4 ms·

Thanks for posting this, I thought it was fascinating research. This sentence in particular was a surprise to me: It’s estimated that we’re born with around 20

by adevine 10y ago

Thanks for posting this, I thought it was fascinating research. This sentence in particular was a surprise to me: It’s estimated that we’re born with around 20,000 blood stem cells, and at any one time, around 1000 are simultaneously active to replenish blood. I was surprised the number was so low. Interesting to think this relatively small number of little factories has to sustain us throughout life.

jrapdx3 10y ago
That is a low number, but not the only example of limited resources in our systems. Dopamine neurons in the brain are essential for normal functioning. Parkinson's develops when dopamine neurons die off with age or due to other injury (toxins, trauma, etc.). It takes only a 15% loss of these irreplaceable cells for the condition to begin to show up. In contrast to the cortex of the brain with its 100's of millions of neurons, we are born with only 250K dopamine cells which are never regenerated. That's a narrow margin to rely on, we skate on very thin ice throughout our lives. Nature is the staunchest conservative and doesn't throw away the things that work. Our bodies still retain the archaic mechanisms of the earliest multicellular organisms in our brains and bodies. Looks like blood forming tissues are similarly following Nature's way.
- btilly 10y ago
  Good evolution and good engineering principles are at odds. In engineering an idea that has been found to work in one place gets adapted in another. Also you often build reliability through adding layers of redundancy. Evolution has no straightforward way for good ideas to be identified and reapplied to another system. It has to evolve again. Also if you have two fully redundant systems, there is nothing to stop deleterious mutations from disrupting one of them. The end result is that one of the redundant systems will become unreliable, and then the other one gets maintained by selection pressure. (This is also why creatures that live in a cave eventually become blind, and parasites lose their ability to use now unnecessary body parts. There is no engineering reason why this would happen, but is a natural result of selection pressures being eliminated.)
  - lucidrains 10y ago
    Brilliant insights
  - enraged_camel 10y ago
    Yep. This is actually one of the better arguments against Intelligent Design: the human body (and in fact all living things) has too many faults and shortcomings to have been designed by an omnipotent, omniscient being. (Of course, it could be argued that said shortcomings are intentional and were part of the design specs, but considering how much pain and suffering they cause (e.g. cancer), it would go against the claim that the designer is omnibenevolent).
  - antisthenes 10y ago
    "Good evolution" is essentially devoting the maximum amount of (usually very limited) resources towards successful procreation and rearing offspring. Hence the "features" or "redundancy" which would otherwise consume resources and not significantly affect procreation chances (like your cave example) eventually get selected against and disappear. "Good Engineering" on the other hand usually devotes additional resources to redundancy, fault-tolerances, which do not necessarily offer a competitive advantage in procreation.
    - datenwolf 10y ago
      > "Good evolution" is essentially devoting the maximum amount of (usually very limited) resources towards successful procreation and rearing offspring. It's not that simple. If producing the maximum amount of offspring were the goal, populations of a species would quickly collapse. It's also a poor principle to explain social behavior and organisms living in hives or strictly hierarchically procreating groups. A much better model principle would be, that evolution optimizes for maximum probability of the _offspring_ to successfully procreate. By adding this layer of indirection a lot more of what we can observe in biology can be explained with less presumptions (Occam's Razor).
      - antisthenes 10y ago
datenwolf 10y ago
I can't help it, but as a physicist the first thing I look at are orders of magnitude and relations in orders of magnitude. And given the total number of cells in a (human) body (~10^13 according to https://www.ncbi.nlm.nih.gov/pubmed/23829164 https://www.ncbi.nlm.nih.gov/pubmed/23829164) compared to the number of stem cells stated in the article that'd would be 10 orders of magnitude in between. Just talking about red blood cells it's still a staggering 10^12 / 10^3, i.e. 9 orders of magnitude. And for white blood cells it's 10^9/10^3, i.e. 6 orders of magnitude. Next question: Does the number of stem cells scale with body mass? Because if that's the case then I give you the Naked Mole Rat (aka the penis with teeth https://en.wikipedia.org/wiki/Naked_mole-rat https://en.wikipedia.org/wiki/Naked_mole-rat ). The Naked Mole Rat is _THE_ model organism for longevity, completely defying extrapolation of ageing models. The average Naked Mole Rat weighs about 30g, the average human weighs about 75kg. So a Naked Mole Rat hat about 1/2000th the body mass of a human. Assuming a mass proportional stem cell count that would make about 1000 stem cells in total and 0.5 stem cells replenishing blood cells at any given moment. Where does these 20000 and 1000 figures come from in the first place? Given my personal experience in TA-ing students of medicine and biology I don't have a lot of confidence in their calculation abilities. These students manage to get their figures wrong by several orders of magnitude on a regular base. So I'd not be surprised if these figures are grossly underestimated.

