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A human arm contains on the order of a trillion cells across bone, cartilage, three-dimensional muscle groups with distinct innervation, vasculature down to capillaries, lymphatics, and skin with appendages. Even granting a functioning blastema, growth would have to proceed for years with correct patterning maintained throughout, in a tissue mass large enough that vascularization and metabolic supply become engineering problems in their own right. These are the same constraints that limit [[organ-bioprinting]] and [[tissue-engineering]].A human arm contains on the order of a trillion cells across bone, cartilage, three-dimensional muscle groups with distinct innervation, vasculature down to capillaries, lymphatics, and skin with appendages. Even granting a functioning blastema, growth would have to proceed for years with correct patterning maintained throughout, in a tissue mass large enough that vascularization and metabolic supply become engineering problems in their own right. These are the same constraints that limit [[organ-bioprinting]] and [[tissue-engineering]]. Clinically, limb loss is addressed by replantation where possible, by vascularized composite allotransplantation of donor hands and arms under lifelong immunosuppression, and by [[myoelectric-prosthetics]] anchored through [[osseointegration]]; where the deficit is weakness rather than absence, a powered [[exoskeleton]] substitutes for the missing force. Replacing individual tissues rather than regrowing them is the province of [[lab-grown-organs]]. Those approaches have improved steadily, while regeneration has not moved from salamanders to mammals.Clinically, limb loss is addressed by replantation where possible, by [[face-and-hand-transplantation|vascularized composite allotransplantation]] of donor hands and arms under lifelong immunosuppression, and by [[myoelectric-prosthetics]] anchored through [[osseointegration]]; where the deficit is weakness rather than absence, a powered [[exoskeleton]] substitutes for the missing force. Replacing individual tissues rather than regrowing them is the province of [[lab-grown-organs]]. Those approaches have improved steadily, while regeneration has not moved from salamanders to mammals. The research question that would most change the picture is narrow and answerable: whether a mammalian amputation site can be made to form a true blastema at all, above the level of the terminal phalanx. Nobody has demonstrated it, and until someone does, every proposal for regrowing a human limb is a proposal about a structure that has never existed in a mammal.The research question that would most change the picture is narrow and answerable: whether a mammalian amputation site can be made to form a true blastema at all, above the level of the terminal phalanx. Nobody has demonstrated it, and until someone does, every proposal for regrowing a human limb is a proposal about a structure that has never existed in a mammal. removed, struck through added, underlinedLine numbers count the serialised markdown of each revision, frontmatter included.
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