A neural, or brain organoid, describes an artificially grown, in vitro, tissue resembling parts of the human brain. Neural organoids are created by culturing pluripotent stem cells into a three-dimensional culture that can be maintained for years. [1] [2] The brain is an extremely complex system of heterogeneous tissues and consists of a diverse array of neurons and glial cells. This complexity has made studying the brain and understanding how it works a difficult task in neuroscience, especially when it comes to neurodevelopmental and neurodegenerative diseases. The purpose of creating an in vitro neurological model is to study these diseases in a more defined setting. This 3D model is free of many potential in vivo limitations. The varying physiology between human and other mammalian models limits the scope of animal studies in neurological disorders. Neural organoids contain several types of nerve cells and have anatomical features that recapitulate regions of the nervous system. [3] Some neural organoids are most similar to neurons of the cortex. In some cases, the retina, spinal cord, thalamus and hippocampus. [1] Other neural organoids are unguided and contain a diversity of neural and non-neural cells. Stem cells have the potential to grow into many different types of tissues, and their fate is dependent on many factors. Below is an image showing some of the chemical factors that can lead stem cells to differentiate into various neural tissues; a more in-depth table of generating specific organoid identity has been published. [3] Similar techniques are used on stem cells used to grow cerebral organoids. [3]
Using human pluripotent stem cells to create in vitro neural organoids allows researchers to analyze current developmental mechanisms for human neural tissue as well as study the roots of human neurological diseases. Neural organoids are an investigative tool used to understand how disease pathology works. These organoids can be used in experiments that current in vitro methods are too simplistic for, while also being more applicable to humans than rodent or other mammalian models might be. Historically, major breakthroughs in how the brain works have resulted from studying injury or disorder in human brain function. An in vitro human brain model permits the next wave in our understanding of the human nervous system. [1]
An embryoid body cultivated from pluripotent stem cells is used to make an organoid. Embryoid bodies are composed of three layers: endoderm, mesoderm and ectoderm, which has the potential to be differentiated into different types of tissue.
A cerebral organoid can be formed by inducing ectoderm cells to differentiate into a cerebral organoids. [4] The general procedure can be broken down into 5 steps. [1] [5] First human pluripotent stem cells are cultured. They are then cultivated into an embryoid body. Next the cell culture is induced to form a neuroectoderm. The neuroectoderm is then grown in a matrigel droplet. The matrigel provides nutrients and the neuroectoderm starts to proliferate and grow. Replication of specific brain regions in cerebral organoid counterparts is achieved by the addition of extracellular signals to the organoid environment during different stages of development; these signals were found to create change in cell differentiation patterns, thus leading to recapitulation of the desired brain region. [3] SMAD inhibition may be used in usual cerebral organoid culturing processes to generate microglia in cerebral organoids. [6] The lack of vasculature limits the size the organoid can grow. This has been the major limitation in organoid development. The use of a spinning bioreactor may improve the availability of nutrients to cells inside the organoid to improve organoid development. [7] Spinning bioreactors have been used increasingly in cell culture and tissue growth applications. The reactor is able to deliver faster cell doubling times, increased cell expansion and increased extra-cellular matrix components when compared to statically cultured cells. [8]
It has been shown that cerebral organoids grown using the spinning bioreactor 3D culture method differentiate into various neural tissue types, such as the optic cup, hippocampus, ventral parts of the teleencephelon and dorsal cortex. [9] Furthermore, it was shown that human brain organoids could intrinsically develop integrated light-sensitive optic cups. [10]
The neural stem/progenitor cells are unique because they are able to self-renew and are multipotent. This means they can generate neurons and glial cells which are the two main components of neural systems. The fate of these cells is controlled by several factors that affect the differentiation process. The spatial location and temporal attributes of neural progenitor cells can influence if the cells form neurons or glial cells. Further differentiation is then controlled by extracellular conditions and cell signaling. [11] The exact conditions and stimuli necessary to differentiate neural progenitor cells into specific neural tissues such as hippocampal tissue, optic nerve, cerebral cortex, etc. are unknown. It is believed that cerebral organoids can be used to study the developmental mechanisms of these processes. [7]
