Leveraging "raw materials" as building blocks and bioactive signals in regenerative medicine

Human umbilical cord mesenchymal stromal cells in regenerative medicine

Cells of the human umbilical cord offer tremendous potential for improving human health. Cells from the Wharton’s jelly (umbilical cord stroma) in particular, referred to as human umbilical cord mesenchymal stromal cells (HUCMSCs), hold several advantages that make them appealing for translational research. In the previous issue of Stem Cell Research & Therapy, Chon and colleagues made an important contribution to the HUCMSC literature not only by presenting HUCMSCs as an emerging cell source for intervertebral disc regeneration in general and the nucleus pulposus in particular, but also by demonstrating that an extracellular matrix-based strategy might be preferred over the use of growth factors. By culturing HUCMSCs under hypoxia in serum-free conditions in the presence of Matrigel with laminin-111, they were able to achieve intense collagen II staining by 21 days without the addition of exogenous growth factors. There is tremendous translational significance here in that such raw materials may alleviate the need for the use of growth factors in some instances, and this may have important ramifications in reducing product cost and streamlining regulatory approval. Chon and colleagues provide a promising example of the potential of HUCMSCs, demonstrating the ability to guide HUCMSC differentiation even in the absence of serum and growth factors and supporting the use of HUCMSCs as a viable alternative in intervertebral disc regeneration.

Tuning mechanical performance of poly(ethylene glycol) and agarose interpenetrating network hydrogels for cartilage tissue engineering

Hydrogels are attractive for tissue engineering applications due to their incredible versatility, but they can be limited in cartilage tissue engineering applications due to inadequate mechanical performance. In an effort to address this limitation, our team previously reported the drastic improvement in the mechanical performance of interpenetrating networks (IPNs) of poly(ethylene glycol) diacrylate (PEG-DA) and agarose relative to pure PEG-DA and agarose networks. The goal of the current study was specifically to determine the relative importance of PEG-DA concentration, agarose concentration, and PEG-DA molecular weight in controlling mechanical performance, swelling characteristics, and network parameters. IPNs consistently had compressive and shear moduli greater than the additive sum of either single network when compared to pure PEG-DA gels with a similar PEG-DA content. IPNs withstood a maximum stress of up to 4.0 MPa in unconfined compression, with increased PEG-DA molecular weight being the greatest contributing factor to improved failure properties. However, aside from failure properties, PEG-DA concentration was the most influential factor for the large majority of properties. Increasing the agarose and PEG-DA concentrations as well as the PEG-DA molecular weight of agarose/PEG-DA IPNs and pure PEG-DA gels improved moduli and maximum stresses by as much as an order of magnitude or greater compared to pure PEG-DA gels in our previous studies. Although the viability of encapsulated chondrocytes was not significantly affected by IPN formulation, glycosaminoglycan (GAG) content was significantly influenced, with a 12-fold increase over a three-week period in gels with a lower PEG-DA concentration. These results suggest that mechanical performance of IPNs may be tuned with partial but not complete independence from biological performance of encapsulated cells.

Subcritical CO2 sintering of microspheres of different polymeric materials to fabricate scaffolds for tissue engineering

The aim of this study was to use CO2 at sub-critical pressures as a tool to sinter 3D, macroporous, microsphere-based scaffolds for bone and cartilage tissue engineering. Porous scaffolds composed of ~200 μm microspheres of either poly(lactic-co-glycolic acid) (PLGA) or polycaprolactone (PCL) were prepared using dense phase CO2 sintering, which were seeded with rat bone marrow mesenchymal stromal cells (rBMSCs), and exposed to either osteogenic (PLGA, PCL) or chondrogenic (PLGA) conditions for 6 weeks. Under osteogenic conditions, the PLGA constructs produced over an order of magnitude more calcium than the PCL constructs, whereas the PCL constructs had far superior mechanical and structural integrity (125 times stiffer than PLGA constructs) at week 6, along with twice the cell content of the PLGA constructs. Chondrogenic cell performance was limited in PLGA constructs, perhaps as a result of the polymer degradation rate being too high. The current study represents the first long-term culture of CO2-sintered microsphere-based scaffolds, and has established important thermodynamic differences in sintering between the selected formulations of PLGA and PCL, with the former requiring adjustment of pressure only, and the latter requiring the adjustment of both pressure and temperature. Based on more straightforward sintering conditions and more favorable cell performance, PLGA may be the material of choice for microspheres in a CO2 sintering application, although a different PLGA formulation with the encapsulation of growth factors, extracellular matrix-derived nanoparticles, and/or buffers in the microspheres may be advantageous for achieving a more superior cell performance than observed here.