Microfluidic engineered high cell density three-dimensional neural cultures

3-D multi-electrode arrays detect early spontaneous electrophysiological activity in 3-D neuronal-astrocytic co-cultures

Three-dimensional (3-D) neural cultures represent a promising platform for studying disease and drug screening. Tools and methodologies for measuring the electrophysiological function in these cultures are needed. Therefore, the purpose of this work was primarily to develop a methodology to interface engineered 3-D dissociated neural cultures with commercially available 3-D multi-electrode arrays (MEAs) reliably over 3 weeks to enable the recording of their electrophysiological activity. We further compared the functional output of these cultures to their structural and synaptic network development over time. We reliably interfaced a primary rodent neuron-astrocyte (2:1) 3-D co-culture (2500 cells/mm³ plating cell density) in Matrigel™ (7.5 mg/mL) that was up to 750 µm thick (30-40 cell-layers) with spiked 3-D MEAs while maintaining high viability. Using these MEAs we successfully recorded the spontaneous development of neural network-level electrophysiological activity and measured the development of putative synapses and neuronal maturation in these co-cultures using immunocytochemistry over 3 weeks in vitro. Planar (2-D) MEAs interfaced with these cultures served as recording controls. Neurons within this interfaced 3-D culture-MEA system exhibited considerable neurite outgrowth, networking, neuronal maturation, synaptogenesis, and culture-wide spontaneous firing of synchronized spikes and bursts of action potentials. Network-wide spikes and synchronized bursts increased rapidly (first detected at 2 days) during the first week in culture, plateaued during the second week, and reduced slightly in the third week, while maintaining high viability throughout the 3-week culturing period. Early electrophysiology activity occurred prior to neuronal process maturation and significant synaptic density increases in the second week. We successfully interfaced 3-D neural co-cultures with 3-D MEAs and recorded the electrophysiological activity of these cultures over 3 weeks. The initial period of rapid increase in electrophysiological activity, followed by a period of neuronal maturation and high-level of synapse formation in these cultures suggests a developmental homeostatic process. This methodology can enable future applications both in fundamental investigations of neural network behavior and in translational studies involving drug testing and neural interfacing.

Stretch growth of motor axons in custom mechanobioreactors to generate long-projecting axonal constructs

The central feature of peripheral motor axons is their remarkable lengths as they project from a motor neuron residing in the spinal cord to distant target muscle. However, current in vitro models have not replicated this feature owing to challenges in generating motor axon tracts beyond a few millimeters in length. To address this, we have developed a novel combination of microtissue engineering and mechanically assisted growth techniques to create long-projecting centimeter-scale motor axon tracts. Here, primary motor neurons were isolated from rat spinal cords and induced to form engineered microspheres via forced aggregation in custom microwells. This technique yielded healthy motor neurons projecting dense, fasciculated axonal tracts. Within our custom-built mechanobioreactors, motor neuron culture conditions, neuronal/axonal architecture, and mechanical growth conditions were optimized to generate parameters for robust and efficient stretch growth of motor axons. We found that axons projecting from motor neuron aggregates were able to tolerate displacement rates at least 10 times greater than those by axons projecting from dissociated motor neurons. The growth and structural characteristics of these stretch-grown motor axons were compared with that of benchmark stretch-grown sensory axons, revealing increased motor axon fasciculation. Finally, motor axons were integrated with myocytes and stretch grown to create novel long-projecting axonal-myocyte constructs that recreate characteristic dimensions of native nerve-muscle anatomy. This is the first demonstration of mechanical elongation of spinal motor axons and may have applications as anatomically inspired in vitro testbeds or as tissue-engineered living scaffolds for targeted axon tract reconstruction following nervous system injury or disease.

Neural tissue engineering with structured hydrogels in CNS models and therapies

The development of techniques to create and use multiphase microstructured hydrogels (granular hydrogels or microgels) has enabled the generation of cultures with more biologically relevant architecture and use of structured hydrogels is especially pertinent to the development of new types of central nervous system (CNS) culture models and therapies. We review material choice and the customisation of hydrogel structure, as well as the use of hydrogels in developmental models. Combining the use of structured hydrogel techniques with developmentally relevant tissue culture approaches will enable the generation of more relevant models and treatments to repair damaged CNS tissue architecture.

