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Why are you not as old as your parents when you were born?

   For some commonplace phenomena in life, it's okay if we don't ask, but if we ask, we may ask about university questions. for example. Before Newton, no one asked a "why" about the phenomenon that "objects thrown into the air will fall back to the ground", but Newton asked, and thus proposed the law of universal gravitation.

a seemingly funny question


  In biology, there is a similar question: why doesn't every baby grow old like his/her parents? Why do children born to parents of any age have the same starting point physiologically, instead of—for example, children born to 40-year-old parents have a biologically different start than children born to 20-year-old parents? How about older? Because the previously sensational cloned sheep "Dolly" died within a few years because its biological age was the same as the age of the sheep that provided the cells.

  You might think I'm joking when you ask such a question. But I'm serious, and here's my reasoning: As we age, the DNA of all the cells in our bodies also accumulates damage over time—it's what makes us age, and germ cells are no exception. Now that our offspring develop from fertilization of these germ cells, I see no reason why they should not "inherit" these DNA lesions. In other words, if a couple had a child in their 20s and another couple had a child in their 40s, there should be a biological gap of more than 20 years when their children are born. But why didn't this happen?

  I am not the first to ask this question; biologists have long asked it as a mystery to be solved. What I want to say now is that recently there has been a tentative answer to this mystery.

markers of cellular senescence


  At first, scientists thought that sex cells (sperm or eggs) might not age. In other words, a person's somatic DNA is accumulating damage—reflecting our bodies' aging—but the sex cells are protected by mechanisms against time. If this answer is true, then the above problem will be solved. Unfortunately, this is wrong. Eggs and sperm also show signs of aging, research shows.

  Therefore, scientists have to make another hypothesis: after the fertilized egg develops into an embryo, at a certain point in time, the embryonic cells clear or repair the DNA damage "inherited" from the parents, thereby resetting the age and rejuvenating.

  So when do embryos reset their age? Recently, Vadim Gladshev of Brigham and Women's Hospital in Boston, USA, studied this question.

  Damage associated with aging often manifests itself in changes in epigenetic marks on cellular DNA. Epigenetic marks are chemical substances (such as methyl groups) attached to DNA that do not change the DNA itself, only the activity of genes on the DNA. For example, they are like gene switches that can disable a previously active gene.

  Epigenetic marks are influenced by environment and lifestyle. What's more, scientists have long discovered that a person's epigenetic markers are more reflective of how old he is than his usual age. For example, a 60-year-old person who looks like he's only 50 years old, if you check his epigenetic markers, he will indeed match the epigenetic markers of a 50-year-old person. So, epigenetic marks can be used to track cellular aging.

"Rejuvenation" of embryonic cells


  Gladshev and his colleagues studied changes in epigenetic marks during embryonic development in mice. They found that when the fertilized egg is formed, the fertilized egg is indeed aged as much as the age of the parents; the aging continues for the next few days; By the time the spheres were hollowed out, the aging process had started to reverse; by around day 10, the aging had been minimal. At this time, the DNA damage "inherited" from the parents has been cleared. In other words, the embryo at this time has completely "rejuvenated". After that, as an individual with a biological age of zero, the normal aging process begins.

  This is what was observed in mice. Since it is ethically forbidden to study the early development of human embryos, we still don't know when human embryos complete "rejuvenation", but the whole process should not be far from that of mice.

  Of course, this is only the first step, and we still have many questions to answer: What mechanism drives the embryonic cells to reset their age? Are there specific genes that drive this process? Do all organisms reset the age of their embryos in this way? Studying these may provide reference or help for how we can slow down the aging process.



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