To test if the neural progenitor cells and stem cells are differentiating into specific neural tissues, several gene markers can be tested. Two markers that are present during pluripotent stages are OCT4 and NANOG. These two markers are diminished during the course of development for the organoid. Neural identity markers that note successful neural induction, SOX1 and PAX6, are upregulated during organoid development. These changes in expression support the case for self-guided differentiation of cerebral organoids. [1] Markers for forebrain and hindbrain can also be tested. Forebrain markers FOXG1 and SIX3 are highly expressed throughout organoid development. However, hindbrain markers EGR2 and ISL1 show early presence but a decrease in the later stages. This imbalance towards forebrain development is similar to the developmental expansion of forebrain tissue in human brain development. [1] To test if organoids develop even further into regional specification, gene markers for cerebral cortex and occipital lobe have been tested. Many regions that have forebrain marker FOXG1, labeling them as regions with cerebral cortical morphology, were also positive for marker EMX1 which indicates dorsal cortical identity. These specific regions can be even further specified by markers AUTS2, TSHZ2, and LMO4 with the first representing cerebral cortex and the two after representing the occipital lobe. [1] Genetic markers for the hippocampus, ventral forebrain, and choroid plexus are also present in cerebral organoids, however, the overall structures of these regions have not yet been formed.
Cerebral organoids also possess functional cerebral cortical neurons. These neurons must form on the radially organized cortical plate. The marker TBR1 is present in the preplate, the precursor to the cortical plate, and is present, along with MAP2, a neuronal marker, in 30-day-old cerebral organoids. These markers are indicative of a basal neural layer similar to a preplate. These cells are also apically adjacent to a neutral zone and are reelin+ positive, which indicates the presence of Cajal-Retzius cells. The Cajal-Retzius cells are important to the generation of cortical plate architecture. [7] The cortical plate is usually generated inside-out such that later-born neurons migrate to the top superficial layers. This organization is also present in cerebral organoids based on genetic marker testing. Neurons that are early born have marker CTIP2 and are located adjacent to the TBR1 exhibiting preplate cells. Late-born neurons with markers SATB2 and BRN2 are located in a superficial layer, further away from the preplate than the early born neurons suggesting cortical plate layer formation. Additionally, after 75 days of formation, cerebral organoids show a rudimentary marginal zone, a cell-poor region. The formation of layered cortical plate is very basic in cerebral organoids and suggests the organoid lacks the cues and factors to induce formation of layer II-VI organization. [1] The cerebral organoid neurons can, however, form axons as shown by GFP staining. GFP labeled axons have been shown to have complex branching and growth cone formation. Additionally, calcium dye imaging has shown cerebral organoids to have Ca2+ oscillations and spontaneous calcium surges in individual cells. The calcium signaling can be enhanced through glutamate and inhibited through tetrodotoxin. [1]
In DishBrain, grown human brain cells were integrated into digital systems to play a simulated Pong via electrophysiological stimulation and recording. The cells "showed significantly improved performance in Pong" when embodied in a virtual game-world. [12] [13] [14] In the 2020s, significant changes in how these electrophysiological systems are made and interact with brain organoids could lead to better stimulation and recording data across the organoind in 3D. [15]
It is not fully understood how individual localized tissues formed by stem cells are able to coordinate with surrounding tissues to develop into a whole organ. [16] It has been shown however that most tissue differentiation requires interactions with surrounding tissues and depends on diffusible induction factors to either inhibit or encourage various differentiation and physical localization. [16] Cerebral organoid differentiation is somewhat localized. The previously mentioned markers for forebrain and hindbrain are physically localized, appearing in clusters. This suggests that local stimuli are released once one or more cells differentiate into a specific type as opposed to a random pathway throughout the tissue. The markers for subspecification of cortical lobes, prefrontal cortex and occipital lobe, are also physically localized. However, the hippocampus and ventral forebrain cells are not physically localized and are randomly located through the cerebral organoid. [1] Cerebral organoids lack blood vessels and are limited in size by nutrient uptake in the innermost cells. Spinning bioreactors and advanced 3D scaffolding techniques are able to increase organoid size, though the integration of in vitro nutrient delivery systems is likely to spark the next major leap in cerebral organoid development. [17]
