Functional recovery from peripheral nerve injury and repair depends on a multitude of factors, both intrinsic and extrinsic to neurons. Neuronal survival after axotomy is a prerequisite for regeneration and is facilitated by an array of trophic factors from multiple sources, including neurotrophins, neuropoietic cytokines, insulin-like growth factors (IGFs), and glial-cell-line-derived neurotrophic factors (GDNFs). Axotomized neurons must switch from a transmitting mode to a growth mode and express growth-associated proteins, such as GAP-43, tubulin, and actin, as well as an array of novel neuropeptides and cytokines, all of which have the potential to promote axonal regeneration. Axonal sprouts must reach the distal nerve stump at a time when its growth support is optimal. Schwann cells in the distal stump undergo proliferation and phenotypical changes to prepare the local environment to be favorable for axonal regeneration. Schwann cells play an indispensable role in promoting regeneration by increasing their synthesis of surface cell adhesion molecules (CAMs), such as N-CAM, Ng-CAM/L1, N-cadherin, and L2/HNK-1, by elaborating basement membrane that contains many extracellular matrix proteins, such as laminin, fibronectin, and tenascin, and by producing many neurotrophic factors and their receptors. However, the growth support provided by the distal nerve stump and the capacity of the axotomized neurons to regenerate axons may not be sustained indefinitely. Axonal regenerations may be facilitated by new strategies that enhance the growth potential of neurons and optimize the growth support of the distal nerve stump in combination with prompt nerve repair.

The effects of prolonged denervation, independent from those of prolonged axotomy, on the recovery of muscle function were examined in a nerve cross-anastomosis paradigm. The tibialis anterior muscle was denervated for various durations by cutting the common peroneal nerve before a freshly cut tibial nerve was cross-sutured to its distal stump. Nerve regeneration and muscle reinnervation were quantified by means of electrophysiological and histochemical methods. Progressively fewer axons reinnervated the muscle with prolonged denervation; for example, beyond 6 months the mean (+/- SE) motor unit number was 15 +/- 4, which was far fewer than that after immediate nerve suture (137 +/- 21). The poor regeneration after prolonged denervation is not due to inability of the long-term denervated muscle to accept reinnervation because each regenerated axon reinnervated three- to fivefold more muscle fibers than normal. Rather, it is due to progressive deterioration of the intramuscular nerve sheaths because the effects of prolonged denervation were simulated by forcing regenerating axons to grow outside the sheaths. Fewer regenerated axons account for reinnervation of less than 50% of the muscle fibers in each muscle and contribute to the progressive decline in muscle force. Reinnervated muscle fibers failed to fully recover from denervation atrophy: muscle fiber cross-sectional area being 1171 +/- 84 microns2 as compared to 2700 +/- 47 microns2 after immediate nerve suture. Thus, the primary cause of the poor recovery after long-term denervation is a profound reduction in the number of axons that successfully regenerate through the deteriorating intramuscular nerve sheaths. Muscle force capacity is further compromised by the incomplete recovery of muscle fibers from denervation atrophy.

Tooth infections or injuries involving dental pulp are treated routinely by root canal therapy. Endodontically treated teeth are devitalized, susceptible to re-infections, fractures, and subsequent tooth loss. Here, we report regeneration of dental-pulp-like tissue by cell homing and without cell transplantation. Upon in vivo implantation of endodontically treated real-size, native human teeth in mouse dorsum for the tested 3 weeks, delivery of basic fibroblast growth factor and/or vascular endothelial growth factor (bFGF and/or VEGF) yielded re-cellularized and revascularized connective tissue that integrated to native dentinal wall in root canals. Further, combined delivery of bFGF, VEGF, or platelet-derived growth factor (PDGF) with a basal set of nerve growth factor (NGF) and bone morphogenetic protein-7 (BMP7) generated cellularized and vascularized tissues positive of VEGF antibody staining and apparent neo-dentin formation over the surface of native dentinal wall in some, but not all, endodontically treated teeth. Newly formed dental pulp tissue appeared dense with disconnected cells surrounded by extracellular matrix. Erythrocyte-filled blood vessels were present with endothelial-like cell lining. Reconstructed, multiple microscopic images showed complete fill of dental-pulp-like tissue in the entire root canal from root apex to pulp chamber with tissue integration to dentinal wall upon delivery of bFGF, VEGF, or PDGF with a basal set of NGF and BMP7. Quantitative ELISA showed that combinatory delivery of bFGF, VEGF, or PDGF with basal NGF and BMP7 elaborated von Willerbrand factor, dentin sialoprotein, and NGF. These findings represent the first demonstration of regenerated dental-pulp-like tissue in endodontically treated root canals of real-size, native human teeth. The present chemotaxis-based approach has potent cell homing effects for re-cellularization and revascularization in endodontically treated root canals in vivo, although in an ectopic model. Regeneration of dental pulp by cell homing, rather than cell delivery, may accelerate clinical translation.

The contribution of prolonged motoneuron axotomy to the poor functional recovery after delayed nerve repair was determined by means of a nerve cross-anastomosis paradigm in the rat. The tibial nerve was axotomized up to 12 months before it was cross-sutured to the distal stump of the freshly cut common peroneal nerve to innervate the freshly denervated tibialis anterior muscle. Three to 17 months later, muscle and motor unit (MU) forces were measured to quantify the number of axons that had successfully regenerated and reinnervated the muscle. The extent of axonal branching was estimated by the innervation ratio (IR) (i.e., the number of muscle fibers innervated by each axon), which was obtained directly by counting muscle fibers in a single glycogen-depleted MU in each muscle and indirectly by calculation. The total number of MUs in each muscle significantly decreased with progression of axotomy and was only 35% of the control when axotomy was prolonged more than 3 months. Concurrently, MU force and IR increased exponentially, with a mean increase of threefold when axotomy was more than 3 months, which largely compensated for the reduction in the number of axons that reinnervated the muscle. Consequently, muscles reinnervated by tibial motor axons that had been axotomized up to 12 months produced as much force as those reinnervated by freshly axotomized tibial motor axons. Muscle weight, size, and muscle fiber size were similar to those after immediate nerve suture. Although prolonged axotomy does not compromise the number of muscle fibers innervated by each axon, it does reduce the capacity of motor axons to regenerate and thus is an important contributing factor to the poor functional recovery in delayed nerve repair.

Dental pulp has intrinsic capacity for self-repair. However, it is not clear whether dental pulp cells can be recruited endogenously for regenerating pulp tissues, including mineralizing into dentin. This work is based on a hypothesis that dental pulp stem/progenitor cells can be induced to migrate by chemotactic cytokines and act as endogenous cell sources for regeneration and mineralization. Dental stem cells (DSCs) were isolated from adult human tooth pulp and seeded on the surfaces of 3D collagen gel cylinders that were incubated in chemically defined media with stromal-derived factor-1α (SDF1), basic fibroblast growth factor (bFGF), or bone morphogenetic protein-7 (BMP7). Significantly more cells were recruited into collagen gel by SDF1 or bFGF than without cytokines in 7 days, whereas BMP7 had little effect on cell recruitment. BMP7, however, was highly effective, equally to dexamethasone, in orchestrating mineralization of cultured DSCs. Cell membrane receptors for SDF1, bFGF, and BMP7 were up-regulated in treated DSCs. Upon in vivo delivery, bFGF induced re-cellularization and re-vascularization in endodontically treated human teeth implanted into the dorsum of rats. Thus, endogenous dental pulp cells, including stem/progenitor cells, may be recruited and subsequently differentiated by chemotaxis of selective cytokines in the regeneration of dental pulp.