Everyone's story begins as a single zygote that generates small set of embryonic stem cells, which reconstruct the body from scratch until reproductive maturity is reached. Then, the newly made fresh mature bodies and tissues are left to be maintained and regenerated for the lifetime of an individual. Unfortunately, as we grow up, this capacity decreases. Aging, then, can be viewed as the progressive downregulation of regenerative capacity. Careful use and engineering of these endogenous regenerative capacities can and has the potential to delay age-related degenerative diseases and therefore postpone death.
Our bodies can repair and regenerate. Regeneration is the proliferation of cells and tissues to replace lost structures  such as the growth of lost fingertips by young children. Much or less, almost all different parts of our body can regenerate, we know our hair follicles and skin are constantly being regenerated. Wounds and broken bones can heal. Our lower rib and our liver grow back. Once thought non-dividing, our body can even regenerate lost neurons and heart muscle cells . In fact most of the cells and tissues in our bodies are much younger than our chronological age, but this process of regeneration gets less efficient with age. Aging therefore can be viewed as the progressive down regulation of regenerative capacities. Careful use and engineering of these endogenous regenerative capacities has the potential to delay age-related diseases and therefore postpone death. In this article we review the underlying molecular mechanisms and mediators involved in in situ regeneration, and their unleashing promise to keep our bodies rejunivated.
Till today, regenerative medicine and especially stem cell transplantation experiments, some of which with great results have been the standard. But in situ regeneration which uses our own endogenous stem cells, can be even more promising. In this article, we try to review these promises of in situ regenerations, its mechanisms and outcomes.
The brain is not solely composed of postmitotic cells, rather than neuronal stem cells, able to migrate in various regions of the brain constantly replace the loss of neurons. Dietary restriction (DR), the most powerful non-genetic anti-aging intervention enhances neurogenesis as well as other stem cell compartments. Apart from DR, reactivating and stimulating the CNS (central nervous system) endogenous stem cells activity has been observed and successfully altered for forming and preventing the death of neurons and glial cells in many neurodegenerative diseases and injuries such as Parkinson's diseases, multiple sclerosis, Alzheimer's diseases, stroke are mostly linked with age. These endogenous recruitment of CNS stem cells, can raise interesting clinical benefits for aged patients [16810245; http://www.bates.edu/Prebuilt/LindvallKokaia2006.pdf].
Newborn heart muscle can regrow. Young animals are capable of regeneration. Young children can regenerate lost fingertips. This capacity decreases with age. Regenerating salamanders which are caught in the juvenile stage have strong regenerative abilities. Portion of the heart removed in mice during the first week after birth grows back wholly and correctly within 3 weeks. A newborn mammal can fix itself, but it forgets how as it gets older (its switched off). Lower organisms like some fish and amphibians can regrow fins and tails as well as portions of their hearts after injury. In young mice, injured beating cardiomyocytes are the major source of the new cells. They stop beating long enough to divide and provide the heart with fresh cardiomyocytes [21350179; http://www.sciencemag.org/content/331/6020/1078.short].
Mammalian muscle culture can be induced to undergo cellularization, proliferation, and dedifferentiation. In amphibians, a critical step in limb regeneration is the cellularization and dedifferentiation of skeletal muscle. Mammalian skeletal muscle does not undergo this response to injury. A stepwise chemical method can induce dedifferentiation and multipotency in mammalian skeletal muscle. The induction of a proliferative response in the cellulite is a crucial step in the dedifferentiation process, which was achieved by p21 downregulation. Downregulation of p27, p57 or p53 failed to induce proliferation and subsequently dedifferentiation. Treatment with reversine during the proliferative window induced the muscle cellulite to differentiate into non muscle cell types and allowed the derivation of adipogenic and osteogenic cells that possessed a degree of functionality .
When we look at an aged individual, maybe one of the best indicators of aging is his/her ectodermal appendages such as hair, teeth, and nails. Although not vital for human beings, they can act as determinants of youth and beauty and thus be desired. Also, the mechanisms and signals involved in morphogenesis of this appendages are strongly similar to limb and other organs morphogenesis, so regeneration of these mini-organs can pave the road for regenerating other organs.
