Integrating Theoretical and Experimental Methods for Multi-Scale Tissue Engineering of the Annulus Fibrosus of the Intervertebral Disc

There is a critical need for tissue engineered replacements for diseased and degenerated intervertebral discs in order to assuage low back pain while restoring function to the spine. Despite progress by many research groups, it remains a challenge to engineer a replacement tissue that can withstand the complex, demanding loading environment of the spine. Due to the hierarchical organization of the intervertebral disc, successful recapitulation of its functional behavior requires replication of anatomic form and physiologic function over a wide range of length scales. In this work, the technology of electrospinning has been employed for tissue engineering of the annulus fibrosus (AF) of the intervertebral disc using a multi-scale approach. The mechanics of electrospun nanofibrous assemblies was first characterized, focusing on how microscopic organization translates to macroscopic mechanical function. Next, engineered tissues were formed by culturing cells on nanofibrous scaffolds, generating aligned, dense collagenous tissues that replicate the single lamellar organization of the AF. This technology was then expanded to engineer angle-ply laminates that replicated both the anatomic form and mechanical function of the native AF. Finally, these results were further extended to engineer an angle-ply fiber-reinforced hydrogel composite that parallels the macroscopic structural organization of the intervertebral disc. Throughout, mechanical testing and mathematical modeling was used to understand material behavior, quantify functional growth, and guide comparison between engineered AF constructs and native tissue benchmarks. Emphasis has been placed on reconciling compositional and structural observations with their macroscopic mechanical implications, utilizing theoretical models to understand these relationships, and using engineered tissues to improve our understanding of structure-function relations within native fiber-reinforced soft tissues

Degeneration and regeneration of the intervertebral disc: lessons from development

Degeneration of the intervertebral discs, a process characterized by a cascade of cellular, biochemical, structural and functional changes, is strongly implicated as a cause of low back pain. Current treatment strategies for disc degeneration typically address the symptoms of low back pain without treating the underlying cause or restoring mechanical function. A more in-depth understanding of disc degeneration, as well as opportunities for therapeutic intervention, can be obtained by considering aspects of intervertebral disc development. Development of the intervertebral disc involves the coalescence of several different cell types through highly orchestrated and complex molecular interactions. The resulting structures must function synergistically in an environment that is subjected to continuous mechanical perturbation throughout the life of an individual. Early postnatal changes, including altered cellularity, vascular regression and altered extracellular matrix composition, might set the disc on a slow course towards symptomatic degeneration. In this Perspective, we review the pathogenesis and treatment of intervertebral disc degeneration in the context of disc development. Within this scope, we examine how model systems have advanced our understanding of embryonic morphogenesis and associated molecular signaling pathways, in addition to the postnatal changes to the cellular, nutritional and mechanical microenvironment. We also discuss the current status of biological therapeutic strategies that promote disc regeneration and repair, and how lessons from development might provide clues for their refinement

Differentiation alters stem cell nuclear architecture, mechanics, and mechano-sensitivity

Mesenchymal stem cell (MSC) differentiation is mediated by soluble and physical cues. In this study, we investigated differentiation-induced transformations in MSC cellular and nuclear biophysical properties and queried their role in mechanosensation. Our data show that nuclei in differentiated bovine and human MSCs stiffen and become resistant to deformation. This attenuated nuclear deformation was governed by restructuring of Lamin A/C and increased heterochromatin content. This change in nuclear stiffness sensitized MSCs to mechanical-loading-induced calcium signaling and differentiated marker expression. This sensitization was reversed when the ‘stiff’ differentiated nucleus was softened and was enhanced when the ‘soft’ undifferentiated nucleus was stiffened through pharmacologic treatment. Interestingly, dynamic loading of undifferentiated MSCs, in the absence of soluble differentiation factors, stiffened and condensed the nucleus, and increased mechanosensitivity more rapidly than soluble factors. These data suggest that the nucleus acts as a mechanostat to modulate cellular mechanosensation during differentiation. DOI: http://dx.doi.org/10.7554/eLife.18207.00