Cerebral organoids have the potential to function as a model with which disease and gene expression might be studied. [18] However, diagnostic tools are needed to evaluate cerebral organoid tissue and create organoids modeling the disease or state of development in question. [19] Transcriptome analysis has been used as an assay to examine the pathology of cerebral organoids derived from individual patients. [20] Additionally, TUNEL assays have been used in studies as an evaluative marker of apoptosis in cerebral organoids. [21] Other assays used to analyze cerebral organoids include the following:
Cerebral organoids can be used to study gene expression via genetic modifications. [18] The degree to which these genetic modifications are present in the entire organoid depends on what stage of development the cerebral organoid is in when these genetic modifications are made; the earlier these modifications are made, such as when the cerebral organoid is in the single cell stage, the more likely these modifications will affect a greater portion of the cells in the cerebral organoid. [18] The degree to which these genetic modifications are present within the cerebral organoid also depends on the process by which these genetic modifications are made. If the genetic information is administered into one cerebral organoid cell's genome via machinery, then the genetic modification will remain present in cells resulting from replication. [18] Crispr/Cas 9 is a method by which this long-lasting genetic modification can be made. [18] A system involving use of transposons has also been suggested as a means to generate long-lasting genetic modifications; however, the extent to which transposons might interact with a cell genome might differs on a cell to cell basis, which would create variable expressivity between cerebral organoid cells. [18] If, however, the genetic modification is made via “genetic cargo” insertion (such as through Adeno-associated virus/ electroporation methods) then it has been found that the genetic modification becomes less present with each round of cell division in cerebral organoids. [18]
Use of computational methods have been called for as a means to help improve the cerebral organoid cultivation process; development of computational methods has also been called for in order to provide necessary detailed renderings of different components of the cerebral organoid (such as cell connectivity) that current methods are unable to provide. [19] Programming designed to model detailed cerebral organoid morphology does not yet exist. [19]
There are many potential applications for cerebral organoid use, such as cell fate potential, cell replacement therapy, and cell-type specific genome assays. [17] Cerebral organoids also provide a unique insight into the timing of development of neural tissues and can be used as a tool to study the differences across species. [17] Further potential applications for cerebral organoids include: [17]
Tissue morphogenesis with respect to cerebral organoids covers how neural organs form in vertebrates. Cerebral organoids can serve as in vitro tools to study the formation, modulate it, and further understand the mechanisms controlling it. [17]
Cerebral organoids can help to study cell migration. Neural glial cells cover a wide variety of neural cells, some of which move around the neurons. The factors that govern their movements, as well as neurons in general, can be studied using cerebral organoids. [4]
Clonal lineage tracing is part of fate mapping, where the lineage of differentiated tissues is traced to the pluripotent progenitors. The local stimuli released and the mechanism of differentiation can be studied using cerebral organoids as a model. [17] Genetic modifications in cerebral organoids could serve as a means to accomplish lineage tracing. [18]
Cerebral organoids can be used to grow specific brain regions and transplant them into regions of neurodegeneration as a therapeutic treatment. [22] [23] They can fuse with host vasculature and be immunologically silent. [24] In some cases, the genomes of these cerebral organoids would first have to be edited. [20] Recent studies have been able to achieve successful transplantation and integration of cerebral organoids into mouse brains; development of cell differentiation and vascularity was also observed after transplantation. [25] Cerebral organoids might serve as the basis for transplantation and rebuilding in the human brain due to the similarity in structure. [25]
Cerebral organoids can be used as simple models of complex brain tissues to study the effects of drugs and to screen them for initial safety and efficacy. Testing new drugs for neurological diseases could also result from this method of applying drug high-throughput screening methods to cerebral organoids. [20] After 2015, significant effort has gone into fabricating microscale devices to generate reproducible cerebral organoids at high-throughput. [15]
Organoids can be used for the study of brain development, for example identifying and investigating genetic switches that have a significant impact on it. [26] [27] [28] This can be used for the prevention and treatment of specific diseases [29] (see below) but also for other purposes such as insights into the genetic factors of recent brain evolution (or the origin of humans and evolved difference to other apes), [30] [31] [32] human enhancement and improving intelligence, identifying detrimental exposome impacts (and protection thereof), or improving brain health spans.