Engineered Axonal Tracts as "Living Electrodes" for Synaptic-Based Modulation of Neural Circuitry

Brain-computer interface and neuromodulation strategies relying on penetrating non-organic electrodes/optrodes are limited by an inflammatory foreign body response that ultimately diminishes performance. A novel "biohybrid" strategy is advanced, whereby living neurons, biomaterials, and microelectrode/optical technology are used together to provide a biologically-based vehicle to probe and modulate nervous-system activity. Microtissue engineering techniques are employed to create axon-based "living electrodes", which are columnar microstructures comprised of neuronal population(s) projecting long axonal tracts within the lumen of a hydrogel designed to chaperone delivery into the brain. Upon microinjection, the axonal segment penetrates to prescribed depth for synaptic integration with local host neurons, with the perikaryal segment remaining externalized below conforming electrical-optical arrays. In this paradigm, only the biological component ultimately remains in the brain, potentially attenuating a chronic foreign-body response. Axon-based living electrodes are constructed using multiple neuronal subtypes, each with differential capacity to stimulate, inhibit, and/or modulate neural circuitry based on specificity uniquely afforded by synaptic integration, yet ultimately computer controlled by optical/electrical components on the brain surface. Current efforts are assessing the efficacy of this biohybrid interface for targeted, synaptic-based neuromodulation, and the specificity, spatial density and long-term fidelity versus conventional microelectronic or optical substrates alone.

A Microfluidic Platform for the Characterisation of CNS Active Compounds

New in vitro technologies that assess neuronal excitability and the derived synaptic activity within a controlled microenvironment would be beneficial for the characterisation of compounds proposed to affect central nervous system (CNS) function. Here, a microfluidic system with computer controlled compound perfusion is presented that offers a novel methodology for the pharmacological profiling of CNS acting compounds based on calcium imaging readouts. Using this system, multiple applications of the excitatory amino acid glutamate (10 nM–1 mM) elicited reproducible and reversible transient increases in intracellular calcium, allowing the generation of a concentration response curve. In addition, the system allows pharmacological investigations to be performed as evidenced by application of glutamatergic receptor antagonists, reversibly inhibiting glutamate-induced increases in intracellular calcium. Importantly, repeated glutamate applications elicited significant increases in the synaptically driven activation of the adjacent, environmentally isolated neuronal network. Therefore, the proposed new methodology will enable neuropharmacological analysis of CNS active compounds whilst simultaneously determining their effect on synaptic connectivity.

Toward modeling the human nervous system in a dish: recent progress and outstanding challenges

Studying the cellular and molecular bases governing development, and normal and abnormal functions of the human CNS is hampered by its complexity and the very limited possibility of experimentally manipulating it in vivo. Development of 3D, tissue-like culture systems offers much promise for boosting our understanding of human neural development, birth defects, neurodegenerative diseases and neural injury, and for providing platforms that will more accurately predict efficacy of putative therapeutic compounds and assess responses to potentially neurotoxic agents. Although novel technological developments and a more interdisciplinary approach to modeling the human CNS are accelerating the pace of discovery, increasing the complexity of in vitro systems increases the ordeals to be overcome to establish highly reproducible models amenable to quantitative analysis.

Mechanical elongation of astrocyte processes to create living scaffolds for nervous system regeneration

