Nerve-mediated FGF-signaling in the early phase of various organ regeneration

Microcomputed tomography staging of bone histolysis in the regenerating mouse digit

Humans and mice have the ability to regenerate the distal digit tip, the terminal phalanx (P3) in response to amputation. What distinguishes P3 regeneration from regenerative failure is formation of the blastema, a proliferative structure that undergoes morphogenesis to regenerate the amputated tissues. P3 regeneration is characterised by the phases of inflammation, tissue histolysis and expansive bone degradation with simultaneous blastema formation, wound closure and finally blastemal differentiation to restore the amputated structures. While each regenerating digit faithfully progresses through all phases of regeneration, phase progression has traditionally been delineated by time, that is, days postamputation (DPA), yet there is widespread variability in the timing of the individual phases. To diminish variability between digits during tissue histolysis and blastema formation, we have established an in-vivo method using microcomputed tomography (micro CT) scanning to identify five distinct stages of the early regeneration response based on anatomical changes of the digit stump. We report that categorising the initial phases of digit regeneration by stage rather than time greatly diminishes the variability between digits with respect to changes in bone volume and length. Also, stages correlate with the levels of cell proliferation, osteoclast recruitment and osteoprogenitor cell recruitment. Importantly, micro CT staging provides a means to estimate open versus closed digit wounds. We demonstrate two spatially distinct and stage specific bone repair/regeneration responses that occur during P3 regeneration. Collectively, these studies showcase the utility of micro CT imaging to infer the composition of radiolucent soft tissues during P3 blastema formation. Specifically, the staging system identifies the onset of cell proliferation, osteoclastogenesis, osteoprogenitor recruitment, the spatial initiation of de novo bone formation and epidermal closure.

One of the Primary Functions of Tissue-Resident Pluripotent Pericytes Cells May Be to Regulate Normal Organ Growth and Maturation: Implications for Attempts to Repair Tissues Later in Life

Adult mesenchymal stem cells were reported more than 30 years ago. Since then, their potential to repair and regenerate damaged or diseased tissues has been studied intensively in both preclinical models and human trials. Most of the need for such tissue repair/regeneration is in older populations, so much of the effort has been performed with autologous cells in older patients. However, success has been difficult to achieve. In the literature, it has been noted that such progenitor cells from younger individuals often behave with more vigorous activity and are functionally enhanced compared to those from older individuals or animals. In addition, cells with the characteristics of mesenchymal stem cells or pluripotent mesenchymal regulatory cells exist in nearly all tissues and organs as pericytes since fetal life. Such evidence raises the possibility that one of the primary roles of these organ-specific cells is to regulate organ growth and maturation, and then subsequently play a role in the maintenance of organ integrity. This review will discuss the evidence to support this concept and the implications of such a concept regarding the use of these progenitor cells for the repair and regeneration of tissues damaged by injury or disease later in life. For the latter, it may be necessary to return the organ-specific progenitor cells to the functional state that contributed to their effectiveness during growth and maturation rather than attempting to use them after alterations imposed during the aging process have been established and their function compromised.

Lmx1b activation in axolotl limb regeneration

BACKGROUND

Axolotls can regenerate their limbs. In their limb regeneration process, developmental genes are re-expressed and reorganize the developmental axes, in which the position-specific genes are properly re-expressed. However, how such position specificity is reorganized in the regeneration processes has not been clarified. To address this issue, we focused on the reactivation process of Lmx1b, which determines the limb dorsal identity in many animals.

RESULTS

Here, we show that Lmx1b expression is maintained in the dorsal skin before amputation and is activated after amputation. Furthermore, we demonstrate that only cells located in the dorsal side prior to limb amputation could reactivate Lmx1b after limb amputation. We also found that Lmx1b activation was achieved by nerve presence. The nerve factors, BMP2 + FGF2 + FGF8 (B2FF), consistently reactivate Lmx1b when applied to the dorsal skin.

CONCLUSIONS

These results imply that the retained Lmx1b expression in the intact skin plays a role in positional memory, which instruct cells about the spatial positioning before amputation. This memory is reactivated by nerves or nerve factors that can trigger the entire limb regeneration process. Our findings highlight the role of nerves in amphibian limb regeneration, including both the initiation of limb regeneration and the reactivation of position-specific gene expression. This article is protected by copyright. All rights reserved.

The amazing and anomalous axolotls as scientific models

Ambystoma mexicanum (axolotl) embryos and juveniles have been used as model organisms for developmental and regenerative research for many years. This neotenic aquatic species maintains the unique capability to regenerate most, if not all, of its tissues well into adulthood. With large externally developing embryos, axolotls were one of the original model species for developmental biology. However, increased access to, and use of, organisms with sequenced and annotated genomes, such as Xenopus laevis and tropicalis and Danio rerio, reduced the prevalence of axolotls as models in embryogenesis studies. Recent sequencing of the large axolotl genome opens up new possibilities for defining the recipes that drive the formation and regeneration of tissues like the limbs and spinal cord. However, to decode the large Ambystoma mexicanum genome will take a herculean effort, community resources, and the development of novel techniques. Here, we provide an updated axolotl-staging chart ranging from 1-cell stage to immature adult paired with a perspective on both historical and current axolotl research that spans from their use in early studies of development to the recent cutting-edge research, employment of transgenesis, high resolution imaging, and study of mechanisms deployed in regeneration. This article is protected by copyright. All rights reserved.