Organoids can be used to study the crucial early stages of brain development, test drugs and, because they can be made from living cells, study individual patients. [33] Additionally, the development of vascularized cerebral organoids could be used for investigating stroke therapy in the future. [34]
Zika virus has been shown to have teratogenic effects, causing defects in fetal neurological development. Cerebral organoids have been used in studies in order to understand the process by which Zika virus affects the fetal brain and, in some cases, causes microcephaly. [20] [21] Cerebral organoids infected with the Zika virus have been found to be smaller in size than their uninfected counterparts, which is reflective of fetal microcephaly. [20] [21] Increased apoptosis was also found in cerebral organoids infected with Zika virus. [35] Another study found that neural progenitor cell (NPC) populations were greatly reduced in these samples. The two methods by which NPC populations were reduced were increased cell death and reduced cell proliferation. TLR3 receptor upregulation was identified in these infected organoids. Inhibition of this TLR3 receptor was shown to partially halt some of the Zika induced effects. [36] Additionally, lumen size was found to be increased in organoids infected with Zika virus. [20] [21] The results found from studying cerebral organoids infected with Zika virus at different stages of maturation suggest that early exposure in developing fetuses can cause greater likelihood of Zika virus-associated neurological birth defects. [21]
Cocaine has also been shown to have teratogenic effects on fetal development. Cerebral organoids have been used to study which enzyme isoforms are necessary for fetal neurological defects caused by cocaine use during pregnancy. [20] One of these enzymes was determined to be cytochrome P450 isoform CYP3A5. [20]
In one case, a cerebral organoid grown from a patient with microcephaly demonstrated related symptoms and revealed that apparently, the cause is overly rapid development, followed by slower brain growth. Microencephaly is a developmental condition in which the brain remains undersized, producing an undersized head and debilitation. Microcephaly is not suitable for mouse models, which do not replicate the condition. [33] The primary form of the disease is thought to be caused by a homozygous mutation in the microcephalin gene. The disease is difficult to reproduce in mouse models because mice lack the developmental stages for an enlarged cerebral cortex that humans have. Naturally, a disease which affects this development would be impossible to show in a model which does not have it to begin with. [37] To use cerebral organoids to model a human's microcephaly, one group of researchers has taken patient skin fibroblasts and reprogrammed them using four well known reprogramming factors. These include OCT4, SOX2, MYC and KLF4. The reprogrammed sample was able to be cloned into induced pluripotent stem cells. The cells were cultured into a cerebral organoid following a process described in the cerebral organoid creation section below. The organoid that resulted had decreased numbers of neural progenitor cells and smaller tissues. Additionally, the patient-derived tissues displayed fewer and less frequent neuroepithelial tissues made of progenitors, decreased radial glial stem cells, and increased neurons. These results suggest that the underlying mechanism of microcephaly is caused by cells prematurely differentiating into neurons leaving a deficit of radial glial cells. [1]
Alzheimer's disease pathology has also been modeled with cerebral organoids. [38] Affected individual's pluripotent stem cells were used to generate brain organoids and then compared to control models, synthesised from healthy individuals. It was found that in the affected models, structures similar to that of plaques caused by amyloid beta proteins and neurofibrillary tangles, that cause the disease's symptoms were observed. [39] Previous attempts to model this so accurately have been unsuccessful, with drugs being developed on the basis of efficacy in pre-clinical murine models demonstrating no effect in human trials. [40]
Cerebral organoids can also be used to study autism spectrum disorders. [41] In one study, cerebral organoids were cultured from cells derived from macrocephaly ASD patients. [41] These cerebral organoids were found to reflect characteristics typical of the ASD-related macrocephaly phenotype found in the patients. [41] By cultivating cerebral organoids from ASD patients with macrocephaly, connections could be made between certain gene mutations and phenotypic expression. [41] Autism has also been studied through the comparison of healthy versus affected synthesised brain organoids. [42] Observation of the two models showed the overexpression of a transcription factor FOXG1 that produced a larger amount of GABAergic inhibitory neurons in the affected models. The significance of this use of brain organoids is that it has added great support for the excitatory/inhibitory imbalance hypothesis [43] which if proven true could help identify targets for drugs so that the condition could be treated.