Following brain injury or neurodegenerative disease, successful regeneration requires orchestrated migration of neurons and reformation of long-distance communication fibres, or axons. Such extensive regeneration does not occur in the mature brain; however, during embryonic development, pathways formed by glial cells extend several millimeters (mm) to create 'living scaffolds' for targeted neural cell migration and axonal pathfinding. Techniques to recapitulate long process outgrowth in glial cells have proven elusive, preventing the exploitation of this developmental mechanism for regeneration. In the current study, astrocytes were induced to form a network of interconnected processes that were subjected to controlled mechanical tension in vitro using custom-built mechanobioreactors. We discovered a specific micron (μm)-scale mechanical growth regime that induced elongation of the astrocytic processes to a remarkable length of 2.5 mm at an optimal rate of 12.5 μm/h. More rapid mechanical regimes (> 20 μm/h) caused greater incidence of process degeneration or outright breakage, whereas slow regimes (< 4 μm/h) led to adaptive motility, thus failing to achieve process elongation. Cellular phenotype for this astrocytic 'stretch-growth' was confirmed based on presentation of the intermediate filament glial fibrillary acidic protein (GFAP). Mechanical elongation resulted in the formation of dense bundles of aligned astrocytic processes. Importantly, seeded neurons readily adhered to, and extended neurites directly along, the elongated astrocytic processes, demonstrating permissiveness to support neuronal growth. This is the first demonstration of the controlled application of mechanical forces to create long astrocytic processes, which may form the backbone of tissue-engineered 'living scaffolds' that structurally emulate radial glia to facilitate neuroregeneration. Copyright © 2016 John Wiley & Sons, Ltd.

A Device for Long-Term Perfusion, Imaging, and Electrical Interfacing of Brain Tissue In vitro

Distributed microelectrode array (MEA) recordings from consistent, viable, ≥500 μm thick tissue preparations over time periods from days to weeks may aid in studying a wide range of problems in neurobiology that require in vivo-like organotypic morphology. Existing tools for electrically interfacing with organotypic slices do not address necrosis that inevitably occurs within thick slices with limited diffusion of nutrients and gas, and limited removal of waste. We developed an integrated device that enables long-term maintenance of thick, functionally active, brain tissue models using interstitial perfusion and distributed recordings from thick sections of explanted tissue on a perforated multi-electrode array. This novel device allows for automated culturing, in situ imaging, and extracellular multi-electrode interfacing with brain slices, 3-D cell cultures, and potentially other tissue culture models. The device is economical, easy to assemble, and integrable with standard electrophysiology tools. We found that convective perfusion through the culture thickness provided a functional benefit to the preparations as firing rates were generally higher in perfused cultures compared to their respective unperfused controls. This work is a step toward the development of integrated tools for days-long experiments with more consistent, healthier, thicker, and functionally more active tissue cultures with built-in distributed electrophysiological recording and stimulation functionality. The results may be useful for the study of normal processes, pathological conditions, and drug screening strategies currently hindered by the limitations of acute (a few hours long) brain slice preparations.

A mathematical model explains saturating axon guidance responses to molecular gradients

Correct wiring is crucial for the proper functioning of the nervous system. Molecular gradients provide critical signals to guide growth cones, which are the motile tips of developing axons, to their targets. However, in vitro, growth cones trace highly stochastic trajectories, and exactly how molecular gradients bias their movement is unclear. Here, we introduce a mathematical model based on persistence, bias, and noise to describe this behaviour, constrained directly by measurements of the detailed statistics of growth cone movements in both attractive and repulsive gradients in a microfluidic device. This model provides a mathematical explanation for why average axon turning angles in gradients in vitro saturate very rapidly with time at relatively small values. This work introduces the most accurate predictive model of growth cone trajectories to date, and deepens our understanding of axon guidance events both in vitro and in vivo.

Advanced biomaterial strategies to transplant preformed micro-tissue engineered neural networks into the brain

OBJECTIVE

Connectome disruption is a hallmark of many neurological diseases and trauma with no current strategies to restore lost long-distance axonal pathways in the brain. We are creating transplantable micro-tissue engineered neural networks (micro-TENNs), which are preformed constructs consisting of embedded neurons and long axonal tracts to integrate with the nervous system to physically reconstitute lost axonal pathways.

APPROACH

We advanced micro-tissue engineering techniques to generate micro-TENNs consisting of discrete populations of mature primary cerebral cortical neurons spanned by long axonal fascicles encased in miniature hydrogel micro-columns. Further, we improved the biomaterial encasement scheme by adding a thin layer of low viscosity carboxymethylcellulose (CMC) to enable needle-less insertion and rapid softening for mechanical similarity with brain tissue.

