question: Why can’t we grow back major body parts?
comment: The other comment is the evolutionary reason, the molecular reason is that EVERY cell in your body with a genetic code baring rbc has the entire genetic blueprint. But to prevent every cell from doing all jobs the cells do something called packing and methylation, this process makes some parts of the gene inaccessible for translation and transcription. Therefore stopping expression. You dont want your brain to make muscle proteins. So when your body is done differentiating and it's over, your body also becomes specialized and only can so 1 function and those cells naturally dont revert to their pluripotent stages. Some animals have the ability to return this function and this results in them regrown limbs like axolotls.
passages: 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.
   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";
   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.
   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 three main causes of CNS injury: stroke, traumatic brain injury (TBI), or developmental complications. Strokes are classified as either hemorrhagic (when a vessel is damaged to the point of bleeding into the brain) or ischemic (when a clot blocks the blood flow through the vessel in the brain). When a hemorrhage occurs, blood seeps into the surrounding tissue, resulting in tissue death, while ischemic hemorrhages result in a lack of blood flow to certain tissues. Traumatic brain injury is caused by external forces impacting the cranium or the spinal cord. Problems with CNS development results in abnormal tissue growth during development, thus decreasing the function of the CNS.
   "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".
   Section::::Use of stem cells for spinal fusion.
   Because there are so many factors that contribute to the success or failure of neural tissue engineering, there are many difficulties that arise in using neural tissue engineering to treat CNS and PNS injuries. First, the therapy needs to be delivered to the site of the injury. This means that the injury site needs to be accessed by surgery or drug delivery. Both of these methods have inherent risks and difficulties in themselves, compounding the problems associated with the treatments. A second concern is keeping the therapy at the site of the injury. Stem cells have a tendency to migrate out of the injury site to other sections of the brain, thus the therapy is not as effective as it could be as when the cells stay at the injury site. Additionally, the delivery of stem cells and other morphogens to the site of injury can cause more harm than good if they induce tumorigenesis, inflammation, or other unforeseen effects. Finally, the findings in laboratories may not translate to practical clinical treatments. Treatments are successful in a lab, or even an animal model of the injury, may not be effective in a human patient.
   Mesenchymal stem cells do not need to come from fetuses, so avoid difficulties around ethics; they come from tissues including bone marrow, adipose tissue, the umbilical cord. They are thought to be pluripotent. Unlike other types of stem cells, mesenchymal cells do not present the threat of tumor formation or triggering an immune system response. Animal studies with injection of bone marrow stem cells have shown improvement in motor function; however not so in a human trial a year post-injury. More trials are underway. Adipose and umbilical tissue stem cells need further study before human trials can be performed, but two Korean studies were begun to investigate adipose cells in SCI patients.
   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.
   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 
   Section::::Research.:Unilateral perforant pathway transection in rats and mice.
   The results of gaining motor function cannot guarantee results. The amount of scarring and size of the gap influences the results of the repair. The larger the gap, the less likelihood of recovery because more axon would have to grow further and astrocytes could develop over the longer length of time it takes to grow than with a shorter gap. In order to reduce the length of time for regeneration, further research is being conducted to speed regeneration such as nerve grafts and stem cells.
  
  Section::::Approach.
   Induced parthenogenesis in mice and monkeys often results in abnormal development. This is because mammals have imprinted genetic regions, where either the maternal or the paternal chromosome is inactivated in the offspring in order for development to proceed normally. A mammal created by parthenogenesis would have double doses of maternally imprinted genes and lack paternally imprinted genes, leading to developmental abnormalities. It has been suggested that defects in placental folding or interdigitation are one cause of swine parthenote abortive development. As a consequence, research on human parthenogenesis is focused on the production of embryonic stem cells for use in medical treatment, not as a reproductive strategy.
   Incorporation success of 3D scaffolds into the CNS has been shown to depend on the stage at which the cells have differentiated. Later stages provide a more efficient implantation, while earlier staged cells need to be exposed to factors that coerce the cells to differentiate and thus respond appropriately to the signals the cells will receive at the CNS injury site. Brain-derived neurotrophic factor is a potential co-factor to promote functional activation of ES cell-derived neurons into the CNS injury sites.
  
  Section::::PNS.
  
