. For example, forebrain organoids derived from cells of individuals with ASD (autism spectrum disorders) display an imbalance of excitatory neuron and inhibitory neuron proportions. They have also developed great interest as potential neurodegenerative diseases models, even though attempts so far have had minimal success. This is mainly due to the fact that many neurodegenerative diseases, such as Alzheimer’s, are age-related and late onset, therefore brain organoids with mimic embryonic brain development may not possess the ideal characteristics to reproduce such development.
In addition to genetic disorders, brain organoids can also provide models for neurotrophic pathogens such as the Zika virus. When brain organoids are exposed to the Zika virus, it results in preferential infection of neural progenitor cells (which suppress proliferation and cause an increase in cell death) leading to what is ultimately drastically reduced organoid size. They then also display a series of other characteristics identified in congenital Zika syndrome, such as the thinning of the neuronal layer, disruption of apical surface junctions and the dilation of the ventricular lumens. This highlights direct evidence of the causal relationship between exposure to the Zika virus and the development of harmful neurological conditions. In this way and many others, brain organoids provide optimistic prospects for the study of various neurodevelopmental diseases—though not without some considerations.
Limitations
The fundamental limiting factor that prevents organoids from being able to fully replicate the late stages of human brain development is their size. Cortical organoids are much smaller in size compared with the full human cerebral cortex. Whereas cortical organoids can at most expand to approximately 4mm in diameter containing 2-3 million cells (about the size of a lentil), the human neocortex is about 15cm in diameter, with the thickness of gray matter alone being 2-4mm. This is a difference of about 50,000 in order. Furthermore, owing to a lack of circulation due to the limited metabolic supply, lack of a circulatory system and the physical distance over which oxygen and nutrients must diffuse, the viable thickness of organoids is restricted.
Notably, cortical folding (gyrification) remains an unachieved ‘holy grail’ for cortical organoids. Gyrification is an essential and unique stage in the development of the human cortical brain in which the cerebral cortex experiences rapid growth and expansion. Due to the stressed of spatial confinement, the cortical layer buckles into wave-like structures, with outward ridges known as gyri and inward furrows called sulci. This stage is unique to humans and some other primates, theorised to be essential to complex behaviours such as language and social communication. In contrast, the brains of small such as rodents exhibit little to no gyrification—and neither do cerebral organoids. This may be because they are unable to reach the stage at which gyrification occurs (the demarcation of ‘primary’ gyri and ‘secondary’ gyri does not occur in humans until the second and third trimester, which is a later stage than what most brain organoids can replicate). Attempts have been made to induce ‘crinkling’ or ‘pseudo-folding’ in early organoid differentiation, but this has not led to the formation of gyrus- and sulcus- like structures.
A better understanding of the mechanism under with gyrification occurs could lead to progress in existing methodologies to engineer the phenomenon in cerebral organoids, however, it is unlikely that the current organoid structure can fully replicate the folding of the human neocortex soon. Statistical analyses have suggested that the degree of folding across mammalian species is scaled with the surface area and thickness of the cortical plate, and organoids—at least in their current form—may simply be too small to achieve this result.
Due to these limitations, many ethical considerations concerning sentience and consciousness remain premature. The vast majority of scientists and ethicists are in agreement that consciousness has never been generated in a lab. Still, concerns over lab-grown brains have highlighted a blind spot: neuroscientists have no agreed upon definition or measurement of consciousness. Furthermore, certain experiments have still drawn scrutiny. In August 2019, a paper in Cell Stem Cell reported the creation of human brain organoids that produced co-ordinated waves of activity, resembling those seen in premature babies. While this was to a very small degree, it still prompted a wave of questions in relation to ethics, autonomy and ownership. Regardless, the waves only continued for a few months before the team shut the experiment down. Though moderate amounts of electrical activity is a sign of consciousness, the vast majority of brain organoids developed today are too far away in sophistication to be considered conscientious, autonomous beings.
Conclusion
Despite compelling data and innovative methodology, the formation of ‘a brain in a dish’ remains out of reach. Current models of brain organoids remain far from reproducing the complex, six-tiered architecture of their natural counterpart, even a foetal one. Presently, the organoids stop growing after a certain period of time and areas mimicking different brain regions are randomly distributed, often lacking the shape and spatial organisation seen in a sophisticated brain. Furthermore, there is also an absence of a necessary circulatory system means their interiors can often accumulate dead cells deprived of oxygen and nutrients.
Yet, even with significant limitations, the potential for cerebral organoids are great. For certain questions, the model provided by this innovation could provide interesting answers and mechanism with which to study early human brain development and the progression of neurodevelopmental disorders. The brain organoid field has made exciting leaps to empower researchers and scientists with new tools to address old questions, and while there is a long path before more faithful in vitro representation of a developing human brain is reached, it is important to consider that no model will likely ever be perfect.
Ishika Jha, Youth Medical Journal 2022
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