MAIN RESULTS

The engineered architecture of cortical micro-TENNs facilitated robust neuronal viability and axonal cytoarchitecture to at least 22 days in vitro. Micro-TENNs displayed discrete neuronal populations spanned by long axonal fasciculation throughout the core, thus mimicking the general systems-level anatomy of gray matter-white matter in the brain. Additionally, micro-columns with thin CMC-coating upon mild dehydration were able to withstand a force of 893 ± 457 mN before buckling, whereas a solid agarose cylinder of similar dimensions was predicted to withstand less than 150 μN of force. This thin CMC coating increased the stiffness by three orders of magnitude, enabling needle-less insertion into brain while significantly reducing the footprint of previous needle-based delivery methods to minimize insertion trauma.

SIGNIFICANCE

Our novel micro-TENNs are the first strategy designed for minimally invasive implantation to facilitate nervous system repair by simultaneously providing neuronal replacement and physical reconstruction of long-distance axon pathways in the brain. The micro-TENN approach may offer the ability to treat several disorders that disrupt the connectome, including Parkinson's disease, traumatic brain injury, stroke, and brain tumor excision.

Microfluidic Chip for Site-Specific Neuropharmacological Treatment and Activity Probing of 3D Neuronal "Optonet" Cultures

The study introduces a "brain-on-a-chip" microfluidic platform that hosts brain-like 3D cultures ("optonets") whose activity and responses to flowing drugs are recorded optically. Optonets are viable, optically accessible 3D neural networks whose characteristics approximate cortical networks. The results demonstrate the ability to monitor complex 3D activity patterns during extended site-specific, reversible neuropharmacogical exposure, suggesting an interesting potential in drug screening.

Restoring nervous system structure and function using tissue engineered living scaffolds

Neural tissue engineering is premised on the integration of engineered living tissue with the host nervous system to directly restore lost function or to augment regenerative capacity following nervous system injury or neurodegenerative disease. Disconnection of axon pathways – the long-distance fibers connecting specialized regions of the central nervous system or relaying peripheral signals – is a common feature of many neurological disorders and injury. However, functional axonal regeneration rarely occurs due to extreme distances to targets, absence of directed guidance, and the presence of inhibitory factors in the central nervous system, resulting in devastating effects on cognitive and sensorimotor function. To address this need, we are pursuing multiple strategies using tissue engineered “living scaffolds”, which are preformed three-dimensional constructs consisting of living neural cells in a defined, often anisotropic architecture. Living scaffolds are designed to restore function by serving as a living labeled pathway for targeted axonal regeneration – mimicking key developmental mechanisms– or by restoring lost neural circuitry via direct replacement of neurons and axonal tracts. We are currently utilizing preformed living scaffolds consisting of neuronal clusters spanned by long axonal tracts as regenerative bridges to facilitate long-distance axonal regeneration and for targeted neurosurgical reconstruction of local circuits in the brain. Although there are formidable challenges in preclinical and clinical advancement, these living tissue engineered constructs represent a promising strategy to facilitate nervous system repair and functional recovery.

From 2D to 3D: novel nanostructured scaffolds to investigate signalling in reconstructed neuronal networks

To recreate in vitro 3D neuronal circuits will ultimately increase the relevance of results from cultured to whole-brain networks and will promote enabling technologies for neuro-engineering applications. Here we fabricate novel elastomeric scaffolds able to instruct 3D growth of living primary neurons. Such systems allow investigating the emerging activity, in terms of calcium signals, of small clusters of neurons as a function of the interplay between the 2D or 3D architectures and network dynamics. We report the ability of 3D geometry to improve functional organization and synchronization in small neuronal assemblies. We propose a mathematical modelling of network dynamics that supports such a result. Entrapping carbon nanotubes in the scaffolds remarkably boosted synaptic activity, thus allowing for the first time to exploit nanomaterial/cell interfacing in 3D growth support. Our 3D system represents a simple and reliable construct, able to improve the complexity of current tissue culture models.

Maintenance and neuronal cell differentiation of neural stem cells C17.2 correlated to medium availability sets design criteria in microfluidic systems

BACKGROUND

Neural stem cells (NSCs) play an important role in developing potential cell-based therapeutics for neurodegenerative disease. Microfluidics has proven a powerful tool in mechanistic studies of NSC differentiation. However, NSCs are prone to differentiate when the nutrients are limited, which occurs unfavorable by fast medium consumption in miniaturized culture environment. For mechanistic studies of NSCs in microfluidics, it is vital that neuronal cell differentiation is triggered by controlled factors only. Thus, we studied the correlation between available cell medium and spontaneous neuronal cell differentiation of C17.2 NSCs in standard culture medium, and proposed the necessary microfluidic design criteria to prevent undesirable cell phenotype changes.

