Advancing models of neural development with biomaterials

Abstract

Human pluripotent stem cells have emerged as a promising in vitro model system for studying the brain. Two-dimensional and three-dimensional cell culture paradigms have provided valuable insights into the pathogenesis of neuropsychiatric disorders, but they remain limited in their capacity to model certain features of human neural development. Specifically, current models do not efficiently incorporate extracellular matrix-derived biochemical and biophysical cues, facilitate multicellular spatio-temporal patterning, or achieve advanced functional maturation. Engineered biomaterials have the capacity to create increasingly biomimetic neural microenvironments, yet further refinement is needed before these approaches are widely implemented. This Review therefore highlights how continued progression and increased integration of engineered biomaterials may be well poised to address intractable challenges in recapitulating human neural development.

Fig. 1 |. The developmental trajectory of in vivo and in vitro model systems.

a | During postconception weeks 3 and 4, a layer of neuroepithelial cells expands, elongates and folds to form the neural tube. Following neurulation, the neural progenitor cells within the neural tube are patterned across both rostrocaudal and dorsoventral axes by morphogen gradients. The first neurons emerge at postconception week 6. Once radially migrating neurons reach their target cortical layer, their axons are guided by chemoattractive and chemorepulsive cues, including secreted molecules, cell surface molecules and the extracellular matrix. Neurogenesis and neuronal migration are followed by the generation and maturation of astrocytes and oligodendrocytes. Last, synaptogenesis, myelination and circuit refinement by synapse elimination continue late into the postnatal years. Several excellent reviews present comprehensive discussions of these processes^,–. b | Pluripotent stem cells can be derived from the inner cell mass of human blastocysts or by reprogramming somatic cells. The process of guiding pluripotent stem cells into neural cell fates is inspired by in vivo neurogenesis, wherein neuroepithelial cells differentiate into neural progenitors and, eventually, functional neurons and glia. To achieve this progression in vitro, two-dimensional (2D) differentiation relies on exposing a monolayer of cells to a defined concentration of small molecules that modulate signalling pathways implicated in neural cell fate acquisition. Three-dimensional (3D) approaches can generally be subdivided into directed and undirected differentiation. In directed differentiation, a series of patterning molecules are used to drive specific brain regionalization and cell fate. These brain region-specific neural organoids can be fused into assembloids, which elicit cellular migration and circuit formation. In undirected differentiation, cellular aggregates are embedded in an exogenous biomaterial and allowed to stochastically pattern. While both 2D and 3D differentiation paradigms broadly recapitulate the emergence of spatio-temporally appropriate cell types, multiple differences between in vivo and in vitro neurodevelopment remain. Of note, the emergence of radially arranged progenitor cells in neural rosettes reflects, but does not completely emulate, the neural tube.

Fig. 2 |. Biochemical and biophysical signalling cues within the neural microenvironment.

The neural microenvironment is multifaceted, spatio-temporally dynamic and, thus far, insufficiently emulated in vitro. Biomaterials have the potential to recapitulate extracellular matrix (ECM)-derived biochemical and biophysical cues, spatio-temporally conserved paracrine and juxtacrine signalling, and electrical stimulation. a | Biomaterials can be engineered to incorporate cell-interactive domains within their backbones to recapitulate the signalling cues presented by the neural ECM and basement membrane. b | Material scaffolds can be engineered to restrict cell geometry. Encapsulated cells can remodel their local niche by secreting proteases to degrade surrounding materials or by exerting strain on surrounding materials. c | Conductive polymers enable electrical signal propagation, which both promotes neurite outgrowth and enhances functional maturation. d | Various cell type-specific growth factors can be tethered to biomaterials to provide spatio-temporal control over cell fate and morphology. The capacity to pattern the local environment with biomaterials, and techniques that use biomaterials such as three-dimensional bioprinting, may facilitate co-cultures with neural and non-neural cells that induce both paracrine and juxtacrine signalling.

Fig. 3 |. Engineered matrices to recreate ecM-derived signalling cues.

a | The native extracellular matrix (ECM) in the brain presents a complex microenvironment composed primarily of hyaluronic acid (HA), proteoglycans, tenascins, link proteins, laminins, fibronectins and collagens. Neural ECM is unique insofar as it contains relatively low levels of fibrous proteins (collagen and fibronectin) and high levels of glycosaminoglycans, including chondroitin sulfate proteoglycans (CSPGs). The pericellular neural ECM is described as a supramolecular assembly of HA–link protein–CSPG–tenascin. b | Various materials have been used for modelling the native brain microenvironment, including decellularized brain matrices, natural biopolymers, protein-engineered biomaterials and synthetic polymers. c | Biomaterial strategies designed to manipulate the local cellular environment include controlling either biochemical or biophysical signalling cues presented to cells. The presentation of certain cell-adhesive ligands can increase neural marker expression. Modulating material stiffness can drive neural differentiation. Both stress relaxation and degradability have been shown to have a role in maintaining neural progenitor cell stemness maintenance through increased cell–cell contact. Matrix confinement can drive wrinkling in neural organoids. HSPG, heparan sulfate proteoglycan.

Fig. 4 |. Biofabrication strategies to modulate multicellular neural maturation.

a | Cellular identity is spatially patterned by morphogen gradients in vivo. Similar morphogen or growth factor gradients can be generated by microfluidics, and growth factor release can be stabilized using drug-releasing microparticles or nanoparticles. Such approaches can enable spatially patterned in vitro cultures in two dimensions and three dimensions. b | The developing brain comprises different cell types and structures, including blood vessels. Bioprinting and biomaterial-based scaffolds may enable the generation of more complex structures and human neurovascularization in three dimensions. c | Electrical activity during development can be emulated by electrical stimulation using engineered materials, such as carbon nanotubes, graphene and conductive polymers. Ultimately, such techniques may enable a multi-electrode array-like structure embedded in three-dimensional (3D) cultures. BMP, bone morphogenetic protein; SHH, sonic hedgehog.