question: What’s actually preventing a human from regenerating complete organs or limbs?
comment: I think you've got it backwards. Regrowing organs and limbs isn't the normal behavior for most life-forms, it's something special that a select few species do. It's not like there's a gene that turns this off in humans, it's something that was *added to* other species.
passages: In 2009, the regeneration of hollow organs and tissues with a long diffusion distance, was a little more challenging. Therefore, to regenerate hollow organs and tissues with a long diffusion distance, the tissue had to be regenerated inside the lab, via the use of a 3D printer.
  
  With printing tissues, by 2012, there were four accepted standard levels of regenerative complexity that were acknowledged in various academic institutions: 
  
  BULLET::::- Level one, "flat tissue" like skin was the simplest to recreate;
  
  BULLET::::- Level two was "tubular structures" such as blood vessels;
  
  BULLET::::- Level three was "hollow non-tubular structures";
   Cystic fibrosis is another disease of the lungs, which is highly fatal and genetically linked to a mutation in the CFTR gene. Through growing patient-specific lung epithelium in vitro, lung tissue expressing the cystic fibrosis phenotype has been achieved. This is so that modelling and drug testing of the disease pathology can be carried out with the hope of regenerative medical applications.
  
  Section::::Induced regeneration in humans.:Penis.
  
  The penis has been successfully regenerated in the lab. The penis is a harder organ to regenerate than skin, the bladder and vagina, due to the structural complexity.
  
  Section::::Induced regeneration in humans.:Spinal nerves.
   Following amputation, most annelids are capable of sealing their body via rapid muscular contraction. Constriction of body muscle can lead to infection prevention. In certain species, such as "Limnodrilus", autolysis can be seen within hours after amputation in the ectoderm and mesoderm. Amputation is also thought to cause a large migration of cells to the injury site, and these form a wound plug.
  
  Section::::In animals.:Echinoderms.
   Some amphibians and certain species of fish and two species of African spiny mice can produce blastemas as adults. For example, salamanders can regenerate many organs after their amputation, including their limbs, tail, retina and intestine. Most animals, however, cannot produce blastemas.
  
  Section::::Limb Regeneration.
   There are now several human tissues that have been successfully or partially induced to regenerate. Many of these examples fall under the topic of regenerative medicine, which includes the methods and research conducted with the aim of regenerating the organs and tissues of humans as a result of injury. The major strategies of regenerative medicine include dedifferentiating injury site cells, transplanting stem cells, implanting lab-grown tissues and organs, and implanting bioartificial tissues.
  
  Section::::Induced regeneration in humans.:Bladder.
   There are some human organs and tissues that regenerate rather than simply scar, as a result of injury. These include the liver, fingertips, and endometrium. More information is now known regarding the passive replacement of tissues in the human body, as well as the mechanics of stem cells. Advances in research have enabled the induced regeneration of many more tissues and organs than previously thought possible. The aim for these techniques is to use these techniques in the near future for the purpose of regenerating any tissue type in the human body.
  
  Section::::History of human tissue regeneration.:History of regeneration techniques.
   A similar story was reported by CNN. A woman named Deepa Kulkarni lost the tip of her little finger and was initially told by doctors that nothing could be done. Her personal research and consultation with several specialists including Badylak eventually resulted in her undergoing regenerative therapy and regaining her fingertip.
  
  Section::::Naturally regenerating appendages and organs.:Kidney.
  
  Regenerative capacity of the kidney has been recently explored.
   In 2016 scientists could transform a skin cell into any other tissue type via the use of drugs. 
  
  The technique was noted as safer than genetic reprogramming which, in 2016, was a concern medically. The technique, used a cocktail of chemicals and enabled efficient on site regeneration without any genetic programming. In 2016 it was hoped to one day use this drug to regenerate tissue at the site of tissue injury.
  
  Section::::Naturally regenerating appendages and organs.
  
  Section::::Naturally regenerating appendages and organs.:Heart.
   Section::::Overview.
  
  Some amphibians and reptiles have the ability to regenerate limbs, eyes, spinal cords, hearts, intestines, and upper and lower jaws. The Japanese fire belly newt can regenerate its eye lens 18 times over a period of 16 years and retain its structural and functional properties. The cells at the site of the injury have the ability to undifferentiate, reproduce rapidly, and differentiate again to create a new limb or organ.
   Almost any piece from a "Schmidtea mediterranea" individual can regenerate an entire organism in a few days. This is in part enabled by the presence of abundant pluripotent stem cells called neoblasts. Transplantation of a single neoblast to a fatally injured animal has been shown to rescue the animal 
   Regenerative medicine also includes the possibility of growing tissues and organs in the laboratory and implanting them when the body cannot heal itself. If a regenerated organ's cells would be derived from the patient's own tissue or cells, this would potentially solve the problem of the shortage of organs available for donation, and the problem of organ transplant rejection.
   Section::::Induced regeneration in humans.:Thymus.
  
  Researchers from the University of Edinburgh have succeeded in regenerating a living organ. The regenerated organ closely resembled a juvenile thymus in terms of architecture and gene expression profile. The thymus gland is one of the first organs to degenerate in normal healthy individuals.
  
  Section::::Induced regeneration in humans.:Vagina.
  
  Between the years 2005 and 2008, four women with vaginal hypoplasia due to Müllerian agenesis were given regenerated vaginas. Up to eight years after the transplants, all organs have normal function and structure.
  
  Section::::Further reading.
   BULLET::::- Level four was "solid organs", which were by far the most complex to recreate due to the vascularity.
  