The hair follicle is an interesting and complex organ, it regenerates within each cycle. This regenerative capacity has been attributed to hair follicle stem cells (HFSCs) which reside in the permanent epithelial part of the hair follicle, just under the arrector pili, which is called bulge region. It has been shown that these stem cells not only contribute to hair follicle regeneration, but from lineage analysis experiments, it has been shown that these HFSCs can also form epithelial parts of skin, in the process of wound healing. Hair follicle develop epithelial-mesenchymal interactions, and this development happens only once in the lifetime of humans and many mammals such as mice, although, de novo follicular neogenesis has been shown to happen in a modified process of wound healing [17507982; http://www.nature.com/nature/journal/v447/n7142/abs/nature05766.html].
This interesting regenerative capacity of HFSCs has a promising therapeutic target, as it has been shown that these cells can dedifferentiate into many other cell types, such as keratinocytes, fat cells, skin fibroblasts, and even neurons.
Teeth are constantly regrowing, In numerous rodents such as voles and guinea pigs as well as rabbits. Even in primates teeth grow more than once. Thus, an induction of the teeth growth process could become reality if the signals and requirements are understood. In fact, it is already possible to culture and implant replacement of natural teeth grown from dental pulp stem cells [21765896; http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0021531]. Fully functional teeth have been grown from stem cells planted in the mouths of mice. It works via the transplantation of bioengineered tooth germ into the alveolar bone in the lost tooth region [21765896; http://www.pnas.org/content/106/32/13475].
Although teeth are not so vital compared to other organs, tooth decay is one of the very few age-related changes to be considered as irreversible. Therefore, it is interesting to note that it can be easily regenerated and also nicely demonstrates the principle.
Intriguingly, suppression of Wnt signaling abrogated hair follicle neogenesis, while overexpression Wnt ligands enhanced this process.
Wnt signaling is causally associated with the age-related decline in adult stem cell capacity. For instance, WNT gene expression decreases in bone-marrow-derived mesenchymal stem cells [Stolzing, et al. unpublished].
Wnt10b deficiency leads to age-dependent reduction in mesenchymal progenitor cells [20499361; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3153316/]. In general deregulation of Wnt signaling with increasing age impairs the function of mesenchymal stem cells [21712954; info:doi%2F10.1371%2Fjournal.pone.0021397'>http://www.plosone.org/article/info:doi%2F10.1371%2Fjournal.pone.0021397] and muscle stem cell regeneration . Also premature aging syndroms affect adult stem cell function via developmental siginaling pathways such as Notch and Wnt [18378774; http://meshorerlab.huji.ac.il/papers/MeshorerGruenbaumJCB08.pdf]. Wnt signaling and its hub protein B-catenin is one of the most important regulators of stem cells, especially in colon crypt stem cells, epithelial stem cells of skin and hair follicles. Altering Wnt signaling can cause organ regeneration. The modification and reactivation of developmental process can happen in individuals but it appears to get harder and harder to do that in aged individuals, as not only stem cells but their niches also age by time.
The MRL strain has extraordinary regenerative capacities. In contrast to normal mice, holes in the ears of this strain completely heal without any scars. The scarless heart - injuring the heart of this strain heals without scar formation [19760323; 9751744; 9683548; 11493713; 14594211; 15293806; 16476076; 17005026; 17327495].
Identifying the causative mutations via next generation sequencing and comparative genomics might enable also humans to get this capability by developing drug treatments or dietary interventions which target the very same gene products mutated in this regenerator strain.
Very few genes are probably controlling endogenous regenerative capacities, which just need to be tuned a little bit. Knocking out a single gene, p21, blesses mice with enormous regenerative abilities [20231440; 21722344]. It is interesting to note that p21 gene activity increases during aging and therefore likely contributes to the age-related loss of wound healing abilities.
Axolotls are salamanders which, due to a mutation, lost the ability to produce a hormone critical for morphogenesis, and are therefore are stuck in the juvenile stage. As juvenile they keep their ability to regenerate throughout life.
Immortal organisms are extremely good regenerators. For instance, the fresh-water animal Hydra is capable of regenerating its whole body. Flatworms with stem cells which have indefinitely regenerative capacity do not age at all, nor are they known to get cancer despite high proliferation rates which are assumed to cause cancer in in humans [22371573; http://www.nottingham.ac.uk/news/pressreleases/2012/february/immortal-worms-defy-ageing.aspx].