The field of epigenetics and how DNA methylation might influence development of ASD has also been of interest in recent years. The traditional method of studying post-mortem neural samples from individuals with ASD poses many challenges, so cerebral organoids have been proposed as an alternate method of studying the potential effect that epigenetic mechanisms may have on the development of autism. This use of the cerebral organoid model to examine ASD and epigenetic patterns might provide insight in regards to epigenetic developmental timelines. However, it is important to note that the conditions in which cerebral organoids are cultured in might affect gene expression, and consequentially affect observations made using this model. Additionally, there is concern over the variability in cerebral organoids cultured from the same sample. [44] Further research into the extent and accuracy by which cerebral organoids recapitulate epigenetic patterns found in primary samples is also needed. [44]
Preterm hypoxic injury remain difficult to study because of limited availability of human fetal brain tissues and inadequate animal models to study human corticogenesis. Cerebral organoid can be used to model prenatal pathophysiology and to compare the susceptibility of the different neural cell types to hypoxia during corticogenesis. Intermediate progenitors seem to be particularly affected, due to the unfolded protein response pathway. [45] It has also been observed that hypoxia resulted in apoptosis in cerebral organoids, with outer radial glia and neuroblasts/immature neurons being particularly affected. [46]
Traditional means of studying glioblastomas come with limitations. One example of such limitations would be the limited sample availability. Because of these challenges that come with using a more traditional approach, cerebral organoids have been used as an alternative means to model the development of brain cancer. In one study, cerebral organoids were simulated to reflect tumor-like qualities using CRISPR CAS-9. Increased cell division was observed in these genetically altered models. Cerebral organoids were also used in mice models to study tumorigenesis and invasiveness. At the same time, the growth of brain cancers is influenced by environmental factors which are not yet replicable in cerebral organoid models. Cerebral organoids have been shown to provide insight into dysregulation of genes responsible for tumor development. [35]
Multiple sclerosis is an auto-immune inflammatory disorder affecting the central nervous system. Environmental and genetic factors contribute to the development of multiple sclerosis, however the etiology of this condition is unknown. Induced pluripotent stem cells from healthy human controls, as well as from patients with multiple sclerosis were grown into cerebral organoids creating an innovative human model of this disease. [47]
Cerebral organoids are preferred over their 3D cell culture counterparts because they can better reflect the structure of the human brain, and because, to a certain extent, they can reflect fetal neocortex development over an extended period of time. While cerebral organoids have a lot of potential, their culturing and development comes with limitations and areas for improvement. [35] For example, it takes several months to create one cerebral organoid, and the methods used to analyze them are also time-consuming. [25] Additionally, cerebral organoids do not have structures typical of a human brain, such as a blood brain barrier. [35] This limits the types of diseases that can be studied. Other limitations include:
Until recently, the central part of organoids have been found to be necrotic due to oxygen as well as nutrients being unable to reach that innermost area. [34] [19] This imposes limitations to cerebral organoids' physiological applicability. [19] Because of this lack of oxygen and nutrients, neural progenitor cells are limited in their growth. [48] However, recent findings suggest that, in the process of culturing a cerebral organoid, a necrotic center could be avoided by using fluidic devices to increase the organoid's exposure to media. [19]
