Why do children not inherit their parents' old age?

Every person starts as a single cell. It divides into two, then four, eight, sixteen – and gradually forms an organism made up of tens of trillions of cells.

But the egg cell and sperm also come from other cells. Those, in turn, appeared as a result of previous divisions. This continuous biological chain stretches through parents, grandparents, and millions of years into the past.

At the same time, we know that cells age: they accumulate DNA damage, manage genes less effectively, lose mitochondrial efficiency, and their telomeres shorten. If each new generation of cells inherits traces from the previous one, humanity should age not only during an individual's life but also from generation to generation.

But that does not happen. A child starts life young, even though the germ cells of the parents cannot be called completely new and undamaged. Between their aging and the development of a new organism, a large-scale restructuring occurs that partially resets the biological clock.

Division itself does not make a cell old

Aging cannot be reduced to the number of cell divisions. Skin, blood, and intestinal cells are constantly renewed, and without this, a person could not survive for even a few weeks.

The problem lies in the gradual accumulation of changes. Errors occur during DNA copying, and the genetic material is affected by reactive oxygen species, radiation, chemicals, and ordinary by-products of metabolism. Proteins lose their correct shape, the cell struggles to rid itself of intracellular debris, and mitochondrial efficiency declines.

The epigenome – a system of chemical tags that regulates gene activity – also changes. The DNA sequence itself may remain the same, but some regions begin to activate at the wrong time, while others become silent. The cell gradually loses the accuracy of its internal program.

The number of divisions affects this process but does not fully explain it. Aging is more akin to the accumulation of errors in a complex system than simple wear and tear of a mechanical part.

Somatic cells and germ line

The cells of the human body can be conditionally divided into two groups.

Somatic cells form the skin, muscles, liver, heart, brain, and other organs. They maintain the life of a specific individual but do not pass on to the next generation.

The germ line consists of cells that ultimately develop into egg cells and sperm. Through them, genetic information is passed to offspring.

From an evolutionary standpoint, the body serves as a temporary carrier of the germ line: it grows, protects itself, finds a mate, and passes on genes. However, belonging to the germ line does not provide cells with absolute protection against aging. Egg cells and sperm also accumulate changes – just in different ways.

The egg cell ages even without constant divisions

Most future egg cells in a woman begin to form during her intrauterine development. These cells enter a special type of division – meiosis – and then stop at one of its stages.

In this state, the egg cell can exist for decades. A cell that participates in conception at 35 or 40 years old may have existed in the woman's body even before her birth.

It hardly divides but still ages. Over time, mitochondrial function declines, metabolism changes, and the molecular system that keeps chromosomes in the correct position weakens.

In this system, the protein complex cohesin is important. It acts like rings or clasps that bind chromosomes. A significant portion of these structures is formed during intrauterine development and then is hardly renewed. After several decades, cohesin may work less effectively, resulting in incorrect chromosome distribution during egg cell maturation.

This leads to cells with an extra or missing chromosome. This is one of the reasons why the likelihood of infertility, early pregnancy loss, and certain chromosomal disorders in the embryo increases with a woman's age. The main mechanisms of this process are already known, although the details of age-related declines in egg quality continue to be studied.

The egg cell is not a perfectly preserved capsule of youth. Long waiting leaves its marks.

In sperm, errors accumulate differently

Sperm does not store in a ready state for decades. They are constantly produced from spermatogonial stem cells, which repeatedly divide and copy DNA.

Copying occurs with high accuracy but not without errors. Hence, with advancing paternal age, there is, on average, an increase in new genetic changes in germ cells.

A large study of family genomes showed that the number of new mutations from the paternal germ line increased by about 1.5 mutations for each year of paternal age. The mother's age was also associated with additional mutations, but the effect in the studied sample was weaker – about 0.37 mutations per year.

This does not mean that late fatherhood will necessarily lead to child illness. Most new mutations do not manifest in any way or occur in genome areas that do not cause significant harm. However, it is impossible to completely avoid the accumulation of changes.

By the time of fertilization, both the egg cell and sperm carry individual mutations, molecular damage, and other traces of the parents' age.

Restructuring begins after fertilization

The merging of the egg and sperm forms a zygote – the first cell of the new organism. It does not simply combine two sets of parental DNA and continue operating with the previous settings.

The genomes of the parents undergo extensive epigenetic reprogramming. A significant portion of the chemical tags that regulate gene activity is removed or rearranged. This frees the embryonic cells from many programs associated with the prior state of the egg and sperm, restoring their ability to form different tissues.

