Insistence of Vision by David Brin

animal’s own stem cells, insert the replacement....
    Whereupon, voila. Step back, and witness a miracle! After some trial and error... and much to my astonishment... George proved right. In those first esophagi we implanted – and in subsequent human tests – my fine patterns of specific growth factors proved unnecessary. No need to command them specifically: “You become a mucus lining cell, you become a support structure...” Somehow, the stem cells divided, differentiated, divided again, growing into a complete adult esophagus. And they did it within the patient!
    “How do they know?” I asked, despite expecting in advance what George would say.
    “They don’t know, Beverly. Each cell is reacting only to its surroundings. To chemical messages and cues from its environment, especially its immediate neighbors. And it emits cues to affect them , as well. Each one is acting as a perfect – if complicated – little...”
    “...cellular automaton. Yes, yes.”
    Others, watching us finish each others’ sentences, would liken us to an affectionate old married couple. Few noticed the undercurrent of scorching rivalry.
    “So,” I continued, “just by jostling against each other in the geometric-chemical pattern of the scaffold, that alone is enough for them to sort themselves out? Differentiating into dozens of types, in just the right geometry?”
    “Geometry, yes.” George nodded vigorously. “Geometrical chemistry. I like that. Good. It’s how cells sort themselves into vastly complex patterns, inside a developing fetal brain. But of course you see what all of this means.”
    He gestured along a row of lab benches at more recent accomplishments, each carefully tended by one or more students.
    – a functioning liver, grown from scaffolding inside a mouse, till we carved it out. The organ now lay in vitro, still working , fed by a nearby blood pump –
    – a cat whose lower intestines had been replaced by polygel tubing... that was now completely lined with all the right cells: in effect two meters of fully functioning gut –
    – two dozen rats with amputated fore-legs, whose stumps were encased in gel-capsules. Along simple frameworks, new limbs could be seen taking shape as the creature’s own cells (with a little coaxing from my selected stem-sims) migrated to correct positions in a coalescing structure of flesh and linear bone. Lifting my gaze, I saw cages where older creatures hobbled about on regrown appendages. So far, they were clumsy, club-like, footless things. Yet, they were astonishing.
    And yes, George, I saw what it meant.
    “We always assumed that mammals had lost the ability to regenerate organs, because it doesn’t happen in nature. Reptiles, amphibians and some fish can regrow whole body parts. But mammals in the wild? They... we... can only do simple damage control, covered by scar tissue.”
    “But if we prevent scarring,” he prompted. “If we lay down scaffolds and nutrient webs –”
    “– then yes, There emerges a level of self-repair far more sophisticated than we ever imagined possible in mammals.”
    I shook my head. “But it makes no sense! Why retain a general capability when nature never supplies the conditions to use it? Only when we provide the right circumstances in our lab, only then do these abilities emerge.”
    George pondered a moment.
    “Beverly, I think you’re asking the wrong question. Have you ever wondered: why did mammals lose... or give up... this ability in the first place?”
    “Of course I have! The answer is obvious. With our fast metabolisms, we have to eat a lot. No mammal in the wild can afford to lay around for weeks, even months, the way a reptile can, while waiting for a major limb or organ to regrow. He’d starve long before it finished. Better to concentrate on things mammals are good at, like speed, agility and brains, to avoid getting damaged in the first place. Mammalian regeneration probably vanished back in the Triassic, over a