METHODOLOGY/PRINCIPAL FINDINGS

A series of microchannels with specific geometric parameters were designed to provide different amount of medium to the cells over time. A medium factor (MF, defined as the volume of stem cell culture medium divided by total number of cells at seeding and number of hours between medium replacement) successfully correlated the amount of medium available to each cell averaged over time to neuronal cell differentiation. MF smaller than 8.3×10(4) µm3/cell⋅hour produced significant neuronal cell differentiation marked by cell morphological change and significantly more cells with positive β-tubulin-III and MAP2 staining than the control. When MF was equal or greater than 8.3×10(4) µm3/cell⋅hour, minimal spontaneous neuronal cell differentiation happened relative to the control. MF had minimal relation with the average neurite length.

SIGNIFICANCE

MFs can be controlled easily to maintain the stem cell status of C17.2 NSCs or to induce spontaneous neuronal cell differentiation in standard stem cell culture medium. This finding is useful in designing microfluidic culture platforms for controllable NSC maintenance and differentiation. This study also offers insight about consumption rate of serum molecules involved in maintaining the stemness of NSCs.

A modular approach to create a neurovascular unit-on-a-chip

In this work, we describe the fabrication and working of a modular microsystem that recapitulates the functions of the "Neurovascular Unit". The microdevice comprised a vertical stack of a poly(dimethylsiloxane) (PDMS) neural parenchymal chamber separated by a vascular channel via a microporous polycarbonate (PC) membrane. The neural chamber housed a mixture of neurons (~4%), astrocytes (~95%), and microglia (~1%). The vascular channel was lined with a layer of rat brain microvascular endothelial cell line (RBE4). Cellular components in the neural chamber and vascular channel showed viability (>90%). The neural cells fired inhibitory as well as excitatory potentials following 10 days of culture. The endothelial cells showed diluted-acetylated low density lipoprotein (dil-a-LDL) uptake, expressed von Willebrand factor (vWF) and zonula occludens (ZO-1) tight junctions, and showed decreased Alexafluor™-conjugated dextran leakage across their barriers significantly compared with controls (p < 0.05). When the vascular layer was stimulated with TNF-α for 6 h, about 75% of resident microglia and astrocytes on the neural side were activated significantly (p < 0.05 compared to controls) recapitulating tissue-mimetic responses resembling neuroinflammation. The impact of this microsystem lies in the fact that this biomimetic neurovascular platform might not only be harnessed for obtaining mechanistic insights for neurodegenerative disorders, but could also serve as a potential screening tool for central nervous system (CNS) therapeutics in toxicology and neuroinfectious diseases.

Brain slice on a chip: opportunities and challenges of applying microfluidic technology to intact tissues

Isolated brain tissue, especially brain slices, are valuable experimental tools for studying neuronal function at the network, cellular, synaptic, and single channel levels. Neuroscientists have refined the methods for preserving brain slice viability and function and converged on principles that strongly resemble the approach taken by engineers in developing microfluidic devices. With respect to brain slices, microfluidic technology may 1) overcome the traditional limitations of conventional interface and submerged slice chambers and improve oxygen/nutrient penetration into slices, 2) provide better spatiotemporal control over solution flow/drug delivery to specific slice regions, and 3) permit successful integration with modern optical and electrophysiological techniques. In this review, we highlight the unique advantages of microfluidic devices for in vitro brain slice research, describe recent advances in the integration of microfluidic devices with optical and electrophysiological instrumentation, and discuss clinical applications of microfluidic technology as applied to brain slices and other non-neuronal tissues. We hope that this review will serve as an interdisciplinary guide for both neuroscientists studying brain tissue in vitro and engineers as they further develop microfluidic chamber technology for neuroscience research.