It is the epigenetic settings that help the liver cell remain a liver cell, and the neuron remain a neuron, although their DNA sequences are almost identical. An early embryo does not yet need such specialization: its cells must have the opportunity to become muscles, nerves, blood, skin, and organs.

However, erasure is not complete. Some tags need to be preserved. For example, the activity of certain genome regions depends on whether they come from the mother or the father. This phenomenon is called genomic imprinting.

And yet, the scale of the changes is immense. The zygote translates the parental genetic material into a special initial state.

The point of biological zero

Scientists have learned to approximately determine the biological age of cells based on characteristic changes in DNA and gene activity. These methods are referred to as epigenetic and molecular clocks.

Research on early embryonic cells using these methods has shown that immediately after fertilization, the measurable age is not necessarily equal to zero. Some age-related traits of the parental germ cells still persist in the cell. Then, as early development progresses, these indicators decline.

In a study using data from mice and humans, scientists identified a point of minimal biological age at an early stage of embryonic development. The authors called it ground zero — a conditional biological zero. After passing this point, age begins to increase again.

It is likely that the new organism does not become completely young at the moment of fertilization but goes through a brief period of natural rejuvenation. During this time, the cellular system is restructured and reaches its youngest state.

Such results should be approached with caution. Epigenetic clocks measure specific molecular changes but do not necessarily reflect all types of cellular damage. They do not identify a single switch that manages age. Yet the deep restructuring of early embryonic cells is beyond doubt.

The embryo restores telomeres

Telomeres are repeating DNA sequences at the ends of chromosomes that protect important genetic information. In many somatic cells, they shorten during division. When telomeres become too short, the cell loses its ability to divide normally or enters a state of aging.

If children simply inherited shortened telomeres from parents, the reserve of cellular divisions would decrease with each generation. However, during early embryonic development, telomeres can lengthen.

Research on mammals shows that first their length is restored by special mechanisms, and later the activity of the enzyme telomerase increases. The embryo gains a new reserve of strength.

Telomere length varies among different children and can depend on genetics, the mother’s health, and conditions of early development. It is not about identical and absolute resetting. The system merely prevents telomeres from shortening indefinitely from generation to generation.

Why the same mechanism cannot simply be activated in adults

For the early embryo, losing cellular specialization is necessary. Its cells must become versatile enough to form the entire organism.

In the adult body, such restructuring is dangerous. A heart cell must remain a heart cell. If it loses its specialization and reverts to too primitive a state, the tissue will stop functioning properly. Uncontrolled division and loss of cellular identity are also associated with the risk of tumors.

Scientists are exploring partial reprogramming. The idea is to return some youthful properties to cells but not erase their specialization entirely. Experiments on cells and animals have already yielded interesting results, but safe rejuvenation of humans is still far off.

Nature restructures one cell from which an organism is yet to be created. At the same time, conducting such a process in the trillions of cells of an established body is much more complex.

Young does not mean flawless

Embryonic rejuvenation does not fix all possible damage. Epigenetic tags can be restructured, telomeres can be lengthened, and damaged molecules and cellular structures can be replaced. But true mutations in the DNA sequence do not necessarily disappear with reprogramming.

A child receives genetic variants from parents, including some harmful ones. New mutations can also arise in the egg, sperm, or during the first divisions of the embryo.

There is also no complete resetting of mitochondria. Almost all are inherited through the egg, so changes in mitochondrial DNA can be passed down the maternal line.

Part of the errors is filtered out by natural selection at the cellular level. Many germ cells do not participate in fertilization, and a significant proportion of embryos with serious abnormalities cease to develop at early stages. This filter is not perfect and does not eliminate all problems.

A new organism begins life young, but not necessarily flawless.

What remains to be clarified

In general terms, the answer to the question of offspring youth is already known. A child does not become a simple copy of worn-out parental cells. Between generations, mechanisms for DNA repair and control operate, the epigenome restructures, telomeres are restored, and cells return to a state from which the organism can be rebuilt.

However, the full picture is still lacking. It is unclear which processes play the main role in embryonic rejuvenation and how accurately epigenetic clocks reflect the age of early embryos. Scientists also do not yet know which damages are completely removed, which are preserved, and which, after reprogramming, simply cease to affect cell function.

The main question now is whether it is possible to separate rejuvenation from embryonic development and safely apply at least part of this mechanism to adult cells. To do this, they need to return to a younger state without losing specialization and without increasing the risk of cancer.

Every person has already gone through natural rejuvenation once – at the very beginning of their development. Nature knows how to turn back biological clocks. It remains to understand how exactly it does this.