by adevine 10y ago

jrapdx3 10y ago

That is a low number, but not the only example of limited resources in our systems. Dopamine neurons in the brain are essential for normal functioning. Parkinson's develops when dopamine neurons die off with age or due to other injury (toxins, trauma, etc.). It takes only a 15% loss of these irreplaceable cells for the condition to begin to show up. In contrast to the cortex of the brain with its 100's of millions of neurons, we are born with only 250K dopamine cells which are never regenerated. That's a narrow margin to rely on, we skate on very thin ice throughout our lives. Nature is the staunchest conservative and doesn't throw away the things that work. Our bodies still retain the archaic mechanisms of the earliest multicellular organisms in our brains and bodies. Looks like blood forming tissues are similarly following Nature's way.

btilly 10y ago
Good evolution and good engineering principles are at odds. In engineering an idea that has been found to work in one place gets adapted in another. Also you often build reliability through adding layers of redundancy. Evolution has no straightforward way for good ideas to be identified and reapplied to another system. It has to evolve again. Also if you have two fully redundant systems, there is nothing to stop deleterious mutations from disrupting one of them. The end result is that one of the redundant systems will become unreliable, and then the other one gets maintained by selection pressure. (This is also why creatures that live in a cave eventually become blind, and parasites lose their ability to use now unnecessary body parts. There is no engineering reason why this would happen, but is a natural result of selection pressures being eliminated.)
- lucidrains 10y ago
  Brilliant insights
- enraged_camel 10y ago
  Yep. This is actually one of the better arguments against Intelligent Design: the human body (and in fact all living things) has too many faults and shortcomings to have been designed by an omnipotent, omniscient being. (Of course, it could be argued that said shortcomings are intentional and were part of the design specs, but considering how much pain and suffering they cause (e.g. cancer), it would go against the claim that the designer is omnibenevolent).
- antisthenes 10y ago
  "Good evolution" is essentially devoting the maximum amount of (usually very limited) resources towards successful procreation and rearing offspring. Hence the "features" or "redundancy" which would otherwise consume resources and not significantly affect procreation chances (like your cave example) eventually get selected against and disappear. "Good Engineering" on the other hand usually devotes additional resources to redundancy, fault-tolerances, which do not necessarily offer a competitive advantage in procreation.
  - datenwolf 10y ago
    > "Good evolution" is essentially devoting the maximum amount of (usually very limited) resources towards successful procreation and rearing offspring. It's not that simple. If producing the maximum amount of offspring were the goal, populations of a species would quickly collapse. It's also a poor principle to explain social behavior and organisms living in hives or strictly hierarchically procreating groups. A much better model principle would be, that evolution optimizes for maximum probability of the _offspring_ to successfully procreate. By adding this layer of indirection a lot more of what we can observe in biology can be explained with less presumptions (Occam's Razor).
    - antisthenes 10y ago

datenwolf 10y ago

I can't help it, but as a physicist the first thing I look at are orders of magnitude and relations in orders of magnitude. And given the total number of cells in a (human) body (~10^13 according to https://www.ncbi.nlm.nih.gov/pubmed/23829164 https://www.ncbi.nlm.nih.gov/pubmed/23829164) compared to the number of stem cells stated in the article that'd would be 10 orders of magnitude in between. Just talking about red blood cells it's still a staggering 10^12 / 10^3, i.e. 9 orders of magnitude. And for white blood cells it's 10^9/10^3, i.e. 6 orders of magnitude. Next question: Does the number of stem cells scale with body mass? Because if that's the case then I give you the Naked Mole Rat (aka the penis with teeth https://en.wikipedia.org/wiki/Naked_mole-rat https://en.wikipedia.org/wiki/Naked_mole-rat ). The Naked Mole Rat is _THE_ model organism for longevity, completely defying extrapolation of ageing models. The average Naked Mole Rat weighs about 30g, the average human weighs about 75kg. So a Naked Mole Rat hat about 1/2000th the body mass of a human. Assuming a mass proportional stem cell count that would make about 1000 stem cells in total and 0.5 stem cells replenishing blood cells at any given moment. Where does these 20000 and 1000 figures come from in the first place? Given my personal experience in TA-ing students of medicine and biology I don't have a lot of confidence in their calculation abilities. These students manage to get their figures wrong by several orders of magnitude on a regular base. So I'd not be surprised if these figures are grossly underestimated.