In vitro models of traumatic brain injury

In vitro models of traumatic brain injury (TBI) are helping elucidate the pathobiological mechanisms responsible for dysfunction and delayed cell death after mechanical stimulation of the brain. Researchers have identified compounds that have the potential to break the chain of molecular events set in motion by traumatic injury. Ultimately, the utility of in vitro models in identifying novel therapeutics will be determined by how closely the in vitro cascades recapitulate the sequence of cellular events that play out in vivo after TBI. Herein, the major in vitro models are reviewed, and a discussion of the physical injury mechanisms and culture preparations is employed. A comparison between the efficacy of compounds tested in vitro and in vivo is presented as a critical evaluation of the fidelity of in vitro models to the complex pathobiology that is TBI. We conclude that in vitro models were greater than 88% predictive of in vivo results.

Ambient nanoelectrospray ionization with in-line microdialysis for spatially resolved transient biochemical monitoring within cell culture environments

We have developed a new mass spectrometry (MS) based approach for continuous, spatially resolved in vitro biochemical detection and demonstrated its utility in a 3-D cell culture system. Extracellular liquid is passively extracted at a low flow rate (~10 nL/s) through a small bore silica capillary (ID 50 μm); inline microdialysis (MD) removes ions that would interfere with mass spectrometric analysis, and the sample is ionized by nanoelectrospray ionization (nano-ESI) and mass analyzed in a time-of-flight mass spectrometer. The system successfully detects low-volume, low-concentration releases of a small protein (8 μL of 5 μM cytochrome-c, molecular mass ~12 kDa) and exhibits ~1 min temporal resolution. The system also displays sensitivity to probe proximity to the sample release point. Due to the sensitivity of ESI-MS and its ability to simultaneously detect and identify multiple unanticipated biochemicals, this approach shows considerable potential as a biomarker discovery tool.

Organs-on-a-chip: a focus on compartmentalized microdevices

Advances in microengineering technologies have enabled a variety of insights into biomedical sciences that would not have been possible with conventional techniques. Engineering microenvironments that simulate in vivo organ systems may provide critical insight into the cellular basis for pathophysiologies, development, and homeostasis in various organs, while curtailing the high experimental costs and complexities associated with in vivo studies. In this article, we aim to survey recent attempts to extend tissue-engineered platforms toward simulating organ structure and function, and discuss the various approaches and technologies utilized in these systems. We specifically focus on microtechnologies that exploit phenomena associated with compartmentalization to create model culture systems that better represent the in vivo organ microenvironment.

Development and characterization of a microfluidic chamber incorporating fluid ports with active suction for localized chemical stimulation of brain slices

We report a novel microfluidic chamber incorporating fluid ports with active suction to achieve localized chemical stimulation of brain slices. A two-level soft-lithography process is used to fabricate fluid ports with integrated injection and suction holes that are connected to underlying microchannels. Fluorescence imaging, particle tracking velocimetry, and cell staining are used to characterize flows around a fluid port with or without active suction to validate effective localization of injected chemicals. To demonstrate biological applicability of the chamber, we show an induction of cortical spreading depression (CSD) waves in mouse brain slices through controlled focal delivery of potassium chloride solution.

Fabrication and optimization of alginate hydrogel constructs for use in 3D neural cell culture

Two-dimensional (2D) culture systems provide useful information about many biological processes. However, some applications including tissue engineering, drug transport studies, and analysis of cell growth and dynamics are better studied using three-dimensional (3D) culture systems. 3D culture systems can potentially offer higher degrees of organization and control of cell growth environments, more physiologically relevant diffusion characteristics, and permit the formation of more extensive 3D networks of cell-cell interactions. A 3D culture system has been developed using alginate as a cell scaffold, capable of maintaining the viability and function of a variety of neural cell types. Alginate was functionalized by the covalent attachment of a variety of whole proteins and peptide epitopes selected to provide sites for cell attachment. Alginate constructs were used to entrap a variety of neural cell types including astroglioma cells, astrocytes, microglia and neurons. Neural cells displayed process outgrowth over time in culture. Cell-seeded scaffolds were characterized in terms of their biochemical and biomechanical properties, effects on seeded neural cells, and suitability for use as 3D neural cell culture models.

