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From my understanding melanocyte stem cells (McSCs) stop being produced and we have a finite reserve that gets depleted with time. They're at the origin of melanocytes that create hair and skin pigments. Our hair turns white when we don't have any more McSCs. So why does our skin stay the same color with aging?

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  • $\begingroup$ Welcome to Biology.SE! Please take the tour and then go through the help pages starting with How to Ask questions effectively on this site and edit your question accordingly. In particular, please add some more details with references to reliable sources (e.g. You say MeSCs stop being produced and are depleted with age. Where did you read this? At what age do they stop renewing themselves? How long do they typically live?) Thanks! 😊 $\endgroup$ – tyersome Jan 28 at 4:37
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    $\begingroup$ Nice question. Will write an answer later, when I have some time. $\endgroup$ – Chris Jan 28 at 8:12
  • $\begingroup$ I thought it was common knowledge among biologists that melanocyte stem cells don't get regenerated. I could throw this link where you can read: "After just a few days, all of the pigment-regenerating stem cells were lost. Once they're gone, you can't regenerate pigment anymore. The damage is permanent." $\endgroup$ – borilla Feb 6 at 2:16
  • $\begingroup$ whether they get depleted or not, the question remains. Hope @Chris gets some time for a basic answer $\endgroup$ – borilla Feb 6 at 2:20
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The answer to this question has its reason in the hair cycle. Our hair goes through a cycle of growth. At the end of this cycle, the cells in the hair follicle die and have to be replenished before a new hair cycle starts, also the hair falls out eventually. See the figure from reference 1:

enter image description here

The replenishment process is done by to two cell types: Epithelial stem cells which replenish the hair follice itself and melanocyte stem cells (MCSC) which replenish the melanocytes in the follicle and produce the pigment of the hair.

The MCSC are located in the bulge region of the hair, to be able to successfully replenish the pool of pigment cells the MCSC need to do things: proliferate and also differentiate. Proliferation makes sure that there are enough cells in the bulge region to repeat this process in further hair cycles, while the differentiation generates the cells who actually produce the pigment. When the MCSC are lost, no further pigment producing cells and hence no pigment can be made.

It has now shown that with age this pool of MCSC is not replenished completely anymore, eventually resulting in the loss of the melanocyte stem cells. The reason that our skin does not become gray at some point is that this process doesn't happen there, so pigment cells are not depleted. Reference 1 summarizes this topic nicely, while reference 2 is the original paper.

References

  1. Melanocyte Stem Cell Maintenance and Hair Graying
  2. Mechanisms of Hair Graying: Incomplete Melanocyte Stem Cell Maintenance in the Niche
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  • $\begingroup$ If I understand correctly, after a certain age hair follicles are not replenished with MCSC. But MCSC are still present somewhere in the skin to differentiate into the melanocytes of the basal layer . And these basal layer melanocytes don't last forever so they need to be replaced. If this is correct then my question is: where are those MCSC that still create melanocytes for the basal skin layer? $\endgroup$ – borilla Apr 7 at 18:36
  • $\begingroup$ MCSC need to proliferate before they can differentiate into melanocytes. This happens to rise the number of cells present in the bulge region and to avoid the depletion of the stem cell niche. If the would only differentiate, then there are no cells left after a few hair cycles. So actually the stem cell niche is empty when you have no pigment anymore. $\endgroup$ – Chris Apr 7 at 19:06
  • $\begingroup$ humm I still don't understand where the melanocytes in the basal layer of the skin come from after all your hair have turned gray. I read both articles and I can't find an explanation on the net. Maybe I am misunderstanding the whole process... $\endgroup$ – borilla Apr 7 at 20:06

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