In mammals, macrophages are known to play a major role in tissue regeneration. These cells contribute to inflammation, histolysis, re-epithelialization, re-vascularization and cell proliferation. While macrophages have been shown to be essential for regeneration in salamanders and fish, their role has not been elucidated in mammalian epimorphic regeneration. Here, using the regenerating mouse digit tip as a mammalian model, we demonstrate that macrophages are essential for the regeneration process. Using cell depletion strategies, we show that regeneration is completely inhibited; bone histolysis does not occur, wound re-epithelization is inhibited and the blastema does not form. While rescue of epidermal wound closure, in the absence of macrophages, promotes blastema accumulation it does not rescue cell differentiation indicating that macrophages play a key role in the re-differentiation of the blastema. Further, we provide additional evidence that while bone degradation is a part of the regenerative process, it is not essential to the overall regenerative process. These findings show that macrophages play an essential role in coordinating the epimorphic regenerative response in mammals.

A major goal of regenerative medicine is to stimulate tissue regeneration after traumatic injury. We previously discovered that treating digit amputation wounds with BMP2 in neonatal mice stimulates endochondral ossification to regenerate the stump bone. Here we show that treating the amputation wound with BMP9 stimulates regeneration of a synovial joint that forms an articulation with the stump bone. Regenerated structures include a skeletal element lined with articular cartilage and a synovial cavity, and we demonstrate that this response requires the Prg4 gene. Combining BMP2 and BMP9 treatments in sequence stimulates the regeneration of bone and joint. These studies provide evidence that treatment of growth factors can be used to engineer a regeneration response from a non-regenerating amputation wound.

In the mouse, digit tip regeneration progresses through a series of discrete stages that include inflammation, histolysis, epidermal closure, blastema formation, and redifferentiation. Recent studies reveal how each regenerative stage influences subsequent stages to establish a blastema that directs the successful regeneration of a complex mammalian structure. The focus of this review is on early events of healing and how an amputation wound transitions into a functional blastema. The stepwise formation of a mammalian blastema is proposed to provide a model for how specific targeted treatments can enhance regenerative performance in humans.

Mammalian digit regeneration progresses through consistent stages: histolysis, inflammation, epidermal closure, blastema formation, and finally redifferentiation. What we do not yet know is how each stage can affect others. Questions of stage timing, tissue interactions, and microenvironmental states are becoming increasingly important as we look toward solutions for whole limb regeneration. This study focuses on the timing of epidermal closure which, in mammals, is delayed compared to more regenerative animals like the axolotl. We use a standard wound closure device, Dermabond (2‐octyl cyanoacrylate), to induce earlier epidermal closure, and we evaluate the effect of fast epidermal closure on histolysis, blastema formation, and redifferentiation. We find that fast epidermal closure is reliant upon a hypoxic microenvironment. Additionally, early epidermal closure eliminates the histolysis stage and results in a regenerate that more closely replicates the amputated structure. We show that tools like Dermabond and oxygen are able to independently influence the various stages of regeneration enabling us to uncouple histolysis, wound closure, and other regenerative events. With this study, we start to understand how each stage of mammalian digit regeneration is controlled.

Regeneration Biology is the study of organisms with endogenous regenerative abilities, whereas Regenerative Medicine focuses on engineering solutions for human injuries that do not regenerate. While the two fields are fundamentally different in their approach, there is an obvious interface involving mammalian regeneration models. The fingertip is the only part of the human limb that is regeneration-competent and the regenerating mouse digit tip has emerged as a model to study a clinically relevant regenerative response. In this article, we discuss how studies of digit tip regeneration have identified critical components of the regenerative response, and how an understanding of endogenous regeneration can lead to expanding the regenerative capabilities of nonregenerative amputation wounds. Such studies demonstrate that regeneration-incompetent wounds can respond to treatment with individual morphogenetic agents by initiating a multi-tissue response that culminates in structural regeneration. In addition, the healing process of nonregenerative wounds are found to cycle through nonresponsive, responsive and nonresponsive phases, and we call the responsive phase the Regeneration Window. We also find the responsiveness of mature healed amputation wounds can be reactivated by reinjury, thus nonregenerated wounds retain a potential for regeneration. We propose that regeneration-incompetent injuries possess dormant regenerative potential that can be activated by targeted treatment with specific morphogenetic agents. We believe that future Regenerative Medicine-based-therapies should be designed to promote, not replace, regenerative responses. Stem Cells Translational Medicine 2018