The requirement and influence of the peripheral nervous system on tissue replacement in mammalian appendages remain largely undefined. To explore this question, we have performed genetic lineage tracing and clonal analysis of individual cells of mouse hind limb tissues devoid of nerve supply during regeneration of the digit tip, normal maintenance, and cutaneous wound healing. We show that cellular turnover, replacement, and cellular differentiation from presumed tissue stem/progenitor cells within hind limb tissues remain largely intact independent of nerve and nerve-derived factors. However, regenerated digit tips in the absence of nerves displayed patterning defects in bone and nail matrix. These nerve-dependent phenotypes mimic clinical observations of patients with nerve damage resulting from spinal cord injury and are of significant interest for translational medicine aimed at understanding the effects of nerves on etiologies of human injury.pattern formation | peripheral nerve | stem cell T he regrowth of vertebrate appendages has been considered a classic model system for interrogating the mechanisms and requirements for tissue/organ generation and regeneration across various phyla (1-5). Urodeles regenerate entire limbs by proliferation of fate-restricted stem and progenitor cells and form a blastema, the collection of cells at the interface of stump and wound epidermis, in a process mediated by nerve-derived factors (6-10). In Urodeles, surgical denervation before amputation results in the inhibition of blastema formation, and ultimately results in regeneration failure. Evidence from fish and amphibians suggests that nerve dependence may be a function of a threshold level of nerve factors with a direct correlation between nerve fibers and degree of regeneration (3,11,12) and supports the idea that nerve dependence may be an evolutionarily conserved requirement for the regeneration of vertebrate limb tissues.Mammalian limbs consist of multiple tissues that are derived from various embryonic origins and germ layers, including embryonic ectoderm, embryonic mesoderm, and embryonic neuralcrest. Recent lineage-tracing studies have demonstrated that tissue-resident stem and progenitor cells that are fate-restricted in their developmental potential are responsible for appendage regeneration of fish, salamanders, and mice (13-16). These cumulative data demonstrate that fate-restricted stem/progenitors as cells of origin, rather than dedifferentiation or transdifferentiation of terminally differentiated cells, represents an evolutionarily conserved cellular mechanism that explains the observed regrowth of the vertebrate appendages.Collectively, the commonalities of nerve dependence and stem/ progenitor fate restriction across vertebrate phyla further highlight the possibility that nerve dependence on appendage regrowth may take place within mammals, and possibly humans. In support of this, human clinical reports indicate that skin complications are manifested in response to spinal cord injury (SCI), suggesting a...
Over 100 million patients acquire scars in the industrialized world each year, primarily as a result of elective operations. Although undefined, the global incidence of scarring is even larger, extending to significant numbers of burn and other trauma-related wounds. Scars have the potential to exert a profound psychological and physical impact on the individual. Beyond aesthetic considerations and potential disfigurement, scarring can result in restriction of movement and reduced quality of life. The formation of a scar following skin injury is a consequence of wound healing occurring through reparative rather than regenerative mechanisms. In this article, the authors review the basic stages of wound healing; differences between adult and fetal wound healing; various mechanical, genetic, and pharmacologic strategies to reduce scarring; and the biology of skin stem/progenitor cells that may hold the key to scarless regeneration.
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