The structure of cerebral organoids across different cultures has been found to be variable; a standardization procedure to ensure uniformity has yet to become common practice. [34] Future steps in revising cerebral organoid production would include creating methods to ensure standardization of cerebral organoid generation. [34] One such step proposed involves regulating the composition and thickness of the gel in which cerebral organoids are cultured in; this might contribute to greater reliability in cerebral organoid production. [19] Additionally, variability in generation of cerebral organoids is introduced due to differences in stem cells used. [20] These differences can arise from different manufacturing methods or host differences. [20] Increased metabolic stress has also been found within organoids. This metabolic stress has been found to restrict organoid specificity. [6] Future steps to streamline organoid culturing include analyzing more than one sample at a time. [25]
At the moment, the development of mature synapses in cerebral organoids is limited because of the media used. [34] Additionally, while some electrophysiological properties have been shown to develop in cerebral organoids, cultivation of separate and distinct organoid regions has been shown to limit the maturation of these electrophysiological properties. Modeling of electrophysiological neurodevelopmental processes typical of development later in the neurodevelopmental timeline, such as synaptogenesis, is not yet suggested in cerebral organoid models. [6] Since cerebral organoids are reflective of what happens during fetal neurodevelopment, there has been concern over how late onset diseases manifest in them. Future improvements include developing a way to recapitulate neurodegenerative diseases in cerebral organoids. [25]
Ethical concerns have been raised with using cerebral organoids as a model for disease due to the potential of them experiencing sensations such as pain or having the ability to develop a consciousness. [49] Currently it is unlikely given the simplicity of synthesised models compared to the complexity of a human brain; however, models have been shown to respond to light-based stimulation, [50] so present models do have some scope of responding to some stimuli.
Steps are being taken towards resolving the grey area such as a 2018 symposium at Oxford University where experts in the field, philosophers and lawyers met to try to clear up the ethical concerns with the new technology. [51] Similarly, projects such as Brainstorm from Case Western University aim to observe the progress of the field by monitoring labs working with brain organoids to try to begin the ‘building of a philosophical framework’ that future guidelines and legislation could be built upon. [52]
Additionally, the "humanization" of animal models has been raised as a topic of concern in transplantation of human stem cell derived organoids into other animal models. [48] For example, potential future concerns of this type were described when human brain tissue organoids were transplanted into baby rats, appearing to be highly functional, to mature and to integrate with the rat brain. Such models can be used to model human brain development and, as demonstrated, to investigate diseases (and their potential therapies) but could be controversial. [53] [54] [55]
Microcephaly is a medical condition involving a smaller-than-normal head. Microcephaly may be present at birth or it may develop in the first few years of life. Brain development is often affected; people with this disorder often have an intellectual disability, poor motor function, poor speech, abnormal facial features, seizures and dwarfism.
Abnormal spindle-like microcephaly-associated protein, also known as abnormal spindle protein homolog or Asp homolog, is a protein that in humans is encoded by the ASPM gene. ASPM is located on chromosome 1, band q31 (1q31). The ASPM gene contains 28 exons and codes for a 3477 amino-acid-long protein. The ASPM protein is conserved across species including human, mouse, Drosophila, and C. elegans. Defective forms of the ASPM gene are associated with autosomal recessive primary microcephaly.
An organoid is a miniaturised and simplified version of an organ produced in vitro in three dimensions that mimics the key functional, structural, and biological complexity of that organ. It is derived from one or a few cells from a tissue, embryonic stem cells, or induced pluripotent stem cells, which can self-organize in three-dimensional culture owing to their self-renewal and differentiation capacities. The technique for growing organoids has rapidly improved since the early 2010s, and The Scientist named it one of the biggest scientific advancements of 2013. Scientists and engineers use organoids to study development and disease in the laboratory, for drug discovery and development in industry, personalized diagnostics and medicine, gene and cell therapies, tissue engineering, and regenerative medicine.