Synapse-to-neuron ratio is inversely related to neuronal density in mature neuronal cultures

Synapse formation is a fundamental process in neurons that occurs throughout development, maturity, and aging. Although these stages contain disparate and fluctuating numbers of mature neurons, tactics employed by neuronal networks to modulate synapse number as a function of neuronal density are not well understood. The goal of this study was to utilize an in vitro model to assess the influence of cell density and neuronal maturity on synapse number and distribution. Specifically, cerebral cortical neurons were plated in planar culture at densities ranging from 10 to 5000 neurons/mm², and synapse number and distribution were evaluated via immunocytochemistry over 21 days in vitro (DIV). High-resolution confocal microscopy revealed an elaborate three-dimensional distribution of neurites and synapses across the heights of high-density neuronal networks by 21 DIV, which were up to 18 μm thick, demonstrating the complex degree of spatial interactions even in planar high-density cultures. At 7 DIV, the mean number of synapses per neuron was less than 5, and this did not vary as a function of neuronal density. However, by 21 DIV, the number of synapses per neuron had jumped 30- to 80-fold, and the synapse-to-neuron ratio was greatest at lower neuronal densities (< 500 neurons/mm²; mean approximately 400 synapses/neuron) compared to mid and higher neuronal densities (500-4500 neurons/mm²; mean of approximately 150 synapses/neuron) (p<0.05). These results suggest a relationship between neuronal density and synapse number that may have implications in the neurobiology of developing neuronal networks as well as processes of cell death and regeneration.

Designing Neural Networks in Culture: Experiments are described for controlled growth, of nerve cells taken from rats, in predesigned geometrical patterns on laboratory culture dishes

Technology has advanced to where it is possible to design and grow-with predefined geometry and surprisingly good fidelity-living networks of neurons in culture dishes. Here we overview the elements of design, emphasizing the lithographic techniques that alter the cell culture surface which in turn influences the attachment and growth of the neural networks. Advanced capability in this area makes it possible to design networks of desired complexity. Other issues addressed include the influence of glial cells and media on activity and the potential for extending the designs into three dimensions. Investigators are advancing the art and science of analyzing and controlling through stimulation the function of the neural networks, including the ability to take advantage of their geometric form in order to influence functional properties.

A microperfused incubator for tissue mimetic 3D cultures

High density, three-dimensional (3D) cultures present physical similarities to in vivo tissue and are invaluable tools for pre-clinical therapeutic discoveries and development of tissue engineered constructs. Unfortunately, the use of dense cultures is hindered by intra-culture transport limits allowing just a few layer thick cultures for reproducible studies. In order to overcome diffusion limits in intra-culture nutrient and gas availability, a simple scalable microfluidic perfusion platform was developed and validated. A novel perfusion approach maintained laminar flow of nutrients through the culture to meet metabolic need, while removing depleted medium and catabolites. Velocity distributions and 3D flow patterns were measured using microscopic particle image velocimetry. The effectiveness of forced convection laminar perfusion was confirmed by culturing 700 microm thick neural-astrocytic (1:1) constructs at cell density approaching that of the brain (50,000 cells/mm(3)). At the optimized flow rate of the nutrient medium, the culture viability reached 90% through the full construct thickness at 2 days of perfusion while unperfused controls exhibited widespread cell death. The membrane aerated perfusion platform was integrated within a miniature, imaging accessible enclosure enabling temperature and gas control of the culture environment. Temperature measurements demonstrated fast feedback response to environmental changes resulting in the maintenance of the physiological temperature within 37 +/- 0.2 degrees C. Reproducible culturing of tissue equivalents within dynamically controlled environments will provide higher fidelity to in vivo function in an in vitro accessible format for cell-based assays and regenerative medicine.

