One of the great mysteries being examined by medical science is what exactly causes people to show signs of aging? As we grow older, hair turns grey (some of it, at least), muscles weaken, and body parts wear out to the point of failure.
Although physical disabilities associated with aging present challenging obstacles to those who experience them – and their families – it is the decline of mental function that many people find particularly disturbing.
Senility is defined as “the physical and mental decline associated with old age; especially the deterioration of cognitive functioning associated with old age.”
To understand physical and mental decline, we need to understand a bit about neurons and how cells work.
A neuron is any cell which is part of the body’s nervous system. It “receives, processes, and transmits information through electrical and chemical signals,” according to Wikipedia. Structures called synapses allow neurons to “pass an electrical or chemical signal to another neuron or to the target efferent cell.”
(An efferent cell conveys or conducts away from an organ or part.)
New neurons are produced in the brain by neural stem cells. The process is called neurogenesis, which is thought to be involved in learning and memory.
Neurons connect to form neural networks. It has been commonly accepted in the medical community that “neurons are generated by special types of stem cells during brain development and childhood. Neurons in the adult brain generally do not undergo cell division.”
Cell division, as the name suggests, occurs when a “parent” cell divides into two or more “daughter” cells. Cancer is the result of overactive cell division.
Normal cell division (also called mitosis) is a feature of all living organisms, including humans, of course. As we grow, some cells die naturally or become damaged and need to be replaced. According to Sciencing, mitosis is the body’s mechanism that lets us “grow and change by expanding the number of total cells.”
In other words, cells die and are replaced constantly in the body. It is the rate of cell division that is thought to determine aging. The National Institute of General Medical Sciences indicates that:
“After cells divide about 50 times, they quit the hard work of dividing and enter a phase in which they no longer behave as they did in their youth.”
Now, this is very interesting:
“Each cell has 92 internal clocks—one at each end of its 46 chromosomes. Before a cell divides, it copies its chromosomes so that each daughter cell will get a complete set. But because of how the copying is done, the very ends of our long, slender chromosomes don’t get copied. It’s as if a photocopier cut off the first and last lines of each page. As a result, our chromosomes shorten with each cell division.”
As a consequence of this incomplete chromosome copying:
“the regions at the ends of our chromosomes — called telomeres — spell out the genetic equivalent of gibberish, so no harm comes from leaving parts of them behind. But once a cell’s telomeres shrink to a critical minimum size, the cell takes notice and stops dividing.”
When cells stop dividing we call it “cell death.” When too many cells die without being replaced by whole, healthy cells, the body can no longer survive.
Scientists believe, based on previous research, that the adult brain stops growing new neurons. However, new research has demonstrated that this may not be true.
An article in Cell Stem Cell, published earlier this month (April 2018), found that “healthy older subjects without cognitive impairment, neuropsychiatric disease, or treatment display preserved neurogenesis.”
Lead author of the Columbia University study, Maura Boldrini, told Science Daily:
“We found that older people have similar ability to make thousands of hippocampal new neurons from progenitor cells as younger people do. We also found equivalent volumes of the hippocampus (a brain structure used for emotion and cognition) across ages. Nevertheless, older individuals had less vascularization and maybe less ability of new neurons to make connections.”
Vascularization is the natural formation of new blood vessels which carry oxygen to all parts of the body.
To summarize the Columbia University study, it appears that adult brains do continue to produce new neurons, but that reduced blood flow and a weakened synaptic function contribute to the symptoms – both physical and mental – of aging.
As the baby boomer generation continues its advance into old age, so does some very promising research into stem cell therapy.
It might be possible, in the not-too-distant future, to minimize the negative effects of aging. This would, literally, gives millions of people around the world a “new lease on life.”