Neural stem cells (NSCs) are self-renewing, multipotent cells that firstly generate the radial glial progenitor cells that generate the neurons and glia of the nervous system of all animals during embryonic development. Some neural progenitor stem cells persist in highly restricted regions in the adult vertebrate brain and continue to produce neurons throughout life. Differences in the size of the central nervous system are among the most important distinctions between the species and thus mutations in the genes that regulate the size of the neural stem cell compartment are among the most important drivers of vertebrate evolution.
Radial glial cells, or radial glial progenitor cells (RGPs), are bipolar-shaped progenitor cells that are responsible for producing all of the neurons in the cerebral cortex. RGPs also produce certain lineages of glia, including astrocytes and oligodendrocytes. Their cell bodies (somata) reside in the embryonic ventricular zone, which lies next to the developing ventricular system.
A neurosphere is a culture system composed of free-floating clusters of neural stem cells. Neurospheres provide a method to investigate neural precursor cells in vitro. Putative neural stem cells are suspended in a medium lacking adherent substrates but containing necessary growth factors, such as epidermal growth factor and fibroblast growth factor. This allows the neural stem cells to form into characteristic 3-D clusters. However, neurospheres are not identical to stem cells; rather, they only contain a small percentage of neural stem cells.
Neural tissue engineering is a specific sub-field of tissue engineering. Neural tissue engineering is primarily a search for strategies to eliminate inflammation and fibrosis upon implantation of foreign substances. Often foreign substances in the form of grafts and scaffolds are implanted to promote nerve regeneration and to repair damage caused to nerves of both the central nervous system (CNS) and peripheral nervous system (PNS) by an injury.
The development of the nervous system in humans, or neural development, or neurodevelopment involves the studies of embryology, developmental biology, and neuroscience. These describe the cellular and molecular mechanisms by which the complex nervous system forms in humans, develops during prenatal development, and continues to develop postnatally.
The evolution of the brain refers to the progressive development and complexity of neural structures over millions of years, resulting in the diverse range of brain sizes and functions observed across different species today, particularly in vertebrates.
Gyrification is the process of forming the characteristic folds of the cerebral cortex. The peak of such a fold is called a gyrus, and its trough is called a sulcus. The neurons of the cerebral cortex reside in a thin layer of gray matter, only 2–4 mm thick, at the surface of the brain. Much of the interior volume is occupied by white matter, which consists of long axonal projections to and from the cortical neurons residing near the surface. Gyrification allows a larger cortical surface area, and hence greater cognitive functionality to fit inside a smaller cranium.
The development of the cerebral cortex, known as corticogenesis is the process during which the cerebral cortex of the brain is formed as part of the development of the nervous system of mammals including its development in humans. The cortex is the outer layer of the brain and is composed of up to six layers. Neurons formed in the ventricular zone migrate to their final locations in one of the six layers of the cortex. The process occurs from embryonic day 10 to 17 in mice and between gestational weeks seven to 18 in humans.
A neuronal lineage marker is an endogenous tag that is expressed in different cells along neurogenesis and differentiated cells such as neurons. It allows detection and identification of cells by using different techniques. A neuronal lineage marker can be either DNA, mRNA or RNA expressed in a cell of interest. It can also be a protein tag, as a partial protein, a protein or an epitope that discriminates between different cell types or different states of a common cell. An ideal marker is specific to a given cell type in normal conditions and/or during injury. Cell markers are very valuable tools for examining the function of cells in normal conditions as well as during disease. The discovery of various proteins specific to certain cells led to the production of cell-type-specific antibodies that have been used to identify cells.
