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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. ==History of human tissues regeneration== In humans with non-injured tissues, the tissue is naturally regenerated over time; by default these tissues have new cells available to replace expended cells. For example, the body regenerates a full bone within 10 years, while non-injured skin tissue is regenerated within two weeks.〔 With injured tissue, the body usually has a different response – this emergency response usually involves building a degree of scar tissue over a time period longer than a regenerative response, as has been proven clinically and via observation. There are however 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 be used in the near future to regenerate any tissue in the human body. ;Regeneration with materials Generally humans, in vivo, can regenerate injured tissues for limited distances. The maximum induced key diffusion distance of regeneration, in 2009, was roughly 1 cm;〔 this regeneration distance standard was achieved, induced and aided by the use of materials that could bridge the wound; this material bridge induced regenerative cells to flow across the wound gap, whereby it then degraded. This technology was first used inside a broken urethra in 1996.〔〔 In 2012, using materials, a full urethra was restored in vivo.〔 ;Regeneration by 3d printing 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: * Level one, ''flat tissue'' like skin was the simplest to recreate;〔 * Level two was ''tubular structures'' such as blood vessels;〔 * Level three was ''hollow non-tubular structures'';〔 * Level four was ''solid organs'', 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. The first organ ever induced and made in the lab was the bladder, which was created in 1999. In 2014, organs that have been various tissues regenerated by the printer; these tissues include 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 a least a decade for any testing of limbs in humans. The limb demonstrated, fully functioning skin, muscles, blood vessels and bones. ==Naturally regenerating appendages and organs== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Regeneration in humans」の詳細全文を読む スポンサード リンク
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