Development-on-chip: in vitro neural tube patterning with a microfluidic device

Design Principles for Pluripotent Stem Cell-Derived Organoid Engineering

Human morphogenesis is a complex process involving distinct microenvironmental and physical signals that are manipulated in space and time to give rise to complex tissues and organs. Advances in pluripotent stem cell (PSC) technology have promoted the in vitro recreation of processes involved in human morphogenesis. The development of organoids from human PSCs represents one reliable source for modeling a large spectrum of human disorders, as well as a promising approach for drug screening and toxicological tests. Based on the "self-organization" capacity of stem cells, different PSC-derived organoids have been created; however, considerable differences between in vitro-generated PSC-derived organoids and their in vivo counterparts have been reported. Advances in the bioengineering field have allowed the manipulation of different components, including cellular and noncellular factors, to better mimic the in vivo microenvironment. In this review, we focus on different examples of bioengineering approaches used to promote the self-organization of stem cells, including assembly, patterning, and morphogenesis in vitro, contributing to tissue-like structure formation.

Grow with the Flow: When Morphogenesis Meets Microfluidics

Developmental biology has advanced the understanding of the intricate and dynamic processes involved in the formation of an organism from a single cell. However, many gaps remain in the knowledge of embryonic development, especially regarding tissue morphogenesis. A possible approach to mimic such phenomena uses pluripotent stem cells in in vitro morphogenetic models. Herein, these systems are summarized with emphasis on the ability to better manipulate and control cellular interfaces with either liquid or solid materials using microengineered tools, which is critical for attaining deeper insights into pattern formation and stem cell differentiation during organogenesis. The role of conventional and customized cell-culture systems in supporting important advances in the field of morphogenesis is discussed, and the fascinating role that material sciences and microengineering currently play and are expected to play in the future is highlighted. In conclusion, it is proffered that continued microfluidics innovations when applied to morphogenesis promise to provide important insights to advance many multidisciplinary fields, including regenerative medicine.

Spatiotemporal control and modeling of morphogen delivery to induce gradient patterning of stem cell differentiation using fluidic channels

The process of cell differentiation in a developing embryo is influenced by numerous factors, including various biological molecules whose presentation varies dramatically over space and time. These morphogens regulate cell fate based on concentration profiles, thus creating discrete populations of cells and ultimately generating large, complex tissues and organs. Recently, several in vitro platforms have attempted to recapitulate the complex presentation of extrinsic signals found in nature. However, it has been a challenge to design versatile platforms that can dynamically control morphogen gradients over extended periods of time. To address some of these issues, we introduce a platform using channels patterned in hydrogels to deliver multiple morphogens to cells in a 3D scaffold, thus creating a spectrum of cell phenotypes based on the resultant morphogen gradients. The diffusion coefficient of a common small molecule morphogen, retinoic acid (RA), was measured within our hydrogel platform using Raman spectroscopy and its diffusion in our platform's geometry was modeled using finite element analysis. The predictive model of spatial gradients was validated in a cell-free hydrogel, and temporal control of morphogen gradients was then demonstrated using a reporter cell line that expresses green fluorescent protein in the presence of RA. Finally, the utility of this approach for regulating cell phenotype was demonstrated by generating opposing morphogen gradients to create a spectrum of mesenchymal stem cell differentiation states.

Brain Organoids-A Bottom-Up Approach for Studying Human Neurodevelopment

Brain organoids have recently emerged as a three-dimensional tissue culture platform to study the principles of neurodevelopment and morphogenesis. Importantly, brain organoids can be derived from human stem cells, and thus offer a model system for early human brain development and human specific disorders. However, there are still major differences between the in vitro systems and in vivo development. This is in part due to the challenge of engineering a suitable culture platform that will support proper development. In this review, we discuss the similarities and differences of human brain organoid systems in comparison to embryonic development. We then describe how organoids are used to model neurodevelopmental diseases. Finally, we describe challenges in organoid systems and how to approach these challenges using complementary bioengineering techniques.

The Neural Crest: A Remarkable Model System for Studying Development and Disease

Neural crest cells are the embryonic precursors of most neurons and all glia of the peripheral nervous system, pigment cells, some endocrine components, and connective tissue of the head, face, neck, and heart. Following induction, crest cells undergo an epithelial to mesenchymal transition that enables them to migrate along specific pathways culminating in their phenotypic differentiation. Researching this unique embryonic population has revealed important understandings of basic biological and developmental principles. These principles are likely to assist in clarifying the etiology and help in finding strategies for the treatment of neural crest diseases, collectively termed neurocristopathies. The progress achieved in neural crest research is made feasible thanks to the continuous development of species-specific in vivo and in vitro paradigms and more recently the possibility to produce neural crest cells and specific derivatives from embryonic or induced pluripotent stem cells. All of the above assist us in elucidating mechanisms that regulate neural crest development using state-of-the art cellular, molecular, and imaging approaches.

