No abstract

Mesenchymal stem cells (MSCs) have the potential to treat early intervertebral disc (IVD) degeneration. However, during intradiscal injection, the vast majority of cells leaked out even in the presence of hydrogel carrier. Recent evidence suggests that annulus puncture is associated with cell leakage and contributes to osteophyte formation, an undesirable side effect. This suggests the significance of developing appropriate carriers for intradiscal delivery of MSCs. We previously developed a collagen microencapsulation platform, which entraps MSCs in a solid microsphere consisting of collagen nanofiber meshwork. These solid yet porous microspheres support MSC attachment, survival, proliferation, migration, differentiation, and matrix remodeling. Here we hypothesize that intradiscal injection of MSCs in collagen microspheres will outperform that of MSCs in saline in terms of better functional outcomes and reduced side effects. Specifically, we induced disc degeneration in rabbits and then intradiscally injected autologous MSCs, either packaged within collagen microspheres or directly suspended in saline, into different disc levels. Functional outcomes including hydration index and disc height were monitored regularly until 6 months. Upon sacrifice, the involved discs were harvested for histological, biochemical, and biomechanical evaluations. MSCs in collagen microspheres showed advantage over MSCs in saline in better maintaining the dynamic mechanical behavior but similar performance in hydration and disc height maintenance and matrix composition. More importantly, upon examination of gross appearance, radiograph, and histology of IVD, delivering MSCs in collagen microspheres significantly reduced the risk of osteophyte formation as compared to that in saline. This work demonstrates the significance of using cell carriers during intradiscal injection of MSCs in treating disc degeneration.

Intervertebral disc degeneration is an important clinical problem but existing treatments have significant drawbacks. The ability to bioengineer the entire spinal motion segment (SMS) offers hope for better motion preservation strategies but is extremely challenging. Here, fabrication of a multicomponent SMS construct with complex hierarchical organization from mesenchymal stem cells and collagen-based biomaterials, using a module-based integrative approach, is reported. The construct consists of two osteochondral subunits, a nucleus pulposus (NP-)-like core and a multi-lamellae annulus fibrosus (AF-)-like component. Chondrogenic medium is crucial for stabilizing the osteochondral subunits, which are shown to allow passive nutrient diffusion, while cyclic compression is necessary for better fiber matrix organization. Cells adhere, survive, and interact with the NP-like core. Cyclic torsional loading stimulates cell alignment in the AF-like lamellae and the number of lamellae affects the mechanical properties of the construct. This work represents an important milestone in SMS tissue engineering and provides a 3D model for studying tissue maturation and functional remodeling.

Matrix remodeling of cells is highly regulated by proteases and their inhibitors. Nevertheless, how would the chondrogenesis of mesenchymal stem cells (MSCs) be affected, when the balance of the matrix remodeling is disturbed by inhibiting matrix proteases, is incompletely known. Using a previously developed collagen microencapsulation platform, we investigated whether exposing chondrogenically differentiating MSCs to intracellular and extracellular protease inhibitors will affect the extracellular matrix remodeling and hence the outcomes of chondrogenesis. Results showed that inhibition of matrix proteases particularly the extracellular ones favors the phenotype of fibrocartilage rather than hyaline cartilage in chondrogenically differentiating hMSCs by upregulating type I collagen protein deposition and type II collagen gene expression without significantly altering the hypertrophic markers at gene level. This study suggests the potential of manipulating extracellular proteases to alter the outcomes of hMSC chondrogenesis, contributing to future development of differentiation protocols for fibrocartilage tissues for intervertebral disc and meniscus tissue engineering.

The first realization of a bioengineered quadri‐phasic spinal motion segment (SMS) is reported by B. P. Chan and co‐workers on page 99 using mesenchymal stem cells (MSC) and collagen as starting materials. The construct consists of two vertebrae‐endplate‐like subunits, sandwiching a multi‐lamellae annulus fibrosus‐like component, which entraps a soft nucleus pulposus‐like core. The complex hierarchical organization ranges from nano‐fibrous collagen meshwork to micrometer‐sized MSC‐collagen beads, and from millimeter‐sized individual tissue components to centimeter‐sized SMS construct.