Macrophage - Regeneration pillar

Deep vast wounds heal in mammals for a long time and hard, and damaged tissues are never completely restored. In the place of muscles and skin, collagen scars are formed, scars remain. The fact that instead of a chopped finger grew up a new one, and there is no question. Meanwhile, axolotle ( Ambystoma Mexicanum , capable of breeding the larva of ambistoma, a close relative of the real salamander) easily restores lost limbs. Naturally, this phenomenon attracts the attention of scientists: we would like that!

Healing of the wound is a complex and multi-stage process in which many genes and biologically active molecules are involved, including growth factors, enzymes, pro- and anti-inflammatory signals, cytokines that attract fibroblasts and cells of the immune system. All these factors in mammals and axolotl are approximately the same. The main difference between them is when they begin to act. So, at axolotl macrophages arrive in the wound significantly earlier than in mammals. Specialists of the Australian Institute of Regenerative Medicine of the University of Monash and the Imperial College of London, led by Professor Nadia A. Rosenthal), established that it was thanks to the early participation of macrophages that Axolotl’s place in the place of the cut foot formed not a stump, but a new limb.

In mammals, the number of macrophages in damaged tissue increases 48-96 hours after injury. Macrophages clean the wound, eating dead cells, secrete pro -inflammatory cytokines (inflammation - the necessary stage of healing), and then factors that suppress inflammation, stimulate the formation of blood vessels and cell division, attract fibroblasts into the wound. It is they who create an environment favorable for the restoration of damaged tissues. If in mice with damaged skeletal muscles to create an artificial deficiency of macrophages, muscle tissue will recover poorly, but fibrous scars are formed in large quantities.

The influence of macrophages on the regeneration of the axolotl paw. The dotted line shows the cut line. The left column - control animals, the right - axolotli, in which, before amputation, they “brought macrophages from the game”. On the fourth day, a blast is formed in control animals - a cluster of cells, pierced by blood vessels, and in experimental fibrous tissue. Without macrophages, regeneration does not go, a stump is formed in the place of a cut limb. But after repeated amputation, a normal limb grows from it. (Drawing from the article)

In the wound of the axolotl, macrophages appear on the very first day after amputation of the foot, reach a peak of number on the 4-6th days and stay in damaged tissues and a regenerating limb of the week, until the end of the early stage of regeneration.

Macrophages in a living organism can be effectively tied by introducing Clodronate packed into liposomes into liposomes. This reagent specifically connects monocytes circulating in the blood and macrophage in tissues. The researchers made axolotles injection of clodronate a day before amputation (the control animals introduced liposomes with the buffer), as a result, the first six days of regeneration took place with an acute deficiency of macrophages and a lack of growth factors and other molecules necessary for the full healing of wounds and limb regeneration. A week later, the number of macrophages returned to normal, but the limb still increased kutsay.

If you make an axilot, not one injection, but three, the number of macrophages in the wound will fall to almost zero. In this case, the regeneration of the limb is completely blocked: the wound is delayed, but instead of the paw, a fibrous stump is formed by scars, like in mammals.

To find out the role of macrophages at the late stage of regeneration, the researchers introduced Clodronate to the 10-13th day after amputation, when the blastem was already formed-the accumulation of homogeneous non-specialized cells that form the tissues of the growing organ. The blastim does not contain thick collagen fibers that cause fibrosis. It turned out that the lack of macrophages at a late stage slows down regeneration, but does not block it. Obviously, the most important events that determine the choice between regeneration and the formation of fibrous tissue occur at the beginning of the formation of a blastem.

In case of unsuccessful regeneration, the program can be reactivated, there would be macrophages. Scientists a secondly amputated the axolotel of the stump, which did not turn into a limb. This time, nothing prevented the regeneration, macrophages were present in the proper amount, and Axolotl grew a normal paw. Recovery is possible even on the 150th day after the first amputation.

Researchers emphasize that for the regeneration of the extremities, it is necessary to create microenvironment at the very early stage, favorable for the restoration of the organ. This environment is created by macrophages. Researchers hope that, having determined the role of all molecules distinguished by macrophages, they will be able to find a tool that will help prevent the formation of scars and contribute to the regeneration of mammalian tissues.

JW Godwin, Ar Pinto, and Na Rosenthal "Macrophages are Required for Adult Salamander Limb Regeneration", PNAS 2013 (23), 9415-9420, DOI: 10.1073/PNAS.1300290110