  In 2012, within 60 days it was possible, inside the lab, to grow tissue the size of half a postage stamp to the size of a football field; and most cell types could be grown and expanded outside of the body, with the exception of the liver, nerve and pancreas, as these tissue types need stem cell populations.
   If the processes involved in forming new tissue can be reverse-engineered into humans, it may be possible to heal injuries of the spinal cord or brain, repair damaged organs and reduce scarring and fibrosis after surgery. Despite the large conservation of the Hox genes through evolution, mammals and humans specifically cannot regenerate any of their limbs. This raises a question as to why humans which also possess an analog to these genes cannot regrow and regenerate limbs. Beside the lack of specific growth factor, studies have shown that something as small as base pair differences between amphibian and human Hox analogs play a crucial role in human inability to reproduce limbs. Undifferentiated stem cells and the ability to have polarity in tissues is vital to this process.
   In 2009, the SENS Foundation was launched, with its stated aim as "the application of regenerative medicine – defined to include the repair of living cells and extracellular material in situ – to the diseases and disabilities of ageing". In 2012, Professor Paolo Macchiarini and his team improved upon the 2008 implant by transplanting a laboratory-made trachea seeded with the patient's own cells.
   In 2013 it was proven in pig tissue that full thickness micro columns of tissue, less than 0.5mm in diameter could be removed and that the replacement tissue, was regenerative tissue, not scar. The tissue was removed in a fractional pattern, with over 40% of a square area removed; and all of the fractional full thickness holes in the square area healed without scarring. In 2016 this fractional pattern technique was also proven in human tissue.
  
  Section::::History of human tissue regeneration.:History of regeneration techniques.:Regeneration with materials.
   The regrowth of lost tissues or organs in the human body is being researched. Some tissues such as skin regrow quite readily; others have been thought to have little or no capacity for regeneration, but ongoing research suggests that there is some hope for a variety of tissues and organs. Human organs that have been regenerated include the bladder, vagina and the penis.
   For example, MDI Biological Laboratory scientists study the tiny nematode "Caenorhabditis elegans", with a lifespan of two to three weeks, to study genes that regulate aging and lifespan. Those same genes are present in humans. They study organisms such as zebrafish and salamanders that are naturally able to regenerate diseased or damaged tissue, organs, and limbs using the same genetic mechanisms found in humans. However, for some reason those mechanisms have been turned off in most mammals. MDI Biological Laboratory scientists are learning how to reactivate these mechanisms to promote healing in humans.
   Despite these examples, it is generally accepted that adult mammals have limited regenerative capacity compared to most vertebrate embryos/larvae, adult salamanders and fish. But the regeneration therapy approach of Robert O. Becker, using electrical stimulation, has shown promising results for rats and mammals in general.
   The first organ ever induced and made in the lab was the bladder, which was created in 1999. In 2014, there had been various tissues regenerated by the 3d printer and these tissues included: the bladder, muscle, vagina, penis and the thymus.
  
  In 2015 researchers developed a proof of principle biolimb inside a laboratory; they also estimated that it would be at least a decade for any testing of limbs in humans. The limb demonstrated, fully functioning skin, muscles, blood vessels and bones.
   A new and rapidly evolving field of medicine surrounds the collection and distribution of tissues for reuse in human patients, including tissues as diverse as eyes, musculoskeletal tissue, eggs, sperm and blood stem cells. Experts agree that the field, in its infancy, has the ability to improve the quality of life, or in some cases preserve life for many patients.
   Regeneration in humans
  
  Regeneration in humans is the regrowth of lost tissues or organs in response to injury. This is in contrast to wound healing, which involves closing up the injury site with a scar. Some tissues such as skin and large organs including the liver regrow quite readily, while others have been thought to have little or no capacity for regeneration. However ongoing research, particularly in the heart and lungs, suggests that there is hope for a variety of tissues and organs to eventually become regeneration-capable.
  
  Section::::History of human tissue regeneration.
   It has been shown that bone marrow-derived cells could be the source of progenitor cells of multiple cell lineages, and a 2004 study suggested that one of these cell types was involved in lung regeneration. Therefore, a potential source of cells for lung regeneration has been found; however, due to advances in inducing stem cells and directing their differentiation, major progress in lung regeneration has consistently featured the use of patient-derived iPSCs and bioscaffolds.
   "C. teleta" is a segmented worm found in North America that is capable of regenerating posterior segments after amputation. This regeneration uses the interaction of several sets of "Hox" genes, as well as blastema formation. All of the "Hox" genes concerned in epimorphosis are present in the abdominal area of the worm, but not in the anterior portion. However, the genes do not, themselves, direct the anterior-posterior patterning of the worm's thorax.
  
  Section::::In invertebrates.:"Planaria vitta".
   In April 2019 researchers 3d printed a human heart. The prototype heart was made by human stem cells but only to the size of a rabbits heart. The researchers hope to one day use a scaled up version of the heart in humans.
  
  Section::::History of human tissue regeneration.:History of regeneration techniques.:Regeneration with drugs.
  
presuppositions: Humans could regenerate organs or limbs, but something is stopping it. Something prevents humans from regrowing organs.
corrections: Humans do not have the genes to regenerate organs or limbs. Regrowing organs is not expected behavior.
raw_presuppositions: Humans could regenerate organs or limbs, but something is stopping it. Something prevents humans from regrowing organs.
raw_corrections: Humans do not have the genes to regenerate organs or limbs. Regrowing organs is not expected behavior.