  Section::::PNS.:Causes of PNS injury.
   Section::::Challenges.
  
  Glial scars can have detrimental effects to neuronal regrowth to aide in the restoration of sensory function. Astrocytes form a barrier preventing further growth by forming gap junctions along with producing molecules that chemically prevent axon extension.
  
  Tension in the rejoined nerve may stretch the epineurium and rip the two parts from each other again. Even tension along the sutured joint is necessary for an even repair.
   After injury, neurons in the adult peripheral nervous system can switch from a dormant state with little axonal growth to robust axon regeneration. DNA demethylation in mature mammalian neurons removes barriers to axonal regeneration. This demethylation, in regenerating mouse peripheral neurons, depends upon TET3 to generate 5-hydroxymethylcytosine (5hmC) in DNA. 5hmC was altered in a large set of regeneration-associated genes (RAGs), including well-known RAGs such as "Atf3", "Bdnf", and "Smad1", that regulate the axon growth potential of neurons.
  
  Section::::See also.
  
  BULLET::::- DNA methylation
  
  BULLET::::- Demethylating agent
  
  Section::::External links.
  
  BULLET::::- "Cell Research" (Nature) for 5-fC and 5-caC antibodies. 
   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.
   Pattern formation in the morphogenesis of an animal is regulated by genetic induction factors that put cells to work after damage has occurred. Neural cells, for example, express growth-associated proteins, such as GAP-43, tubulin, actin, an array of novel neuropeptides, and cytokines that induce a cellular physiological response to regenerate from the damage. Many of the genes that are involved in the original development of tissues are reinitialized during the regenerative process. Cells in the primordia of zebrafish fins, for example, express four genes from the homeobox "msx" family during development and regeneration.
  
  Section::::Tissues.
   In fact, when higher expression of CMAH is forced in mouse brains, it proves lethal. Human brains are different from most primate brains in that they continue to grow for some time postnatally, unlike primate brains that stop growing soon after birth. It is therefore possible that even the small amounts of Neu5Gc present in most mammalian brains could be inhibiting their brain growth; losing the CMAH gene may have released the human brain from this constraint.
   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.
   Section::::Assembly methods.
  
  One of the continuing, persistent problems with tissue engineering is mass transport limitations. Engineered tissues generally lack an initial blood supply, thus making it difficult for any implanted cells to obtain sufficient oxygen and nutrients to survive, or function properly.
  
  Section::::Assembly methods.:Self-assembly.
   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.
   Regeneration indicates the ability to regrow a missing part. This is very prevalent amongst plants, which show continuous growth, and also among colonial animals such as hydroids and ascidians. But most interest by developmental biologists has been shown in the regeneration of parts in free living animals. In particular four models have been the subject of much investigation. Two of these have the ability to regenerate whole bodies: "Hydra", which can regenerate any part of the polyp from a small fragment, and planarian worms, which can usually regenerate both heads and tails. Both of these examples have continuous cell turnover fed by stem cells and, at least in planaria, at least some of the stem cells have been shown to be pluripotent. The other two models show only distal regeneration of appendages. These are the insect appendages, usually the legs of hemimetabolous insects such as the cricket, and the limbs of urodele amphibians. Considerable information is now available about amphibian limb regeneration and it is known that each cell type regenerates itself, except for connective tissues where there is considerable interconversion between cartilage, dermis and tendons. In terms of the pattern of structures, this is controlled by a re-activation of signals active in the embryo.
   The need for neural tissue engineering arises from the difficulty of the nerve cells and neural tissues to regenerate on their own after neural damage has occurred. The PNS has some, but limited, regeneration of neural cells. Adult stem cell neurogenesis in the CNS has been found to occur in the hippocampus, the subventricular zone (SVZ), and spinal cord. CNS injuries can be caused by stroke, neurodegenerative disorders, trauma, or encephalopathy. A few methods currently being investigated to treat CNS injuries are: implanting stem cells directly into the injury site, delivering morphogens to the injury site, or growing neural tissue "in vitro" with neural stem or progenitor cells in a 3D scaffold. Proposed use of electrospun polymeric fibrous scaffolds for neural repair substrates dates back to at least 1986 in a NIH SBIR application from Simon. For the PNS, a severed nerve can be reconnected and reinnervated using grafts or guidance of the existing nerve through a channel.
  