In vertebrates, the ventricular zone (VZ) is a transient embryonic layer of tissue containing neural stem cells, principally radial glial cells, of the central nervous system (CNS). The VZ is so named because it lines the ventricular system, which contains cerebrospinal fluid (CSF). The embryonic ventricular system contains growth factors and other nutrients needed for the proper function of neural stem cells. Neurogenesis, or the generation of neurons, occurs in the VZ during embryonic and fetal development as a function of the Notch pathway, and the newborn neurons must migrate substantial distances to their final destination in the developing brain or spinal cord where they will establish neural circuits. A secondary proliferative zone, the subventricular zone (SVZ), lies adjacent to the VZ. In the embryonic cerebral cortex, the SVZ contains intermediate neuronal progenitors that continue to divide into post-mitotic neurons. Through the process of neurogenesis, the parent neural stem cell pool is depleted and the VZ disappears. The balance between the rates of stem cell proliferation and neurogenesis changes during development, and species from mouse to human show large differences in the number of cell cycles, cell cycle length, and other parameters, which is thought to give rise to the large diversity in brain size and structure.
Neurogenesis is the process by which nervous system cells, the neurons, are produced by neural stem cells (NSCs). This occurs in all species of animals except the porifera (sponges) and placozoans. Types of NSCs include neuroepithelial cells (NECs), radial glial cells (RGCs), basal progenitors (BPs), intermediate neuronal precursors (INPs), subventricular zone astrocytes, and subgranular zone radial astrocytes, among others.
Sergiu P. Pașca is a Romanian-American scientist and physician at Stanford University in California. He is renowned for his groundbreaking work creating and developing stem cell-based models of the human brain to gain insights into neuropsychiatric disease. His lab was the first to develop and name assembloids: multi-unit self-organizing structures created in 3D cultures that allow for the study of human neural circuit and systems functions in vitro. Pașca’s lab generated and published human cortico-striatal and cortico-motor assembloids in 2020. Combining regionalized neural organoids pioneered in the lab and studies with human forebrain assembloids and transplantation, in 2024, Pașca developed a therapeutic for a severe genetic disorder called Timothy Syndrome, which was published on the cover of Nature.
Madeline Lancaster is an American developmental biologist studying neurological development and diseases of the brain. Lancaster is a group leader at the Medical Research Council (MRC) Laboratory of Molecular Biology in Cambridge, UK.
Jürgen Knoblich is a German molecular biologist. Since 2018, he is the interim Scientific Director of the Institute of Molecular Biotechnology (IMBA) of the Austrian Academy of Sciences in Vienna.
Experimental models of Alzheimer's disease are organism or cellular models used in research to investigate biological questions about Alzheimer's disease as well as develop and test novel therapeutic treatments. Alzheimer's disease is a progressive neurodegenerative disorder associated with aging, which occurs both sporadically or due to familial passed mutations in genes associated with Alzheimer's pathology. Common symptoms associated with Alzheimer's disease include: memory loss, confusion, and mood changes.
A myelinoid or myelin organoid is a three dimensional in vitro cultured model derived from human pluripotent stem cells (hPSCs) that represents various brain regions, the spinal cord or the peripheral nervous system in early fetal human development. Myelinoids have the capacity to recapitulate aspects of brain developmental processes, microenvironments, cell to cell interaction, structural organization and cellular composition. The differentiating aspect dictating whether an organoid is deemed a cerebral organoid/brain organoid or myelinoid is the presence of myelination and compact myelin formation that is a defining feature of myelinoids. Due to the complex nature of the human brain, there is a need for model systems which can closely mimic complicated biological processes. Myelinoids provide a unique in vitro model through which myelin pathology, neurodegenerative diseases, developmental processes and therapeutic screening can be accomplished.
An assembloid is an in vitro model that combines two or more organoids, spheroids, or cultured cell types to recapitulate structural and functional properties of an organ. They are typically derived from induced pluripotent stem cells. Assembloids have been used to study cell migration, neural circuit assembly, neuro-immune interactions, metastasis, and other complex tissue processes. The term "assembloid" was coined by Sergiu P. Pașca's lab in 2017.
Table 1: Protocols for brain organoid generation
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