Bone regeneration and neovascularization processes in a pellet culture system for periosteal cells

Reliable bone regeneration can be achieved with a pellet culture system using bovine periosteal cells. However, bone regeneration and neovascularization processes in this system have remained unclear. The present study aimed to clarify the extracellular environment and neovascularization process. To detect components of the extracellular matrix secreted by cells and to identify the conditions necessary for bone regeneration in the body, Western blotting and in vivo tests in nude mice were performed. Cells were cultured with or without ascorbic acid and culture supernatant was precipitated. Western blotting showed that culture supernatant contained collagen type I, procollagen type I, and procollagen type I C-terminus when cells were cultured with ascorbic acid. Cells cultured with ascorbic acid formed partial bony tissues at 2 weeks after grafting to nude mice, while bone formation was missing without ascorbic acid. Immunostaining was performed using species-specific vascular endothelial cell markers to ascertain whether vascular endothelial cells were bovine or murine (nude mouse). Immunohistological methods showed vascular endothelial cells in osseous tissue formed in the subcutaneous tissue of nude mice were murine. Extracellular matrix synthesis in vitro and host blood flow in vivo are essential for bone regeneration.

Culturing thick brain slices: an interstitial 3D microperfusion system for enhanced viability

Brain slice preparations are well-established models for a wide spectrum of in vitro investigations in the neuroscience discipline. However, these investigations are limited to acute preparations or thin organotypic culture preparations due to the lack of a successful method that allows culturing of thick organotypic brain slices. Thick brain slice cultures suffer necrosis due to ischemia deep in the tissue resulting from a destroyed circulatory system and subsequent diffusion-limited supply of nutrients and oxygen. Although thin organotypic brain slice cultures can be successfully cultured using a well-established roller-tube method (a monolayer organotypic culture) (Gahwiler B H. Organotypic monolayer cultures of nervous tissue. J Neurosci Methods. 1981; 4: 329-342) or a membrane-insert method (up to 1-4 cell layers, <150 microm) (Stoppini L, Buchs PA, Muller D. A simple method for organotypic cultures of neural tissue. J Neurosci Methods 1991; 37: 173-182), these methods fail to support thick tissue preparations. A few perfusion methods (using submerged or interface/microfluidic chambers) have been reported to enhance the longevity (up to few hours) of acute slice preparations (up to 600 microm thick) (Hass HL, Schaerer B, Vosmansky M. A simple perfusion chamber for study of nervous tissue slices in vitro. J Neurosci Methods 1979; 1: 323-325; Nicoll RA, Alger BE. A simple chamber for recording from submerged brain slices. J Neurosci Methods 1981; 4: 153-156; Passeraub PA, Almeida AC, Thakor NV. Design, microfabrication and characterization of a microfluidic chamber for the perfusion of brain tissue slices. J Biomed Dev 2003; 5: 147-155). Here, we report a unique interstitial microfluidic perfusion technique to culture thick (700 microm) organotypic brain slices. The design of the custom-made microperfusion chamber facilitates laminar, interstitial perfusion of oxygenated nutrient medium throughout the tissue thickness with concomitant removal of depleted medium and catabolites. We examined the utility of this perfusion method to enhance the viability of the thick organotypic brain slice cultures after 2 days and 5 days in vitro (DIV). We investigated the range of amenable flow rates that enhance the viability of 700 microm thick organotypic brain slices compared to the unperfused control cultures. Our perfusion method allows up to 84.6% viability (p<0.01) and up to 700 microm thickness, even after 5 DIV. Our results also confirm that these cultures are functionally active and have their in vivo cyto-architecture preserved. Prolonged viability of thick organotypic brain slice cultures will benefit scientists investigating network properties of intact organotypic neuronal networks in a reliable and repeatable manner.

Microfluidics: a new cosset for neurobiology

Recently, microfluidic systems have shown great potential in the study of molecular and cellular biology. With its excellent properties, such as miniaturization, integration and automation, to name just a few, microfluidics creates new opportunities for the spatial and temporal control of cell growth and environmental stimuli in vitro. In the field of neuroscience, microfluidic devices offer precise control of the microenvironment surrounding individual cells, and the delivery of biochemical or physical cues to neural networks or single neurons. The intent of this review is to outline recent advances in microfluidic-based applications in neurobiology, with emphasis on neuron culture, neuron manipulation, neural stem cell differentiation, neuropharmacology, neuroelectrophysiology, and neuron biosensors. It also aims to stimulate development of microfluidic-based applications in neurobiology by involving scientists from various disciplines, especially neurobiology and microtechnology.