Formation of spheroids by dental pulp cells in the presence of hypoxia and hypoxia mimetic agents

This study explored the novel strategy of hypoxic preconditioning of bone marrow mesenchymal stem cells before transplantation into the infarcted heart to promote their survival and therapeutic potential of mesenchymal stem cell transplantation after myocardial ischemia. Mesenchymal stem cells from green fluorescent protein transgenic mice were cultured under normoxic or hypoxic (0.5% oxygen for 24 hours) conditions. Expression of growth factors and anti-apoptotic genes were examined by immunoblot. Normoxic or hypoxic stem cells were intramyocardially injected into the peri-infarct region of rats 30 minutes after permanent myocardial infarction. Death of mesenchymal stem cells was assessed in vitro and in vivo after transplantation. Angiogenesis, infarct size, and heart function were measured 6 weeks after transplantation. Hypoxic preconditioning increased expression of pro-survival and pro-angiogenic factors including hypoxia-inducible factor 1, angiopoietin-1, vascular endothelial growth factor and its receptor, Flk-1, erythropoietin, Bcl-2, and Bcl-xL. Cell death of hypoxic stem cells and caspase-3 activation in these cells were significantly lower compared with that in normoxic stem cells both in vitro and in vivo. Transplantation of hypoxic versus normoxic mesenchymal stem cells after myocardial infarction resulted in an increase in angiogenesis, as well as enhanced morphologic and functional benefits of stem cell therapy. Hypoxic preconditioning enhances the capacity of mesenchymal stem cells to repair infarcted myocardium, attributable to reduced cell death and apoptosis of implanted cells, increased angiogenesis/vascularization, and paracrine effects.

The clinical translation of stem-cell-based dental pulp regeneration will require the use of injectable scaffolds. Here, we tested the hypothesis that stem cells from exfoliated deciduous teeth (SHED) can generate a functional dental pulp when injected into full-length root canals. SHED survived and began to express putative markers of odontoblastic differentiation after 7 days when mixed with Puramatrix™ (peptide hydrogel), or after 14 days when mixed with recombinant human Collagen (rhCollagen) type I, and injected into the root canals of human premolars in vitro. Roots of human premolars injected with scaffolds (Puramatrix™ or rhCollagen) containing SHED were implanted subcutaneously into immunodeficient mice (CB-17 SCID). We observed pulp-like tissues with odontoblasts capable of generating new tubular dentin throughout the root canals. Notably, the pulp tissue engineered with SHED injected with either Puramatrix™ or rhCollagen type I presented similar cellularity and vascularization when compared with control human dental pulps. Analysis of these data, collectively, demonstrates that SHED injected into full-length human root canals differentiate into functional odontoblasts, and suggests that such a strategy might facilitate the completion of root formation in necrotic immature permanent teeth.

Stemming from in vitro and in vivo pre-clinical and human models, tissue-engineering-based strategies continue to demonstrate great potential for the regeneration of the pulp-dentin complex, particularly in necrotic, immature permanent teeth. Nanofibrous scaffolds, which closely resemble the native extracellular matrix, have been successfully synthesized by various techniques, including but not limited to electrospinning. A common goal in scaffold synthesis has been the notion of promoting cell guidance through the careful design and use of a collection of biochemical and physical cues capable of governing and stimulating specific events at the cellular and tissue levels. The latest advances in processing technologies allow for the fabrication of scaffolds where selected bioactive molecules can be delivered locally, thus increasing the possibilities for clinical success. Though electrospun scaffolds have not yet been tested in vivo in either human or animal pulpless models in immature permanent teeth, recent studies have highlighted their regenerative potential both from an in vitro and in vivo (i.e., subcutaneous model) standpoint. Possible applications for these bioactive scaffolds continue to evolve, with significant prospects related to the regeneration of both dentin and pulp tissue and, more recently, to root canal disinfection. Nonetheless, no single implantable scaffold can consistently guide the coordinated growth and development of the multiple tissue types involved in the functional regeneration of the pulp-dentin complex. The purpose of this review is to provide a comprehensive perspective on the latest discoveries related to the use of scaffolds and/or stem cells in regenerative endodontics. The authors focused this review on bioactive nanofibrous scaffolds, injectable scaffolds and stem cells, and pre-clinical findings using stem-cell-based strategies. These topics are discussed in detail in an attempt to provide future direction and to shed light on their potential translation to clinical settings.