When a hobby-store owner in Cincinnati sliced off his fingertip in 2005 while showing a customer why the motor on his model plane was dangerous, he went to the emergency room without the missing tip. He couldn't find it anywhere. The doctor bandaged the wound and recommended a skin graft to cover the top of his right-middle stub for cosmetic purposes, since nothing could be done to rebuild the finger.
Months later, he had regrown it, tissue, nerves, skin, fingernail and all.
This particular hobbyist happened to have a brother in the tissue-regeneration business, who told him to forego the skin graft and instead apply a powdered extract taken from pig's bladder to the raw finger tip. The extract, called extracellular matrix, lays the framework that cells use to generate any given body part. It's like a cellular scaffolding, and all animals have it. It holds the signals that direct cells to divide, differentiate and build themselves into a specific form.
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