Organs to Cells and Cells to Organoids: The Evolution of in vitro Central Nervous System Modelling

Abstract

With 100 billion neurons and 100 trillion synapses, the human brain is not just the most complex organ in the human body, but has also been described as “the most complex thing in the universe.” The limited availability of human living brain tissue for the study of neurogenesis, neural processes and neurological disorders has resulted in more than a century-long strive from researchers worldwide to model the central nervous system (CNS) and dissect both its striking physiology and enigmatic pathophysiology. The invaluable knowledge gained with the use of animal models and post mortem human tissue remains limited to cross-species similarities and structural features, respectively. The advent of human induced pluripotent stem cell (hiPSC) and 3-D organoid technologies has revolutionised the approach to the study of human brain and CNS in vitro , presenting great potential for disease modelling and translational adoption in drug screening and regenerative medicine, also contributing beneficially to clinical research. We have surveyed more than 100 years of research in CNS modelling and provide in this review an historical excursus of its evolution, from early neural tissue explants and organotypic cultures, to 2-D patient-derived cell monolayers, to the latest development of 3-D cerebral organoids. We have generated a comprehensive summary of CNS modelling techniques and approaches, protocol refinements throughout the course of decades and developments in the study of specific neuropathologies. Current limitations and caveats such as clonal variation, developmental stage, validation of pluripotency and chromosomal stability, functional assessment, reproducibility, accuracy and scalability of these models are also discussed.

In vitro modeling of the neurovascular unit: advances in the field

“…An organoid is an in vitro organotypic preparation consisting of various cells grown together under appropriate conditions to generate a miniature artificial version of an organ of interest, including the brain [97]. BBB organoids consist of human primary brain endothelial cells, astrocytes, and pericytes [98] assembling under low-adherence conditions into a multicellular structure resembling the blood-brain barrier [99].…”

Recent Overview of the Use of iPSCs Huntington’s Disease Modeling and Therapy

“…The progress in tissue engineering technologies during the last years has eminently improved neurodegenerative disease modeling by the generation of 3D cellular complexes resembling human brain tissue termed brain organoids. Brain organoids derived from pluripotent stem cells can effectively model human cortical development and recapitulate its 3D cytoarchitecture; thus, opening a new window of opportunities to further investigate disease pathogenesis in vitro [40,41]. Moreover, the 3D brain organoids can reveal particular disease phenotypes, which are barely observed in 2D culture systems [42].…”

Centrosomes: The good and the bad for brain development

“…2013; Pacitti et al. 2019] or brain organoids infected with Zika virus [Qian et al. 2016] were already used to recapitulate the aetiology of microcephaly.…”

“…Some restrictions still exist regarding the analysis of different developmental stages, the reproducibility of brain organoid cultures, the clonality of these 3D‐cultures and the absence of blood vessels and of an immune system, which might change the penetrance of the phenotypes, to cite only some caveats [Pacitti et al. 2019]. Solving all these issues appears as the next challenge for future basic research.…”