It seems that with organisms getting more complex, the ability to regenerate decreases. Hydra and flatworms are considered truly regenerative and they can regenerate their whole body, fishes and amphibians can regrow fins and tails, salamanders, somehow, never get old. Newborn mammals can also fix themselves, unfortunately they forget how as they grow older which is genetically programmed. In mammals the responsibility for tissue homeostasis falls upon adult somatic stem cells. So it is possible that age-related loss of regenerative capacity is due to adult stem cells changes, since characteristics of stem cells such as replicative capacity which is dependent tissue turn over. These changes can also be very tissue specific, and must be viewed separately.
A repeated theme is that age-related loss of regenerative capacity is due to adult stem cell changes. The question arises what are these stem cell changes?
Stem cell aging can, theoretically, be due to three different reasons:
It can be due to a decrease in the number of stem cells, intrinsic reduction of stem cell functions, or changes in the niche and environment in which stem cells reside or even a combination of all. Several evidence support the notation that the changes in the niche of stem cells is most crucial, as systemic factors in the stem cells environment impact then on the number and function of adult stem cells (their tenants). For instance, hematopoietic stem cell (HSC) numbers is significantly higher and even more active in bone-marrow derived from old mice than young mice, but aged HSCs possess an aged-genome which is epigenetically different. These epigenetic changes in aged HSCs most likely reflect the influence of an aging niche on these cells, since epigenetic status of stem cells are dependent on the environment and its signaling factors. It has also been shown that putting old stem cells onto ECM of young restores their stemness to that of young levels. This can have a great impact on how we view aging of stem cells and loss of regenerative power. In analogy: stem cells are like humans, if a person does not have a tidy home, to eat and shower, or his/her home is unhealthy and deformed, s/he gets sick and mad.
Very small embryonic like stem cells (VSELs) are pluripotent stem cells (i.e. able to differentiate into all three germ-layers) present throughout the adulthood and are altered in their function by hormones which exhibit age-related changes and respond to the nutritional state (insulin/insulin-like growth hormone axis) [21566652; 22023227; 22648539]. Their activity is actually determined by DNA-methylation. In such it appears that endocrine changes affect the epigenomic state of stem cells and curtails their function with increasing age, while dietary restriction reactivates these cells by partly reversing these changes.
Ideally we might be able to find ways to induce regeneration in situ without the need for artificial ex vivo techniques. One way is the screening for drugs that vivify endogenous regenerative processes [http://www1.easl.eu/easl2011/program/Orals/261.htm].
Progressive decline and loss of regenerative capacity of adult stem cells plays an important role in decline of tissue homeostasis and repair capacities of aged-tissues. Maintaining regenerative capacity of stem cells as we age is a promising avenue for keeping tissues young. The regenerative capacity of adult stem cells decreases as we grow older, but environmental conditions and stimuli can rejuvenate these cells. in situ epigenetic engineering and reprogramming of cells and stem cells can be a good candidate, and has been found to control organismal longevity [20555324; 21441951; http://www.stanford.edu/group/brunet/Pollina,%202011.pdf]. Also epigenetic drugs can target many well known regulators of organisms longevity, such as FOXO, a known regulator of DNA-repair through chromatin-modification [http://benthamscience.com/open/toleukemiaj/articles/V003/34TOLEUKEMIAJ.pdf].
Apart from why stem cells milieu age over time (possibly extrinsic factors, cells waste materials, and no evolutionary pressure for a mechanism to avoid it) there are possible ways to renew the stem cell environment  and apart from epigenetic reprogramming, cytoplasmic reprogramming can also be a useful therapy to regenerate a tissue function  as they have turned adult pancreatic cells into β islet cells.
So in general, we can 1) in situ regenerate from activating, and actually recruiting already present stem cells to regenerate an organ, or recruit them from another organ to the desired place by some factors , or we can activate and rejuvenate adult cells by 2) putting them into a new niche, or 3) we can in situ regenerate by activating regenerating powers through in vivo reprogramming them (nuclear/epigenetic, or cytoplasmic)  which can directly change one cell to another cell, or transdifferentiate or a combination of all can create even a novel organ (de novo hair follicle regeneration).