Microhexagon gradient array directs spatial diversification of spinal motor neurons

Motor neuron diversification and regionalization are important hallmarks of spinal cord development and rely on fine spatiotemporal release of molecular cues. Here, we present a dedicated platform to engineer complex molecular profiles for directed neuronal differentiation. Methods: The technology, termed microhexagon interlace for generation of versatile and fine gradients (microHIVE), leverages on an interlocking honeycomb lattice of microstructures to dynamically pattern molecular profiles at a high spatial resolution. By packing the microhexagons as a divergent, mirrored array, the platform not only enables maximal mixing efficiency but also maintains a small device footprint. Results: Employing the microHIVE platform, we developed optimized profiles of growth factors to induce rostral-caudal patterning of spinal motor neurons, and directed stem cell differentiation in situ into a spatial continuum of different motor neuron subtypes. Conclusions: The differentiated cells showed progressive RNA and protein signatures, consistent with that of representative brachial, thoracic and lumbar regions of the human spinal cord. The microHIVE platform can thus be utilized to develop advanced biomimetic systems for the study of diseases in vitro.

Coordination of Morphogenesis and Cell-Fate Specification in Development

During animal development, cell-fate-specific changes in gene expression can modify the material properties of a tissue and drive tissue morphogenesis. While mechanistic insights into the genetic control of tissue-shaping events are beginning to emerge, how tissue morphogenesis and mechanics can reciprocally impact cell-fate specification remains relatively unexplored. Here we review recent findings reporting how multicellular morphogenetic events and their underlying mechanical forces can feed back into gene regulatory pathways to specify cell fate. We further discuss emerging techniques that allow for the direct measurement and manipulation of mechanical signals in vivo, offering unprecedented access to study mechanotransduction during development. Examination of the mechanical control of cell fate during tissue morphogenesis will pave the way to an integrated understanding of the design principles that underlie robust tissue patterning in embryonic development.

In Vitro Microfluidic Models for Neurodegenerative Disorders

Microfluidic devices enable novel means of emulating neurodegenerative disease pathophysiology in vitro. These organ-on-a-chip systems can potentially reduce animal testing and substitute (or augment) simple 2D culture systems. Reconstituting critical features of neurodegenerative diseases in a biomimetic system using microfluidics can thereby accelerate drug discovery and improve our understanding of the mechanisms of several currently incurable diseases. This review describes latest advances in modeling neurodegenerative diseases in the central nervous system and the peripheral nervous system. First, this study summarizes fundamental advantages of microfluidic devices in the creation of compartmentalized cell culture microenvironments for the co-culture of neurons, glial cells, endothelial cells, and skeletal muscle cells and in their recapitulation of spatiotemporal chemical gradients and mechanical microenvironments. Then, this reviews neurodegenerative-disease-on-a-chip models focusing on Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. Finally, this study discusses about current drawbacks of these models and strategies that may overcome them. These organ-on-chip technologies can be useful to be the first line of testing line in drug development and toxicology studies, which can contribute significantly to minimize the phase of animal testing steps.

The case for applying tissue engineering methodologies to instruct human organoid morphogenesis

Three-dimensional organoids derived from human pluripotent stem cell (hPSC) derivatives have become widely used in vitro models for studying development and disease. Their ability to recapitulate facets of normal human development during in vitro morphogenesis produces tissue structures with unprecedented biomimicry. Current organoid derivation protocols primarily rely on spontaneous morphogenesis processes to occur within 3-D spherical cell aggregates with minimal to no exogenous control. This yields organoids containing microscale regions of biomimetic tissues, but at the macroscale (i.e. 100's of microns to millimeters), the organoids' morphology, cytoarchitecture, and cellular composition are non-biomimetic and variable. The current lack of control over in vitro organoid morphogenesis at the microscale induces aberrations at the macroscale, which impedes realization of the technology's potential to reproducibly form anatomically correct human tissue units that could serve as optimal human in vitro models and even transplants. Here, we review tissue engineering methodologies that could be used to develop powerful approaches for instructing multiscale, 3-D human organoid morphogenesis. Such technological mergers are critically needed to harness organoid morphogenesis as a tool for engineering functional human tissues with biomimetic anatomy and physiology.

STATEMENT OF SIGNIFICANCE

Human PSC-derived 3-D organoids are revolutionizing the biomedical sciences. They enable the study of development and disease within patient-specific genetic backgrounds and unprecedented biomimetic tissue microenvironments. However, their uncontrolled, spontaneous morphogenesis at the microscale yields inconsistences in macroscale organoid morphology, cytoarchitecture, and cellular composition that limits their standardization and application. Integration of tissue engineering methods with organoid derivation protocols could allow us to harness their potential by instructing standardized in vitro morphogenesis to generate organoids with biomimicry at all scales. Such advancements would enable the use of organoids as a basis for 'next-generation' tissue engineering of functional, anatomically mimetic human tissues and potentially novel organ transplants. Here, we discuss critical aspects of organoid morphogenesis where application of innovative tissue engineering methodologies would yield significant advancement towards this goal.