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You are a mosaic of mutations
A cloned start
A person gets 23 chromosomes from each parent, and at birth every cell is a clone with the same information. DNA copying is not perfect, and skin cells may pick up somewhere between 10 and 30,000 mutations per cell per day from basic sun exposure. Against a genome with terabytes of information each hit is small, but across years no cell stays identical to the one next to it, like the intricate tile in Morocco.
Some cells last unusually long. Neurons in the brain and spinal cord are mostly the ones a person was born with, and their DNA keeps collecting mutations depending on how deep they sit.
Where stem cells hide
The stem cells that renew the immune system live in the hollow long bones, and the marrow is thought to be their home because it protects them from radiation and chemical cues that cause mutations. The skull and spine shelter neurons in the same way.
Pruning by immunity
The immune system also prunes the results. Around age 40 or 50 these white spots appear on the skin, and they are thought to be places where the immune system sensed a cluster of precancerous cells and wiped out the pigment-making melanocytes there. That area is white instead of as dark as the rest of the skin.
Why do skin cells gather so many mutations?
Ultraviolet light fuses neighboring letters of DNA into abnormal bonds, and the cell's repair machinery fixes most but not all of them. Errors that slip through are copied when the cell divides and are inherited by its descendants. A 2015 study in Science by Inigo Martincorena and colleagues examined normal sun-exposed eyelid skin and found small patches of cells carrying mutations also seen in skin cancers. Healthy-looking skin is therefore a patchwork of genetically distinct clones.
How does bone marrow protect its stem cells?
Blood-forming stem cells sit in specialized niches in the marrow, many of them in low-oxygen pockets. Low oxygen reduces the oxidative damage that can harm DNA. These stem cells also stay mostly dormant and divide rarely, which gives errors fewer chances to occur during copying. The bone around them blocks a large share of outside radiation.
Why do neurons keep their mutations?
Most neurons stop dividing during development and last for life. Without cell division to dilute or remove damage, any mutation that happens in a neuron stays in that neuron. A 2018 study in Science by Michael Lodato and colleagues sequenced single human neurons and found that the number of mutations rose with age. Each neuron therefore carries its own set of changes.
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